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.

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.

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:

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!