Keeping cool in a heatwave | Sustainable Witchford

Guide Cambridge Carbon Footprint Heatwaves and overheating

Written by Tom Bragg, a Cambridge Carbon Footprint volunteer, and reproduced here for Witchford residents. It covers shading, ventilation, reflective surfaces, insulation, thermal mass and air conditioning, along with advice on staying safe in hot weather.

heatwave
overheating
adaptation
shading
ventilation
insulation
health

Expect more frequent, more severe heatwaves

2026’s extreme June heatwaves emphasise that Britain’s outdated housing, including most new-builds, have an overheating crisis. Homes and habits need to adapt to become more Mediterranean. Many recent UK homes are still poorly designed to prevent overheating, as required by Building Regulations since 2022.

Shade your home

Minimise the sun’s heat getting in through windows and rooflights.

Internal curtains or blinds, especially white or reflective ones, can help reduce the effect of sun coming in a window, but external shading that keeps the sun right out is even better. See FamilyRollo Roller Blinds.
External shutters are traditional in southern Europe for a good reason. External blinds can be electrically operated. See many varied examples.
DIY: re-sized white sheets are effective and let some light through. Try old sheets from charity shops or FreeCycle. See the Heatwave Toolkit.
Awnings, retractable or DIY (video), can shade windows or a patio. DIY awnings can be deployed from an upper window, but take them down if it gets too windy.
Remote-controlled, solar-powered electric awnings can automatically deploy when sunny and retract if too windy. See more choices and further ideas for external shading.
Rooflights angled towards the sun let in a lot of heat. Most Velux roof windows can have external anti-heat blinds retro-fitted. Their interior blinds can help too. Other manufacturers have their own products.
Trees that shade your home really help. Deciduous trees let you benefit from winter sun too. Could you plant one or more for the future? Or build an arbour or pergola to shade your home or patio?
Solar-control film on windows reduces the sun’s heat coming in, but it cuts some light too. It is a low-cost solution that you can fit yourself. See video.
Low-G glass, or solar-control glass, lets in much less radiant heat from the sun, with a small reduction in light transmission. Specify it for new or replacement sunny windows.

Note: solar-control film and Low-G glass also reduce useful solar gain in winter. Controllable shading is better in this respect.

Green external shutters on a window, illustrating external shading.

External shutters and blinds keep solar heat out before it reaches the glass.

Ventilate right

On a hot day, as soon as the air is hotter outside than in, close all doors, windows and vents that would let hot air in. You may be able to keep ventilating longer on the shady side of your home.
Cool off at night. To sleep better and cool down your whole home, let hot air out by opening windows or skylights at the top of your home. Draw in cool air by opening ground-floor windows or vents where the air is cooler outside. Fans, including upper-floor extractor fans, may help.
If your home is only on one floor, purge the hot air by opening windows or vents on opposite sides if possible. If you only have openable windows on one side, can they be opened at the top and the bottom? If so, place a fan to draw in cool air at the bottom. Hot air will rise and be expelled through the top.
Security: many windows can be part-opened in secure ways that will not let a burglar in. For example, restrictors like these can be fitted to wooden sash windows, letting them only open a little when needed.
If mosquitos or other insects are a problem, consider fitting fly screens to keep them out. More options are available here, including magnetic ones.
DIY using Midge Mesh. It is also good for solar screening while still enabling you to have a view out. See DIY video; information is in the YouTube description.

White walls and roofs absorb much less heat from the sun

Thermal and visible images of two neighbouring houses, showing a white wall surface 21 degrees Celsius cooler than a plain brick wall during a heatwave.

This thermal image shows a white wall’s surface in a heatwave that is 21°C cooler than the neighbour’s plain brick wall.

Of course, the rooms inside will not be 21°C different, but the rooms with a white wall will be significantly cooler. Both homes are making good use of white internal blinds and shutters. The white home on the right has a very hot grey front door that would be cooler if painted white. See White Walls are Cooler in a Heatwave.

Flat roofs get very hot in the sun too. This Aluminium Solar Reflective Paint suits asphalt or bituminous roofs.

Thermal image of a slate roof showing a surface temperature of 74 degrees Celsius.

A slate roof recorded at 74°C in a thermal image.

Avoid unwanted heating from appliances, lighting and cooking

Choose low-energy appliances and lights. Use them only as needed.
Are your hot water pipes and tank, if you have one, well insulated? Adding insulation will save you energy in the winter too.
Is your fridge or freezer in a hot place? It will be inefficient and make that place even hotter. Fridges and freezers work best in cool places, although check their user guides for operating ambient temperature range. For example, a garage may be too cold in winter.
Eat salads and minimise cooking, especially ovens. Use the extractor fan if that removes cooking heat. Barbecue outside or cook in the cool evening with the door and windows open.

Insulate walls and roofs, and use thermal mass

Insulate walls and roofs, especially those that get hot in the sun. Reflective insulation, such as multifoil, is effective for reducing heat radiating down from a hot roof. Draught-proofing keeps hot air out. These measures also save energy used in winter heating.

Thermal mass from brick, stone and concrete stabilises indoor temperatures, reducing peaks, as in cathedrals. External wall insulation, with the thermal mass of the walls inside, is good for this. You can benefit from the thermal mass of solid floors too by covering them with tiles rather than thick carpets. Cooling the thermal mass in your home with night-time ventilation helps keep those rooms cool in the day.

Air conditioning

Try all the passive cooling measures above that work for your home and budget. If you are still too hot, you probably need active cooling, consuming energy, usually meaning air conditioning.

Air-to-air heat pumps are very similar, and most can cool you in summer like air conditioning, and heat you in winter, providing 3-4 kW of cooling or heating from 1 kW of electricity. In a heatwave, your outdoor air-conditioning unit blows out hot air, which may heat your neighbours and worsen the urban heat island.

Cool yourself

Spray or sponge yourself with water, or put a damp towel around your neck.
A fan can help you stay comfortable, especially with a spray. Its electricity consumption is low compared with air conditioning.
Similarly, cool your home by hanging up damp towels or sheets, maybe combined with a fan. This works well if it is not too humid.
Take a cool shower.
Use a cold pack around your neck or under your arm, in a sock if too cold. Put a nearly full water bottle in the freezer; later, cover it and take it to bed.
Wear loose, thin clothing, a wide-brimmed hat and sunglasses outside.
Avoid going out or exerting yourself in the heat of the day. Plan ahead.
Take a water bottle and sun shade when you go out. Check the weather forecast first.
Do not get dehydrated. Have cold drinks regularly.

Avoid heat exhaustion and heatstroke. See NHS advice.

Water running from a shower head.

A cool shower can help lower body temperature.

A cold drink with ice, mint and lime.

Cold drinks help reduce dehydration risk.

Human choices

If your home is too hot, is there anywhere cooler you can go? Can you sleep in a cooler room, on the north side or ground floor?

See this NHS advice on staying safe in hot weather. Older people and young children are most at risk.

Some people, like those in sunny, single-aspect flats, have few options to stay cool and suffer badly in heatwaves. If global heating reaches 2°C above pre-industrial levels, as expected by 2040, 9 in 10 existing UK homes will be at risk of overheating, but many of the improvements needed are fairly simple for most homes. Ones that are difficult to improve will need air conditioning.