Thermal Imaging in a Heatwave

Find good pointers on how to keep a home cool by thermal imaging it during a heatwave

Insulation for keeping a home warm in winter will also keep it cool in a heatwave. But depending on how the sun heats the building, extra protection may be needed to avoid overheating.

Look for the hotter outside surfaces of the building

They’ll vary with the time of day and the sun’s angle. Can they be shaded?
Can the rooms on the inside of these hot surfaces be kept cooler by better insulation or ventilation?

During a heatwave, roofs and walls get HOT, especially if they're facing the sun.  This thermal image shows a slate roof in Cambridge (UK) with a surface temperature [Sp1] of 74.4°C.  Good insulation is needed under a roof like this to avoid it heating the house below. If the insulation is in the loft floor, rather than the roof , it will have a very hot loft space, probably radiating and conducting heat down into the house.
When heat flows downwards like this, convection is unimportant and radiation becomes more significant. Multifoil insulation that reflects thermal radiation can be effective.  Follow their installation instructions regarding air-gaps. Avoid putting a vapour barrier in the wrong place.

It may be possible to cool a hot loft space by ventilation.  We’ve put vents (with insect-proof mesh) under the eaves and also into an unused chimney near the top of the loft space to create convective ventilation, drawing in cooler air from under the eaves.

The wall in this first image at 38°C [Sp2] is less hot than the roof because it's not angled towards the sun and it's white-painted, reflecting some of the solar energy (see below).  Also being an uninsulated solid brick wall, more of the sun's heat on the wall is probably being conducted inside, lowering its surface temperature, compared to the roof. Or the thermal mass of the brick may still be heating up after the cooler night.

When thermal imaging in heatwaves, be prepared for transient heat flows like this, when the sun hits a wall or stops heating it.  Heat flows are rarely steady-state and the surface temperatures of brick walls, etc, with thermal mass will often be affected by their  bulk temperatures, resulting from previous conditions.

Compare temperatures in the room inside

Where a wall or roof is hot outside, is its surface temperature in the room inside significantly hotter than other surfaces in that room? If so, heat flowing in through that wall is heating up the room.

Can you benefit from shading?

The thermal image of the front of these terraced houses shows the advantage of shading: the coolest spot 33.4 °C [Sp1] is under the tree, by the left home's front door.

The roof slates [Sp2] at 68.2° are the hottest, in the full glare of the sun.

Both houses have uninsulated solid brick walls, the left one is plain brick at 48.3°C [Sp3], whereas the white-painted right house is at 40.8°C [SP4], probably because its white surface reflects more of the sun's heat.


This image shows the benefit of two DIY fabic awnings, made as described in this article.

The brick wall in the sun [Sp1] is at 51.3°C, while the nearby window frame, shaded by the top awning, is at 33.3°C [Sp2].  The window frame shaded by the lower awning [Sp3] is at 35.9 °C.
We can sit at the table, shaded by the lower awning, which is much more comfortable than in the full sun, during hot weather.

The hottest part of this image is the roof of the extension, sloping towards the camera, which is about 70°C.  It has 2 Velux rooflights which are open a little. It would be better to shut them with the surrounding roof tiles so hot.  These Velux windows are fitted with external awning blinds  useful for reducing solar heating of the room below.

Make use of cooler spaces

Which rooms are cooler and more comfortable? This may vary with the time of day.

The suspended wooden floor [Sp1], which is insulated underneath, is at 23.7°C. In fact the sub-floor below these floorboards and insulation was at 16°C on this day - a useful cold area. During a heatwave we open a hatch in the floor under the stairs, along with windows and skylights at the top of the house, to draw cool air from the subfloor up into the house.  It's hard to persuade this cool air to rise, so I'm going to try a fan, fitted in a board, to place over the hatch opening.

Tom BraggCambridge Carbon Footprint

See Keeping Cool in a Heatwave for more general advice