Month: July 2020

Five minute read.


Our house is like many houses in Scotland; from the outside it looks like a white-rendered masonry wall. But in fact the rendered concrete is a 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). 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.


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 0.2 W/m2K instead of about 0.14 W/m2K, so about 40% 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 perfectly functional wall 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 50 or 60mm 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 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.