How to Keep Your House Cooler in Summer — With Cork Insulation

The Met Office's last five summers have all landed in the UK's top ten warmest on record. Bedrooms under uninsulated roofs are routinely hitting 28–32°C overnight. South and west-facing brick walls are radiating that heat back into rooms long after sunset. If you want to keep your house cooler without bolting on air conditioning, the single most effective fabric intervention is the same one that keeps it warmer in winter — proper insulation. The difference is in what kind.

Why most UK houses overheat

British housing stock was designed around one problem: keeping heat in. Solid brick walls, slate roofs, single-skin extensions — all of it was built for a climate that rarely pushed past 25°C. The thermal mass of a brick wall absorbs solar heat all day and dumps it inside through the evening. A black or dark grey roof slope can hit 60–70°C surface temperature on a clear July afternoon. With nothing to slow that heat transfer down, you get the classic August scenario: rooms that are still 27°C at midnight.

Insulation works both ways — but only if you pick the right kind

There's a stubborn myth that insulation 'traps heat in' and therefore makes a house hotter in summer. It does the opposite. Insulation slows heat transfer in whichever direction the heat is flowing. In winter that's outward; in summer that's inward. The reason so many UK loft conversions still cook in summer is that the cheapest mineral-wool fits the legal minimum U-value but has very little thermal mass or solar reflectivity — heat passes through it within a few hours.

Cork is unusual because it does three things at once: it has a low thermal conductivity (around 0.065 W/mK), it has meaningful thermal mass for the thickness, and the surface reflects a significant proportion of incident solar radiation rather than absorbing it. Translated into July: a sprayed cork roof or wall coating delays peak heat transfer by several hours and reduces the overall amount that gets through. The bedroom that used to peak at 30°C at 11 pm now peaks at 26°C — which is the difference between sleeping and not.

Where to spray cork to keep a UK house cooler

1. The roof or loft conversion ceiling

The single biggest summer-heat entry point in a typical UK house is the roof. South-facing slate or tile roofs absorb extreme solar radiation and conduct it straight into the loft space — which on a converted loft is your bedroom ceiling. Sprayed cork applied to the underside of the roof deck or to the external slate/tile surface (conservatory roofs especially) cuts that solar gain dramatically.

2. South and west-facing external walls

Brick walls facing the afternoon sun are the second biggest contributor. External sprayed cork at 3–6 mm sits between the brick and the sun, reflecting part of the solar load and slowing what gets absorbed. The same coating that cuts winter heat loss by up to 30% cuts summer heat gain on those elevations.

3. Single-skin extensions and garden rooms

Single-skin block, timber-frame and SIPs extensions have almost no thermal mass. They heat up within an hour of direct sun and stay hot. An internal or external cork coating buys you several degrees of buffer — often the difference between using the room in August and abandoning it.

Cork vs the other 'keep cool' options

What about Part O and overheating regulations?

Since June 2022, Part O of the Building Regulations has required new dwellings to be designed to limit summer overheating. Existing homes aren't directly captured, but the same principles apply: reduce solar gain on glazing, increase fabric thermal performance, allow purge ventilation at night. Cork doesn't replace good ventilation or appropriate shading, but it does the heavy lifting on the fabric side — and on retrofit projects where adding shading is impossible (listed buildings, conservation areas) it's often the only intervention available.

A realistic checklist for a cooler UK home

• Insulate the roof or loft properly — not the legal minimum, the genuine minimum for comfort.
• Address south and west-facing walls with a coating that reflects and slows solar gain.
• Add internal or external shading on the worst-affected windows (Velux blinds, external louvres, planting).
• Open windows on opposing elevations late evening for purge ventilation, close them by 8 am.
• Avoid running heat-producing appliances (oven, tumble dryer) during the afternoon peak.

The bottom line

A cooler house in July and a warmer house in January are the same problem solved twice. The wrong insulation (thin mineral wool, foam board with no thermal mass) helps with one and barely touches the other. Sprayed cork is one of the few fabric upgrades on the UK market that genuinely does both — and it does it for 40+ years with no repaint cycle.