How to Choose Window Treatments to Keep Your Home Cool
Do curtains cut it? Are shutters sufficient? We asked the experts for some heat-reducing advice
Professional advice from: Kit Knowles of Ecospheric; Tim O’Callaghan of nimtim Architects; Rob Bohm of CLPM
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“The climate crisis and the increasing summer temperatures we are beginning to see pose a huge challenge, because homes in the UK have been designed in the context of a much cooler, more temperate climate,” architect Tim O’Callaghan says.
Homes in warmer countries are traditionally built with features that prevent the interior from overheating, such as external shutters, large roof overhangs, solar shading and cross ventilation, but, Tim says, “this hasn’t really been part of our domestic architectural language”.
“The high temperatures we’ve experienced have certainly highlighted the deficiencies in a large proportion of our housing,” heating and energy consultant Rob Bohm says. “This needs to be addressed, both to improve comfort levels and prevent us relying on air conditioning, with the result that we reduce our energy usage.”
“Windows contribute the vast majority of any direct solar gain input in a house, in combination with air temperature and internal heat gains (from people and appliances),” managing director and sustainability consultant Kit Knowles says. Our preference for large windows and lots of glazing is, of course, exacerbating that.
If you’re in the happy position of designing a new home, modifying the amount of glazing and number of windows it has will have an impact on heat loss and heat gain.
In an existing property, shading is the principal approach, but it’s helpful to understand how effective – or not – this can be. “You can block sunlight in three ways,” Kit says. “Externally, which has the potential to keep 100% of all heat out; mid-pane, by fitting glazing that has integral Venetian blinds between the panes of glass, which can keep 60% to 80% of heat out; and internally, with curtains or blinds. This method can only block up to 40% of the heat, so at least 60% of the heat hitting your window will make it inside.”
“External shutters are one of the most effective ways to reduce heat gain,” Tim says. “When sunlight comes through our windows and hits our home’s interior, it turns directly into heat. We call this solar gain. Shutters will block this.”
However, it’s not always straightforward to fit shutters on UK properties. “There can be planning issues,” Tim warns, “as it’s not something we’ve traditionally required. Our homes haven’t been designed with shutters in mind, unlike in southern Europe, for example, so there isn’t always the space for them or an appropriate place to fix them. If it’s possible to do it, it’s a great idea.”
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Other forms of external solar shading include fins – vertical elements that project from the main wall – and brise-soleil (’sun breakers’, pictured), which typically extend out horizontally from an external wall to deflect direct summer sunlight.
“If you’re building a new house or an extension, now’s the time to add architectural details such an overhang or a brise-soleil,” Rob says. “These can be very effective, as they will cut out the high summer sun but allow the lower, weaker winter sun in, which is desirable.”
This modern extension on a house in Australia has shades built around its windows to provide effective shading.
With any of these shading options, there are potential downsides. “Consider the cost of design, purchase and installation,” Kit says. “There can also be conflict between the principal views and effective shading, and between maximising daylight and effective shading.”
Curtains and blinds hung inside your home can certainly help. “The obvious point is to close these existing window coverings before the sun rises too high,” Rob says.
One other good solution is to fit solar reflective film. “It can be fitted internally or externally to existing windows and it’s relatively inexpensive at around £15 to £20 per square metre, although the external film can be more, as it needs to be weather-resistant,” Rob says.
“Just keep in mind that the film will also reduce the solar gain from the winter sun,” he adds. This means the house isn’t being warmed naturally by sunlight as much, which may increase the need for heating in the chillier months.
“On a very simple level, lighter colours on the rear face of the curtains or blinds, particularly white, will reflect more light than black or dark colours, so specify a white lining or choose white blinds if possible,” Tim says.
Many roof windows are fitted with blinds that will block some heat, but its their position in the roof that matters most. “The main thing about roof lights is to be careful about where you place them,” Tim says. “Any that are south-facing will create a large amount of heat gain.
“If you want to reduce heat gain on a roof light that’s already installed, films are a great option,” he says, “and you can probably use ones that are more reflective and darker than would be noticeable on a vertical window.”
Deciduous planting schemes are a cost-effective way to provide natural shading in summer while allowing light in during winter.
“Many trees and climbers shed their leaves in winter and will grow up a simple trellis or wire on walls without damaging the building,” Kit says. “Their leaves increase the depth of the reveal by the window, shading the glass, and then drop in time for winter when you want that sun to come in and contribute its warmth.”
The wisteria on this house, for instance, is a deciduous climber that’s covered in fragrant flowers between April and June.
There are other benefits, too. “While shutters may need to be manually opened and closed, this is an automatic system that doesn’t require your input,” he says, “plus trees or plants look good and contribute to biodiversity.”
Be careful planting on or near buildings, though, as this can shorten the life of the structure or create root impact. And remember that planting will block views in summer.
One final window treatment is to simply open them, but only at certain times. “Night-time ventilation is a key strategy for keeping your home cool,” Kit says. “When it’s very hot, opening windows during the day doesn’t work. If it’s 30ºC outside, you don’t want to let that hot air in, but it can work at night.”
Do you know of any other window treatments that reduce overheating? Please share them in the Comments.