Category: Home retrofit

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!

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.