Category: Passivhaus

In a previous post I described how all of the heating in our modest sized (100 m2TFA) EnerPHit house is provided by a single air-to-air minisplit heat pump. In the right situation this can be a very efficient and cost-effective heating solution, but it leaves a question of how to provide the hot water – unlike air-to-water heat pumps (the type of heat pump usually installed in the UK), minisplits aren't usually able to provide hot water*.

 

We went for a stand-alone heat pump water heater (HPWH); an insulated hot water tank with a small heat pump on top. This combination of air-to-air minisplit and heat pump water heater, along with very good fabric performance, has given us exceptionally low monitored heating costs; £175 for heating and hot water for the first 12 months of monitoring (second 12 months data coming soon!). Not bad for a house built in 1975 and in one of the windiest, coldest and least sunny climates of the UK.

 

Designers of passive houses are sometimes tempted to use direct heating (or infra-red panels) because of the low installation cost. In this scenario you've gone to all the effort of building a passive house but you end up with heating bills similar to what you would have had if you'd built a building-regs house with a heat pump. I'm hopeful that the combination of air-to-air heat pumps and heat pump water heaters can offer a similarly priced but radically more efficient (>4x) alternative in situations where spending many thousands of pounds on an air-to-water heat pump is hard to stomach because the heating demand is so low.

 

Heat pump water heaters are usually ducted to use outside air as the heat source, and this is originally how I intended to do ours. However, after coring the holes for the ventilation ducts through the concrete blockwork rainscreen of our house my shoulders were keen that I avoided doing that again, and I started to wonder whether taking heat from inside the house might work, and even whether it might be better than taking it from outside. I'm often now asked about why I did this, and whether I think it's a good idea, so what follows is my attempt to answer that. Apologies if you find it extraordinarily niche and geeky!

 

On first glance you might expect the efficiency of the HPWH to be better if it is taking heat from the warm house, rather than the cold outside. This is the case for summer, when our house is warm enough that it can spare enough heat to heat the hot water without making the room it is taking heat from too cool, but in the winter things are a little more complicated. In winter the house heating system has to provide any heat that the HPWH takes from the house, otherwise the house will get colder. If the main heating system is another heat pump, as it is in our house, then effectively this works as a two-stage heat pump – one heat pump moves heat from the air outside into the house, and another heat pump moves heat from inside the house into the hot water. If we know the coefficient of performance for each heat pump we can work out the overall coefficient of performance, the schematic diagram below shows how.

 

Schematic diagram showing flow of heat from outside the house into the hot water tank via a 2-stage heat pump.
If the heat pump water heater is taking heat from the house, then during the heating season this will increase the heating demand. In our house this additional heating demand is then met by the air-to-air minisplit heat pump, which effectively gives us a two-stage heat pump. Assuming a COP of 4 for the HPWH and 5 for the A2A minisplit this gives an overall COP for hot water production of 2.5.

 

For the above example I've assumed that the HPWH is operating at a COP of 4 (which looks about right from the datasheet for a source temperature of 20°C and a water temperature of 55°C) and that the A2A minisplit is operating at a COP of 5, which is approximately what the datasheet suggests for an outdoor temperature of 0°C, this gives an overall hot water COP of 2.5**. This is a bit lower than the 2.8 the datasheet suggests for the COP of the HPWH if it was taking its heat from outside air at 0°C, and it would be even lower if the A2A wasn't operating at such a heroic COP (COP of 4 for the A2A would give an overall COP of 2.3). These numbers will of course vary with the weather to give an overall seasonal COP.

 

So if the winter COP is worse, but the summer COP is better, what's the balance? PHPP allows you to calculate this with its HPWH tool, and for my house it predicts an additional 1.4 kWh/m2a of PER demand if the HPWH is taking heat from inside rather than outside. So slightly worse to be taking the heat from inside than out, but this isn't the whole picture, there are some other important losses associated with the HPWH that are eliminated if you take heat from inside. Specifically:

  • The ductwork between the unit and the external wall will contain circulating cold air whenever the unit is running, and to a lesser extent when it is not running. There are heat losses from the house to this ductwork.
  • The casing around the heat pump is poorly insulated and doesn't look very airtight. This will leak heat from the building all the time via both conduction and air leakage.

 

PHPP have a tool for estimating heat losses from the ducts as well, and I can also use this to estimate conduction heat loss from the casing. I can assume that the house as a whole is slightly less airtight because of the leaky casing. Doing all that gets us to a point where the two answers (heat source inside or heat source outside) are within one kWh/m2a PER of each other, close enough in my book to call it evens given the uncertainty in some of these estimates. Is there anything else to consider?

 

In favour of using the indoor air as a heat source I would have:

  • Shorter reheat times, especially in winter, meaning we are reliably able to do all of our water heating in the 5h window of very cheap electricity our dual-rate tariff offers
  • Avoids the unit reverting to electric resistance at very low outdoor temperatures. Our unit does this when the source temperature goes below -7°C, and I think other units are similar. That doesn't happen often where we are (we're too close to the sea) but it would be a much more significant consideration somewhere higher and/or further from the sea
  • Provides some useful 'free' cooling to the house in the summer, especially during heat waves. Indeed, this is significant enough that you can see the influence on the PHPP overheating assessment (see below). In a warmer climate than mine this would be especially beneficial:

Comparison of overheating risk for my house with the heat pump water heater taking heat from the house or taking heat from outside.
Under normal modelling assumptions the overheating risk for my house is small whether the HPWH takes heat from inside or outside the house, but under a stress test (no window opening, doubled internal heat gains, increase of 2°C in summer temperatures) the HPWH takes the overheating risk from what the Passivhaus Institute define as 'catastrophic' back down to merely 'poor'.

 

  • Potentially improved longevity due to the HPWH not having to work so hard
  • Simplicity – two fewer penetrations through the external walls

 

Other considerations

  • If the HPWH is taking heat from inside then plan for it to take heat from as large a space as possible. The instructions for my unit say a minimum of 30 m3, but the larger a space it takes heat from the less likely you'll end up needing to heat one cold room when the rest of the house is plenty warm enough. This may involve some creative ducting (see diagram and photo below).
  • Make sure the heat source for the house is able to directly compensate for the heat the HPWH is removing from the house.
  • I think if the house is not at or close to Passivhaus/EnerPHit levels of performance, and the hot water demand is also minimised (shower water heat recovery, radial microbore distribution), taking the heat from inside the house might be really annoying because you'd often end up with a house or a room that was uncomfortably cold outside of the heating season. Our house has 'heat to spare', regulated by the MVHR bypass opening or closing, for 5 or 6 months of the year, most houses don't have this.
  • If the main space heating is direct electric resistance or infra-red panels then having the HPWH take heat from the house is a terrible idea – the COP of heating water in winter drops to 1, the same as the space-heating COP.
  • Consider if the air flows to and from the HPWH are going to impact the ventilation system. These can be much higher than typical ventilation rates to or from the rooms they are in (while running, our HPWH has a flow rate in excess of 400 m3/h, according to the datasheet).

 

Because the utility room is small and has no direct heat source, I ducted the exhaust from the HPWH into the kitchen/dining room (via a silencer) and added additional vents in the wall between the kitchen and the utility room for supply air to the HPWH. This means the space that the HPWH is taking heat from is much larger, and has a direct source of make-up heat (the A2A heat pump) so the room does not get too cold in winter.

 

Overall, for my house at least, I don't think there's a lot in it either way, so perhaps it does all come down to whether you can be bothered to core two more holes through your external walls or not! Do the modelling in PHPP and see where it gets you, since your situation may be significantly different to mine.

 

*I've heard rumours of some models that are able to also provide hot water, I've not seen any of these.

**Thanks to Alan Clarke for helping me understand how to work this out, back when Twitter was useful.

Super efficient, simple and cheap heating, what's not to like?

Pros and cons of an air-to-air minisplit heat pump in a deep retrofit

In the UK, when we talk about heat pumps, we are normally talking about air-to-water (A2W) heat pumps (outside air is the heat source, heat is delivered to the home through water via radiators or underfloor heating), or sometimes ground-to-water (as above but with ground as the heat source). There also exist air-to-air heat pumps whereby the heat from outside air is delivered into the house via a fan-coil unit (a fan blows air from the room over the condenser coils). People often think these are bringing fresh air into the house; they aren't, only circulating indoor air over a heating coil. A separate system for ventilation is still needed.

 

The following are notes based on my own experience of using an air-to-air minisplit heat pump as the only heating over the past two years since moving back into our EnerPHit (Passivhaus retrofit) of a 1970s timber-frame house. I've not blogged for the last few years, so for a deep dive on what we did to our house, check out this podcast from the Energy Transition Show, this one from House Planning Help and this one from Building Sustainability. If you prefer videos check out this from Passive House Accelerator and  this from House Planning Help. There's also a podcast I recorded with Betatalk about our experience with an air to air minisplit here.

 

Outdoor unit of air to air minisplit heat pump
The outdoor unit of our air to air minisplit is significantly smaller than the smallest air-to-water heat pumps.

 

Indoor fan-coil unit of the air to air minisplit heat pump.
The indoor fan-coil unit sits in our dining room, which is centrally located on the ground floor of our two-storey, four-bedroom house. This provides all the heating for the whole house.

 

An air-to-air (A2A) minisplit heat pump (aka an air conditioner) can work well as a very inexpensive and efficient source of heating in homes with low heat demands. In our case we have a single minisplit, located in a downstairs dining room, and this provides all the heating for the whole house. This can be coupled with a heat pump water heater (HPWH) for hot water, either ducted to the outside or taking heat from the house. Based on my experience this system has the following advantages:

 

  • Very low capital cost (our A2A was £1,500 installed in 2023, plus £2,500 for the HPWH)
  • Air-to-air mini-splits are available in smaller sizes than air-to-water units, meaning the sizing is more closely aligned with typical Passivhaus heat demands
  • No space is lost to radiators
  • The outdoor units of small A2A heat pumps are considerably smaller than the smallest A2W units
  • It's straightforward to get to work efficiently in a very low demand house (I couldn’t find an A2W installer locally that would take me seriously about our post-retrofit heat load being under 2 kW and the things that needed to be done to get a 5 kW heat pump to work efficiently in such a house)
  • Minisplits can provide cooling in summer if this is needed
  • The heat pump water heater can provide ‘free’ cooling in summer if it is taking heat from the house
  • The room where the heat is being delivered can be ‘overheated’ on a cheap overnight tariff, and because bedroom doors are closed these stay cooler, the temperature evens out in the morning when bedroom doors open. This enables further savings by doing a greater proportion of the heating on very cheap tariffs

 

And the following disadvantages:

 

  • Less local control – in winter our bedrooms will typically be a degree or two cooler than the room where the heat is being delivered, although a degree or two cooler is often how people prefer bedrooms, so may not be a disadvantage. In very cold weather it's probably realistic/conservative to assume you'll want a bit of local direct electric heating for rooms that are far from the heat source, especially for sedentary activities (e.g. home office)
  • Internal doors need to be left open for a room to receive significant heat. For us this works well for the bedrooms – we don’t mind leaving the bedroom doors open during the day, and they get enough heat this way that they stay warm enough overnight. However, this will be heavily dependent on the fabric performance of the house – our bedrooms are in the new upstairs bit of our house, so are effectively new build Passivhaus standard
  • Some noticeable air movement and noise in the room where the heat is being delivered. In our house, where the heat pump is rarely working hard this is usually barely noticable (my wife comments that she genuinely never notices), but with higher heat demands it could be a problem
  • Slower hot water reheat times, although this is mitigated if HWHP is taking heat from the house (rather than ducting outside)
  • Imprecise thermostat (because it’s in the fan-coil unit) on A2A minisplit means the target temperature has to be adjusted to achieve the same room temperature, depending on how hard the heat pump is working. I’ve improved this by pulling the thermostat out so it is outside the unit. This has improved the situation somewhat, but it's still not very precise, I'm working on a solution to this
  • Some cycling at very low loads (I'm working on a solution to this along with the imprecise thermostat)
  • Ineligible for funding, and may impact eligibility for funding for other eco measures since A2A is not covered by MCS
  • The limit in terms of how much heat can be delivered to the house is not necessarily the output of the unit. For our house it is how quickly the heat can get around to the rest of the house. This is not usually a problem, but after a winter holiday where we’d had the heating off for ten days it did mean a long (2-3 days) reheat time for the house to get comfortable again
  • The controls and programming are not very sophisticated on my unit. I suspect because the manufacturers are not expecting people to use them as a main heat source. E.g. I can have a different programme for every day of the week, but I can’t tell the heating to stay off for two weeks and then come on 3 days before we get back from holiday (see above!). Other units might have more sophisticated controls. I’m aiming to solve this with the same solution that solves the cycling at very low loads and imprecise thermostat – stay tuned next winter!
  • For higher heat loads, or more spread-out houses, consider a multi-split (one outdoor unit, more than one indoor unit). The indoor units are typically the same size for units with different heating powers, so logically they must either be running at higher flow temperatures (less efficient) or higher fan speed (noisier and more air movement); adding indoor units should mitigate this

How efficient is our system, on a seasonal basis? This is not such an easy question to answer as it is with an air-to-water heat pump, because it is much harder to measure the heat output. However, I do have separate metering of the electricity use for the air-to-air heat pump and for the hot-water heat pump, and I have the modelled heat demand from PHPP. From this I can reverse engineer a SCOP. Based on this monitoring and modelling, I estimate that I'm getting somewhere between a SCOP of 4 and 5 for space heating. If it were much higher than 5 then this would imply much higher heat demand than PHPP predicts and be hard to believe in terms of heat pump thermodynamics, and if it were much lower than 4 then that would imply truly crazy low heating demand (significantly better than PHPP predicts). An SCOP of between 4 and 5 is pretty good, up there with the best performing air-to-water heat pumps on Heat Pump Monitor. In fact, in combination with the excellent fabric performance of our house it meant our house had the lowest heating costs for 2024 of any monitored on Heat Pump Monitor! We totalled £175 for heating and hot water for the first 12 months that I had monitoring, not bad for a 1970s house so far north (near Fort William).

 

I've got temperature monitoring in three rooms of our house, the dining room (where the heat is delivered by the air-to-air heat pump), the living room (adjacent to this room) and the main bedroom, which is the furthest room from where the heat is being delivered. We've also got temperature monitoring for outside, although because this is under a porch it tends not to get the extremes of temperature, and it tends to lag the peaks and troughs slightly (in winter it gets a bit colder than the monitoring suggests, and in summer a bit hotter).

 

Here is the coldest week of the winter:

 

Graph showing temperatures in three rooms and outside during the coldest week of the 2024-25 winter.
Temperatures in three rooms and outside during the coldest week of the 2024-25 winter. The lowest indoor temperatures (17°C) are seen in the morning in the bedroom, and begin to climb rapidly immediately after this when we get up and open the door between the bedroom and the (warmer) rest of the house. Overnight spikes in temperature for the dining room and living room can be seen - this is us using a very cheap overnight tariff to 'pre-heat' the house, which we can do without making the bedrooms too warm.

 

And here is a more typical winter week:

 

Graph showing temperatures in three rooms and outside during the a typical winter week.
During a typical winter week you can see the same trend of the bedroom being colder than other rooms overnight (down to 18°C), but it recovers more easily during the day and is generally closer in temperature to the rest of the house.

 

In summary, an air-to-air minisplit can work very well, and be very cheap. This is attractive for Passivhaus buildings where saving thousands of pounds on the heating system can help pay for some of the other things you wouldn't be paying for on a conventional build. However, I would be very wary of using such a system in a conventional leaky house. It works well in our house precisely because it never has to work very hard or move very much heat between rooms. In most houses I would expect an air-to-water heat pump to be more suitable.

 

I'll write a separate post soon on the pros and cons of hot water heat pumps and whether to have them taking heat from inside or outside the house.

 

Some other folk have had some useful things to say about air-to-air heat pumps. See this from John Ewbank, and this and this from John Cantor.

 

As I write this my feet and legs feel chilly, despite the cosy slippers and the radiator a metre behind me being so hot that I cannot hold a bare hand to it for more than a second or so. It's cold and windy outside today and the floorboards and carpet that are the sum total of the insulation and draughtproofing in my floor are clearly inadequate in terms of thermal comfort.

There really isn't any insulation under the floor is there, Daddy!?!

 

Insulating a suspended floor can be a DIY job, but it's often done in a way that performs poorly. There are two common pitfalls. Firstly when fibrous insulation is used (for example mineral wool) it isn't protected from air movement in the solum (the space under the floor, which is ventilated to remove moisture). This reduces the performance of the insulation in a similar way to the wind whipping through a wooly jumper on a windy day - the insulation is excellent but it only works to its full potential when you add a windproof layer on top. The second common pitfall is to use rigid insulation, which is difficult to fit between the floor joists without gaps, and these gaps substantially reduce the real-world performance since air movement can simply short-circuit the insulation (this is called 'thermal bypass'). This problem is present whether or not there is a windproof layer outside the insulation, but is especially severe when there is not. There is some excellent guidance on insulating suspended floors that addresses these pitfalls, and also moisture risk, from Ecological Building Systems, here.

 

I wanted to do something similar to what EBS suggest in their blog, but I wanted to add more insulation below the floor joists. To achieve Passivhaus levels of thermal comfort I need to get really low U values wherever I can. I'm already compromising a fair bit on the ground floor walls because of insulating them internally, and only adding insulation between the joists would give a similar, relatively poor U value of 0.23 W/m2K. That seemed a shame when there was still a lot of space underneath the joists in which I could add insulation while still maintaining a decent (150mm) depth of well-ventilated solum. Adding insulation beneath the joists also means that they are not exposed to the cold solum, reducing moisture risk to them.

 

In the past I've seen special joist extenders that look like half an I joist fixed to the existing joists and used to support a board that is used to hold the insulation in. This seems like a good, simple idea, but on further investigation it seemed impossible to get hold of these and the alternative of chopping I joists in half seemed expensive. While playing with some insulation samples I came up with an alternative idea; extend the joists using wood fibre insulation board, and use this to hold the wind-tight breather membrane in place. This would allow me to add an additional 100mm of insulation below the joists, bringing the U value to 0.14 W/m2K.

Proposed floor detail in cross section

I'm going to be using loosefill wood fibre insulation since it will fill the spaces well and because it is much cheaper than wood fibre or Jute fibre batts. All of these options are very good from a moisture point of view since they are both vapour open (they allow water vapour to pass through them) and hygroscopic (they will absorb liquid water and allow it to dissipate). This reduces the risk of damage should any moisture find its way into the build up, but care is needed to reduce the risk of this happening in the first place.

No insulation or draughtproofing under the floorboards makes for chilly feet. Thankfully no sign of damp so far in the places I've pulled up boards.

From the three places I've looked under the ground floor, the solum seems very dry, which is a great start, but I'll be taking the following measures to reduce the risk of moisture getting into the floor build up.

  • Airtight vapour barrier installed on the top (warm side) of the insulation; this stops both vapour diffusion and bulk air movement which if not controlled can bring significant quantities of water into the build up
  • Wind-tight, breathable membrane installed underneath (cold side) the insulation to ensure any moisture that gets into the build up can escape into the solum, while protecting the insulation from wind-wash
  • Check there are sufficient ventilation bricks into the solum, and that ventilation paths are not blocked by insulation
  • Extend the damp-proof course where necessary to make sure that insulation is not touching foundation walls below it
  • Ensure there is a 150mm clear space underneath the insulation for ventilation to take moisture to escape from the solum

The last few months have been pretty busy getting the building warrant completed and ironing out some complicated design choices about what to do with the first floor walls and roof (for another blog!). Planning permission has been granted and the building warrant is now in. I'm looking forward to getting started on the floor and starting to see some dramatic improvements in the comfort of our house.

 

 

This blog is a bit of a departure for me, a change from previous blogs that have all been about homes.

 

I've come to this issue because of my involvement, through my kids, with schools - seeing what their school is doing with regards to ventilation to try and make school safer during the coronavirus pandemic, and how similar measures have been covered in the media. I'm going to discuss ventilation in relation to schools, but much of what I'm covering applies to workplaces as well.

 

I'm not an expert on the risks of transmission of Coronavirus, so I'm not going to comment on that, I'm just going to comment on what the current ventilation strategies mean in terms of thermal comfort and energy use for different types of ventilation.

What is being asked of schools?

Schools are being asked to keep rooms well ventilated at all times, either through natural ventiation (opening windows) or through increasing mechanical ventilation rates, and dress code requirements have been relaxed in recognition that this will mean rooms may be colder than usual. Beyond this general advice there is more detail on how ventilation systems should be operated to minimise the risk of Coronavirus transmission in this guidance from the Federation of European heating, ventilation and air-conditioning associations (REHVA). This suggests actions to increase the ventilation rate in order to reduce the risk of transmission through aerosols. For mechanical ventilation systems the guidance recommends increasing the ventilation rate to the maximum, with no recirculation of air, for all occupied hours, and for two hours before and after occupancy. For natural ventilation it recommends opening windows much more than usual, even if this results in thermal discomfort. For buildings with mechanical ventilation the guidance recommends that window opening can be used to further increase ventilation rates.

 

This increase in ventilation rates has caused complaints from parents in some schools who are worried about their children having to learn while being cold (and potentially damp from being outside more than usual), and the impact this will have on both their learning environment and their health. Teachers are also concerned about the impact of opening windows.

What sorts of ventilation systems are in schools?

There are lots of different types of ventilation, but for the purposes of this commentary they can broadly be separated into mechanical ventilation with heat recovery (MVHR), and ventilation without heat recovery, which includes both mechanical ventilation without heat recovery and natural ventilation (fresh air supplied and extracted through opening windows or vents).

 

For MVHR systems the extract air does not mix with the fresh supply air, but there is a transfer of heat from one to the other. This is the type of ventilation that is installed in Passivhaus buildings. 100% of the air being supplied is fresh air, there is no recirculation, only the recovery of heat that would otherwise be lost. This is a good overview of how MVHR systems work.

Impacts of increasing ventilation rates in systems with no heat recovery

The vast majority of schools (and workplaces) have ventilation systems without heat recovery. For these schools, the impact of increasing ventilation rates during cold weather will be to decrease comfort levels and increase energy use for heating, dramatically so during cold weather. Thermal comfort is reduced by introducing cold draughts and by increasing air speeds (if you're interested there's a detailed discussion on thermal comfort here). As outdoor temperatures get colder in the winter, schools may also struggle to maintain the target air temperatures, and this will decrease comfort yet further. There is even a chance that during extremely cold weather schools will be unable to maintain temperatures above 16°C. If this happens schools may be required to close due to health and safety regulations.

 

Impacts of increasing ventilation rates in schools with heat recovery ventilation (MVHR)

It's worth starting the discussion on MVHR with noting that schools with well-designed and commissioned MVHR systems typically have better air quality all the time than naturally ventilated schools, due to more effective ventilation, especially in winter, when people in naturally ventilated schools are likely to close windows and vents because of discomfort. This has a big impact on the quality of education provision in normal times. There is a good summary of what is known about the impact of air-quality on learning outcomes for students, and how Passivhaus schools compare to naturally ventilated schools here. So before even considering increasing the ventilation rate these schools are starting from a safer point than their naturally ventilated cousins since they are better ventilated.

 

 

What happens when you increase the ventilation rate on MVHR systems? In terms of comfort the impact will be very small - the air speed will increase slightly, but because there is heat recovery the supply air is nearly as warm as the air in the room, so there are no cold draughts. For an illustration of this imagine a cold winter's day, with 0°C air outside and 20°C inside the classrooms. In this situation, with a 90% efficient heat recovery system (this is typical for a Passivhaus MVHR system) the supply air will be delivered at 18°C. Compared to a system with natural ventilation or mechanical ventilation without heat recovery, where the supply air will be at 0°C, the comfort difference is considerable. What about the impact on the heating energy demand? This is also small; in energy terms doubling the ventilation rate with MVHR is the same as increasing the ventilation rate by 10% in a system without heat recovery. There will be an increase in electricity use for the fans but this is also likely to be small compared to the additional heating energy required in a system without heat recovery.

 

There is an active discussion in Passivhaus circles about whether simply increasing ventilation rates through the MVHR is enough to reduce Coronavirus risk to tolerable levels, and even whether increasing rates considerably is desirable since very high ventilation rates during cold winter weather will lead to very dry indoor air, which carries some of its own problems in terms of increasing the risk of infection. I don't know enough about Coronavirus risk to comment on whether the standard ventilation rates for Passivhaus schools are sufficient, or whether they should be increased through increasing the mechanical ventilation rate, or even further through also opening windows. What is certain, however, is that in all three situations a Passivhaus school has a huge comfort and energy advantage over schools ventilated without heat recovery.

 

Better thermal comfort, even with open windows

We've already seen that increasing the ventilation rate through the MVHR, even by large amounts, will have only a small impact on energy use and thermal comfort. But what about opening windows?

 

Opening windows will certainly have an energy and comfort impact, but both of these impacts will be smaller than in 'normal' schools. This is because the very high standards of insulation required of Passivhaus buildings mean that the heat losses are so small that the length of the 'window opening season' (the months in which windows can be open while maintaining comfortable conditions inside) is much longer. Because heat losses through the building fabric are so low, and surface temperatures are close to the internal air tempertature, windows can be opened more of the time without occupants feeling cold. Furthermore, the number of days in which opening the windows means the heating has to come on is lower. In both comfort and energy terms Passivhaus buildings win, even if window opening is required.

 

What does this mean for how we should be building in the future?

 

Coronavirus should be a wake-up call for the construction industry - we knew before that most new buildings are inadequate in terms of comfort, energy use, climate impacts and various health risks (such as asthma). We now know that they are also inadequate in the face of global pandemics. Coronavirus makes the argument for Passivhaus standard even stronger - for new buildings we should be building to that standard, for existing buildings we should be undertaking deep, whole-building retrofits, where we can to EnerPHit standard (the Passivhaus standard for retrofit), and for health reasons we should be starting with the ventilation system.

Five minute read.

 

Our house is like many houses in Scotland; from the outside the walls look like white-rendered masonry walls. But in fact the rendered concrete is primarily a weather screen - the roof and floors are held up by a timber frame. There is a ventilated cavity between the timber frame and the rendered concrete block wall.

 

A plan view of the existing wall (what you would see if you chopped the top off the wall and looked down on it). The insulation (red squiggles) between the studs is very thin, about 15mm at most.

 

 

 

Investigations show that in the case of our house the insulation in the timber frame is extremely minimal (about 15mm thick). What should we do to improve it?

 

Some of the most obvious solutions don't work very well at all; If we add insulation to the cavity between the timber frame and the blockwork wall then we lose the important function that cavity plays in taking moisture away from the timber frame (this is why it is ventilated) and stopping moisture from the outside from reaching the timber frame. This has already been done, inadvertently, in at least one part of the house where an old doorway has been closed up and insulation simply fitted against the blockwork wall that fills the old doorway. This has resulted in a damp problem, pictured below, likely due to moisture moving through the wall (probably via both bulk air movement and vapour diffusion through the structure) from the inside and condensing on the cold blockwork, and maybe also some rain getting through the blockwork.

 

This is an old external doorway that has been filled in. Insulation has been fitted up against the external blockwork wall, resulting in a damp patch seen here as darkened insulation at the base of the opening. This is probably due to moisture condensing on the cold blockwork and trickling down to the bottom of the cavity.

 

What about adding external wall insulation to the blockwork wall? This is an attractive option since it would involve no loss of space and minimal disruption. But that insulation would be outside the ventilated cavity. If ventilation to the cavity is maintained then the insulation will be almost completely useless since cold air will be able to bypass the insulation straight to the cavity. If the ventilation is blocked then we risk a moisture problem in the wall since that would remove the current route that moisture takes to escape the wall. Damp in structural walls is never a great situation, but it's especially worrying in a timber frame house, where the structural integrity of the house depends on the timber staying reasonably dry.

 

If neither of the two obvious and simple solutions to improving the thermal performance of the walls will work, what are our options?

 

Initially, to avoid losing space internally, because it would give better U values, and because it would be less mess internally, I was keen to find a solution that we could do from the outside. We developed a really good detail that involved removing the blockwork wall, removing the sheathing board on the outside of the timber frame, adding insulation between the studs, adding an airtight sheathing board, adding a lot more insulation outside of this and then cladding with a timber rainscreen. The only one small problem with this strategy was the cost. The quantity surveyor estimated that this option was going to be £33k more than insulating from the inside!

 

So that left us to come up with a detail for insulating the wall from the inside. In the end we came up with something similar to that suggested for timber frame walls by Chris Morgan (who's working for John Gilbert Architects on this project) in his excellent guide to domestic retrofit (detail from page 80 onwards). I'm actually really pleased with this detail now. The U value will be quite a lot higher (about 0.22 W/m2K instead of about 0.14 W/m2K, so about 60% more heat loss through the walls) than the previous strategy, and some of the thermal bridging details will be trickier, but I'll be able to do a lot of this work myself (which I wouldn't have been able to do for the 'from the outside' plan), it won't involve removing the perfectly functional external blockwork and it will be a good opportunity to redecorate the rooms. Here's the plan:

  • Remove the existing plasterboard and insulation
  • Insulate between the studs using woodfibre or jute insulation batts. Using insulation batts, rather than rigid insulation boards, allows the insulation to be fitted tight on all sides, eliminating gaps that can dramatically worsen real-world performance. Wood fibre and jute insulation are both vapour permeable (meaning water vapour can move through them) and hygrosopic (meaning they can wick liquid water so it can disperse instead of causing damage). These properties are important since, by insulating so well, we are reducing the amount of heat flowing through the wall that can dry out damp areas (although we're reducing the risk in other ways, see below). They're also ecological options, with low embodied carbon emissions (the carbon emissions associated with manufacturing, transporting, installing, disposing of, etc.) and low toxicity.
  • Add another 40mm of woodfibre insulation board, fixed to the studs in the walls
  • Add airtightness and vapour-barrier membrane on the inside of the insulation board. This will be an 'intelligent' membrane that will allow drying to the inside when conditions permit. Getting the airtightness and vapour control right is crucial to reducing the moisture risk to the wall, see this excellent blog by my former MSc lecturer for a good explanation of why.
  • Add battened and insulated service cavity
  • Add new plasterboard to finish the walls

We may have to add another sheathing board internally (dependent on an assessment from the structural engineer following finalisation of what we're doing in the rest of the house), and we might have to do some remediation to the existing sheathing board to prevent wind passing through the new insulation (wind washing), but this is the gist of what we'll do.

 

Here's how the proposed wall will look, in plan view:

We'll be insulating the existing stud wall properly, then adding more insulation, and an airtightness line, inside of that.

In total this solution will 'cost' us about 100mm of space internally on each external wall on the ground floor. Not a huge amount, but the house already feels small so I don't want to lose more than this. This plan will radically improve the comfort of the downstairs rooms - improving the U values from over 1.3 W/m2K (current wall, which likely performs considerably worse than this in reality due to poor installation) to 0.2 W/m2K will increase the internal surface temperature during cold weather (0°C outside, 20°C inside) from 16.5°C to 19.5°C. For an explanation of why this is important see my blog post on thermal comfort Why do I feel chilly? We'll lose a little space downstairs, but we'll be adding space upstairs. That will require a different solution for the first floor walls, a topic for a future blog post!

Talking to people about low-energy buildings, I’m often struck by how the conversation frequently swings to thermal mass as a way to reduce heating demand. To my mind this focus in the general public’s perception is out of all proportion to the ability of thermal mass to reduce heating demand; the key performance indicator for low-energy housing in the UK. I hope this blog post might go a little way to correcting that by bringing a bit of clarity to some often-misunderstood building physics.

 

Mass doesn't 'trap heat', it resists temperature change

When heat is added or taken away from a substance, that substance warms up (its temperature rises) or cools down (its temperature falls). How much the temperature changes for a given amount of heat added depends on the specific heat capacity and the mass of the substance. Put a pan of water on a hob for twenty seconds and it only warms a little, heat the same pan for the same length of time on the same hob with just air in (with the lid on, we’re interested in efficiency after all!) and the temperature will rise a lot more. Both the density (and thus the mass in the pan) and the specific heat capacity of water are higher, so the same amount of heat energy added to it causes its temperature to rise less than for air. We can say that water is more ‘thermally massive’ than air. Similarly a litre of warm water will cool slower than a litre of air starting at the same temperature.

 

The rate at which heat flows through walls, floors and roofs is determined by their U value, and taking this equation apart is informative. The U value describes the heat flow in Watts per square metre per degree Kelvin (or Celcius) difference between the internal and external temperature (W/m2-K). There is no mention of mass in this equation. For a wall with a U value of 0.10 W/m2-K (typical of a Passive House standard building in Scotland), if the temperature difference between inside and outside is 20°C, then heat loss through the walls will be 2 Watts per square metre of wall. A similar formula describes the heat lost through replacing the air in a building with fresh air. This depends on the temperature difference between the fresh and stale air and the rate of change. In both cases the thermal mass of the building makes no difference to this rate of heat loss.

 

If two buildings have the same U values and air-change rates but one is thermally massive (it has a lot of thermal mass within the insulated envelope of the building) and one is thermally lightweight, then the rate at which the temperature in the building changes if heat losses are not balanced by heat gains will be different (the thermally massive building will change temperature more slowly). For a given amount of heat loss or gain, the temperature of the high-mass building will change less than that of the low-mass building. Conversely it will take much more heat energy to warm a high-mass building up by the same amount as a low-mass building. So high-mass buildings don’t hold on to heat, but they do hold on to temperature.

 

To understand the impact this has on the heating demand of buildings let’s do a little thought experiment on two imaginary buildings to understand how thermal mass affects the heat demand in different scenarios. One building has high thermal mass and one low thermal mass, but with identical construction of the walls, roofs, floors (let’s say all the additional thermal mass is in intermediate floors and internal partitions), identical windows, shading, airtightness, MVHR, etc.

 

Scenario one - heat losses higher than gains, heating on

In the first scenario it is cold and cloudy, heat loss from the building exceeds heat gains from solar gains, occupants, appliances and hot water systems and both houses are being kept at the same constant internal temperature by the heating system. The heating required to keep a constant temperature will be determined by the rate at which the buildings lose heat, which will be determined by the U values and air change rates, both of which are identical for both buildings. In this first scenario the thermal mass makes no difference to the heat demand of the building.

 

Scenario two - heat losses higher than gains, heating off

In the second scenario the heating is off (let’s say the owners are away for the weekend) and the weather is, again, cold (0°C) and overcast. Even though the building is well insulated and airtight it is losing more heat than it is gaining from the sun and appliances (fridges, etc. running even when occupants are out). Both of our buildings start this second scenario at the same temperature of 20°C.

 

Because the internal temperatures are the same, and the U values and air change rates are the same, the two buildings lose heat at the same rate at the start of this period. However, the lightweight building will change temperature more quickly than the heavyweight building. Let’s say that after 24 hours the internal temperature of the heavyweight building has dropped to 19°C, while the lightweight building has dropped to 18°C. Because they are now at different temperatures, they are no longer losing heat at the same rate. Our heavyweight building is now losing heat at a rate of 19°C x 0.1 W/m2-K, or 1.9 W for every square metre of wall. Our lightweight building is now losing heat slightly slower, at 1.8 W/m2. Similar differences will be present for the roof, floor, ventilation, infiltration etc. The longer this situation of heat loss exceeding heat gains persists, the greater the difference between the heat loss rates of the two buildings. Thus, after a short period of cold weather with the heating off, our lightweight building will be colder, but it will have lost less heat energy than our heavyweight building. It will therefore take less heat energy to warm it back up to a comfortable temperature. In this case thermal mass kept the heavyweight building warmer (often misunderstood as reducing heat loss) but actually increased heat loss and subsequent heating demand. In this second scenario the additional thermal mass has led to an increase in heating demand for the building.

 

Buildings built to very stringent thermal standards, such as Passive Houses, lose heat so slowly (regardless of how thermally massive they are) that they can be kept at comfortable temperatures all winter, 24 hours a day. Turning the heating off for a few hours a day doesn’t save much energy in buildings that lose heat so slowly, and having a consistent, small input of heat allows for smaller, simpler heating systems, and potentially more efficient operation of those systems. In buildings operated in this way the situation outlined in scenario two should be rare.

 

In a building that is never allowed to drop below comfortable temperatures, can adding thermal mass reduce heating demand? If so how, and by how much?

 

Thermal mass can reduce heating demand. But if mass doesn’t trap heat, how does it reduce heating demand? For scenario three let’s take a look at the way in which it does this.

 

Scenario three - heat gains higher than heat losses

Counter-intuitively, mass reduces heating demand by resisting temperature rise when gains are high. In a super-efficient house, on a sunny winter’s day, or when lots of people come to visit (each person adding about 100 W of heat) heat gains will be higher than heat losses and the temperature will rise above the heating setpoint (and the heating will switch off). For the same amount of net heat gains, a lightweight building will get warmer than an otherwise identical heavyweight one. Because it is warmer the difference between temperatures inside and outside will be greater and thus the rate of heat loss from the building will be higher. In an extreme case (maybe a really good party!) the lightweight building might get so warm that the occupants open the windows to maintain comfortable conditions, leading to even higher losses, whereas the heavyweight building can maintain comfort without opening windows (although with all those people it might be a good idea for air quality, the ventilation system in a Passive House is only sized for typical occupancy, or a bit more on ‘boost’). In this situation the heavyweight building has lost less of its gained heat, and will stay above the heating setpoint for longer once the sun has gone in/everyone has gone home.  In this situation the thermal mass has reduced heat demand.

 

So we've identified one way in which thermal mass can reduce heating demand, but how much difference does this make over the course of the year? Taking a sample of the five PHPP models I have for projects that are currently on site and which are all lightweight buildings, making them 'thermally massive' in PHPP reduces the annual heat demand by just 0.9 kWh/m2. For a 100m2 home that's a saving of just over five pounds a year on your heating bill (assuming a heat pump with a COP of 2.5 and an electricity cost of 15p/kWh). Small beer. "Ah", I hear you say, "but PHPP can't accurately take into account thermal mass because it is a static simulation". This is true, the effect of thermal mass in PHPP (as I understand it) is added in as an approximation gained from testing on dynamic simulations. But in my experience, when I've modelled buildings in both PHPP and dynamic tools (such as I did for this paper) if anything PHPP predicts slightly higher savings through thermal mass than dynamic simulation does for this part of the world.

 

What about inter-seasonal storage?

People get very excited about the idea that we can store some of the heat from summer and use it to ride out some of the cold of the autumn and winter. You can look into this question in a lot of detail and try and calculate how much mass would be needed, how much it would reduce heating demand and so on, but I think a simpler and quicker way to think about it is to flip this around: we know that it is possible to build lightweight buildings that require no more than 15 kWh/m2 of heating each year (this is the Passivhaus standard and many lightweight Passivhaus buildings are in existence). Heated with a heat pump with a coefficient of performance of 2.5, and a UK grid carbon intensity of 200 gCO2/kWh (higher than the rolling average for a year in the UK, see here) this equates to just 120 kg of CO2 per year for a 100 m2 house. In reality, since the carbon intensity of the grid is decreasing year on year, this will likely be an overestimate over the lifetime of the building. Let's be generous to mass and assume that by adding a lot of it we can reduce that heating demand to zero (we probably can't). Buildings I have seen that try to do this have done so with a lot of concrete - 50 tonnes in intermediate floors, dense concrete in the walls and so on. If we assume 200 tonnes of concrete for a 100m2 house, and 180 kg of CO2 emitted to make each tonne of concrete, that's 36 tonnes of upfront CO2. Even with making generous assumptions for mass, that's a 300 year pay back on the up-front CO2 for the additional concrete required.

 

On top of the nonsensical carbon maths of adding concrete to reduce heating demand, this approach will make other aspects of construction much more complicated: because of the increased mass the structural engineering required makes thermal-bridge-free construction harder to achieve, foundations have to be stronger (yet more embodied CO2) and so on.

 

Wrapping up (warmly!)

In summary; If you want a building that loses heat very slowly, make it super-insulated, glazed optimally, super airtight and ventilated with efficient MVHR. Chasing thermal mass as a way to reduce your heating demand is inefficient in terms of embodied carbon, cost and complication.

 

I've only talked about winter performance here, because I feel it is this that is most often misunderstood when it comes to thermal mass (people see it as a good way to reduce winter heating demand). Some mass can help summer performance in some circumstances, but it's not a panacea. A topic for another blog post!

Five minute read.

 

The plan for our house is to retrofit it to meet the rigorous EnerPHit standard, the Passivhaus standard for existing properties. But before starting that it's important to have a good idea of how the home works (or doesn't work!) at the moment, so as to understand what will be needed to reach the levels of performance required by the EnerPHit standard.

Our house, pre-retrofit, from the front.

Our house is a small (about 84m2 treated floor area, a Passivhaus measure of useful floor area), 1.5 storey, three bedroom timber frame house built in 1975. Without doing any investigation of the fabric I know that we use about 1,100 litres of heating oil, costing about £600 per year and emitting over three tonnes of CO2 each year. Almost all of this oil is for heating the house - the oil boiler does do hot water but it is not very effective meaning we usually wash our hands with cold water, fill the kitchen sink from the kettle and the bath from the electric shower. That's three tonnes of CO2 for a house that we deliberately run colder than we would like (thermostat set at 18°C), that feels uncomfortable even when the heated to 20/21°C (Why do I feel Chilly?) and that suffers from damp and mould problems.

 

So why is the house so inefficient? The EPC says the walls and roof are insulated, and there's pretty new double glazing on all but one window. Let's do some investigations...

The EPC for our house (produced before we bought the house) assumes that the loft and walls are pretty well insulated. 4 out of 5 stars sounds pretty good, right?

First of all let's look in the loft. The EPC for that assumes that there is 200mm of insulation throughout. When you pop your head into the loft it indeed looks like there is relatively new, thick insulation on the flat (because the house is 1.5 storeys some of the roof insulation is sloping), go a metre or so in either direction from the loft hatch and the insulation changes and thins to about 15mm. I suspect someone has been paid to insulate the loft and has deliberately bodged it to deceive either an EPC assessor, the home owner, or both. The construction industry is crazy.

Thick insulation next to the loft hatch, not so thick beyond there...
Thick insulation next to the loft hatch, not so thick beyond there...

So we've got 15mm of insulation in the loft, what about the walls, which the EPC also says are insulated? Looking at the gables in the loft it looks like the same insulation as is in the loft continues down into the timber stud walls, and removing a few bits of plasterboard in key places confirms this - 100mm deep timber frame but with only ~15mm of insulation. Couple this derisory thickness with the fact that the insulation is fibrous, in many places has no wind protection and is in a very leaky house (more on this below) and you end up with a house that is performing a long way below the assumptions in the EPC.

Current loft at the west gable. 15mm of insulation above the ceiling and the same in the studwork walls (to the right here)

Thin insulation in the timber stud wall, also stopping short of the timber stud.

This is looking into the roofspace at the eaves. This space is well ventilated (as it should be), but the insulation visible is fibrous and very thin. The wind washing of this insulation will render it almost completely useless during windy weather (think fleece or wooly jumper on a windy day with no windproof layer on top).

What about under the floorboards? Well the good news is that the EPC was, for once, extremely accurate here; there is no insulation whatsoever, nor any draughtproofing, under the floorboards. Remove a section of floorboard and you can feel a strong breeze, this ventilation is critical to maintaining the timbers used in the floor, and we'll need to be careful with the design of retrofit measures here to not increase the moisture risk to these timbers. No insulation, and air able to enter or leave the house directly through gaps in the floorboards explains why, even with thick carpets, the floor feels cold most of the time.

No insulation or draught proofing under the floorboards makes for chilly feet. Thankfully no sign of damp so far in the places I've pulled up boards.

We also had an old friend Ben Wear (who you can find at Ben@skyedesign.co.uk) do a pressure test to establish how airtight the existing building is. Not suprisingly it's super leaky. Depressurise the house and all the carpets lift (as air comes in from the underfloor space), and in some places the air just pours in. What's perhaps more surprising is that despite being leaky our house is not what I would call 'well ventilated'. Bedrooms are stuffy in the morning, some cupboards suffer from damp and mould and it is very difficult to dry clothes indoors, even in summer, unless the heating is on. All this despite our house being much more leaky than is sufficient for trickle vents+extractor fans to be deemed an acceptable method of ventilation by building regulations. Our pressure test result was somewhere around 15 m3/m2/hr at 50 pascals, three times the 5 m3/m2/hr above which natural ventilation is deemed acceptable. The video below shows considerable leakage from under the kitchen sink (where services come into and leave the house), from above a sliding door and through the window/door seals.

 

 

 

 

You’ve heard about Passivhaus and the superb comfort, excellent air quality, super-low energy costs and tiny CO2 emissions all sound great. You’ve decided Passivhaus is what you want for your dream home, so when should you get a Passivhaus expert on board? (more…)

 

In the previous post I looked at the main factors other than internal air temperature that affect how thermally comfortable we feel in a room. Internal surface temperature played a big role, both because of its influence on radiant heat loss and because of the ability of cold surfaces to cause fast-moving air through down draughts.

(more…)