Suntraps: how do I stop my glazed extension from overheating?
I wanted a light-filled home so I introduced lots of glazing. Now London is sweltering and my home is too hot! What should I do?
In the heat of the summer, the light-filled kitchen you dreamed of has taken on a life of its own. Sun pours through the rooflights, pools on the tiled floor and climbs the walls. Outside, the garden is bright but bearable; inside, the room built to bring the outdoors closer has become a greenhouse. Side-return extensions with expanses of glazed roofs were flung up across London as a nifty way to get light into the long plan of a Victorian terraced house. In our heating climate, perhaps that was the wrong approach. So what should we do now? It would be irresponsible to knock it down and start over, so here are some other approaches.
The glass is rarely the whole problem. A south or west-facing wall of glazing, a rooflight without shade, doors left exposed to the afternoon sun, a sealed-up old house with little cross-ventilation: together, these decisions create a suntrap. Many extensions were designed for a climate in which overheating was treated as a minor inconvenience. London’s hotter summers bring those design decisions into sharp relief. Here are some suggestions of what to do in the short, medium and long term.
This summer: quick ways to cool a glass extension
The first job is shade, before cooling. Stop the sun before it reaches the glass and the room has far less heat to lose later. A pale sheet stretched above an accessible rooflight externally can make an immediate difference. So can a shade sail rigged across the garden-facing side of the extension. Leave a gap between the fabric and the glass so that hot air can escape. Do not climb onto a glazed roof or lean dangerously out of a window to install it.
Those fold-out reflective car windscreen shades, held on with suction cups, are useful on vertical panes for a few brutal afternoons. They are a stopgap, but one that costs very little. Internal blinds or curtains help too, though they are fighting heat already inside the room.
Consider painting a temporary coating onto the external surface of your windows. There are various products on the market, but they may contain some nasty chemicals and not be so reversible. The Heatwave Toolkit suggest using yoghurt instead! Make sure it is high in fat. It turns the glass milky, reflects some sunlight and can be washed away after the heat breaks. It is best reserved for windows you can safely reach from the ground.
Change how you use the house for the hottest few days. Close blinds and curtains in rooms facing the sun. Keep windows shut once the air outside is hotter than the air indoors, then open them at night when it is cooler outside. Crank up the fans and purge that hot air! Move sleeping arrangements or work into the coolest room. Check on friends and neighbours who may struggle in the heat, particularly older people, and those with long-term health conditions, and help them implement some of these measures!
Most importantly, slow down. Heat is a health risk and the climate crisis demands action but also slowness. Batten down the hatches!
Over the next one to three years: shade your rooflights and glazing
Heatwaves won’t be a novelty for long, and we must face our new reality head on. Retractable external blinds can be retrofitted onto windows and rooflights and can include solar sensors to activate them when the sun comes out. If your house is south-facing, consider adding a canopy or pergola above large areas of vertical glazing, to block high summer sun while still allowing lower winter light into the room.
Planting has been shading us long before retractable blinds. A deciduous climber trained over a pergola will shade the extension through the hottest months, then die back in winter and let daylight back in. Solar-control film may help with particularly troublesome patches of glazing, though test a sample first: it will alter the character of the light.
Over the next three to ten years: plan for ventilation and cooling
Mechanical cooling has a place where heat creates a health risk, particularly in top-floor rooms or homes occupied by people who are vulnerable to high temperatures. An air-to-air heat pump can provide cooling in summer and heating in winter, making it a more useful long-term investment than a portable air-conditioning unit hauled out for a few bad weeks each year.
The larger task is to help the whole house lose heat after dark. Future works could create a more reliable route for cross-ventilation, with high-level openings to release warm air and secure lower windows to draw cooler air in. Replace the worst-performing glazing only when it has reached the end of its life, or once shade and ventilation have proved insufficient. It may also be time to rethink the plan: move work and sleep into the coolest rooms in high summer and change how you occupy the space.
Make a light-filled home work in every season
A light-filled home should not become unusable once summer arrives. Before taking out the glazing or rebuilding the extension, look at how the sun gets in, where it heats up and whether it has a way to cool down after dark. The temperatures we are experiencing in London this summer can be really dangerous, so start with quick DIY solutions to shade the glass outside, but think also about long term solutions to make your house more liveable in all seasons. Mechanical cooling has a place in some homes, especially where heat creates a health risk, but it should support a building that is already working with the seasons.