09/09/2026
Summer overheating in UK homes is usually treated as a ventilation problem, or increasingly an air conditioning one. But it starts earlier than that, with the fabric of the building.
The belief is that British homes trap heat because they were built to keep it in. That gets the physics backwards, and it flatters our housing stock more than it deserves.
Insulation slows the flow of energy from the warm side to the cool side, whether it’s cold outside and cosy inside, or the opposite. Around 80% of European homes have no air conditioning, and the housing that copes best in a heatwave tends to be the housing with the best fabric, not the leakiest.
In a loft conversion the build-up is squeezed by the existing structure and the headroom you have to keep. Insulation depth comes first, since a U-value missed is not recovered by anything else. Beyond that, material choice matters more in a roof than anywhere else, because a sunlit roof surface runs far hotter than the air and the heat arrives in a daily cycle. Dense insulation such as woodfibre absorbs more of that heat before passing it on, flattening and delaying the peak. It needs roughly 1.7 times the thickness of PIR for the same U-value, so in a constrained roof there is a real trade to make.
In winter this is a gift, reducing the work the heating has to do. In summer, unmanaged, it is the main driver of overheating. The same physics, the same glass, a different season. Shading and glazing in the next post.
Full article on summer comfort at mountainfold.uk. Link in bio.
