Why does my house overheat even with good insulation? – Can I Just Ask? | Ep.42

Overheating in well-insulated homes is a growing issue in the UK, and one that catches many homeowners and builders off guard. As the climate shifts and sharp heat events become more frequent, the assumption that good insulation equals a comfortable home year-round is being tested. The problem, as discussed in this episode, often comes down to an over-reliance on U-values as the primary measure of a building’s thermal performance — a single metric that says very little about how a building actually behaves in summer.

Several factors compound the problem. Airtightness is a major one — without it, hot external air simply pushes through the fabric and warms the interior regardless of how well-insulated the walls are. Glazing orientation is another often-overlooked issue: east and west-facing windows allow low-angle sun to penetrate deep into a space even when shading is present, making them significantly worse for overheating than south-facing glazing, which is far easier to shade effectively.

The material choice for insulation plays a bigger role than most people realise. Low-density insulations can achieve impressive U-values on paper but have poor thermal mass, meaning heat moves through them quickly. Wood fibre, by contrast, combines high specific heat capacity with relatively low thermal conductivity, resulting in a thermal diffusivity that slows heat movement dramatically — up to 16 hours compared to around two and a half to three hours for fibreglass in a roof build-up. That time-shift means the heat from a hot summer day doesn’t reach the interior until temperatures outside have already begun to drop, keeping internal conditions stable and comfortable.