How to Keep Your Cool: Reduce summer overheating through windows
Windows are a really important cause of heat gain in many homes, so the first thing we’re going to look at is how to reduce heat gain through windows (and doors and any other glass or transparent areas your home may have).
This is the basics of what happens when the sun shines through a window:
The first thing we need to think about to keep our homes cool is… What do people in hot countries already do?
In hot countries, houses usually have small windows, especially on the sides of the house where the sun can shine in (East, West and South in the northern hemisphere).
Where windows aren’t small, the roof often overhangs a long way, preventing the sun shining directly into the windows- or on lower floors there are often balconies, pergolas or other shading such as brise soleil.
External shutters (traditionally wooden, now more often meta, either painted a pale colour or reflective) are often used to prevent the sun from shining in.
Less traditionally, special glazing films or specialist glass can be used that reflect a lot of the sun’s light from the window- like sunglasses for windows.
We can use similar approaches here.
This is how they work:
With internal shading, most of the light and heat energy from the sun is reflected out of the window instead of heating up the room, but a small “greenhouse” area which still heats up remains between the window and the curtain or blind.
Almost all the heat and light energy from the sun is reflected from the shutters (depending on the shutter material- pale shiny shutters will be more effective than dark wood).
So, the best sort of shading to keep the heat out would be reflective external shutters- just like the metal shutters you see on shops, or on thousands of homes all over Europe. Combine these with some nice big roof overhangs and a pergola (covered in grapes, of course) to sit under outside and your house will stay much cooler.
Of course, the roof overhang and pergola are not going to be possible for most people and certainly aren’t a quick fix (though by all means go for it if you have the space- it sounds lovely…), while external shutters are expensive to install and, since windows in the UK usually open outwards, often would mean replacing windows too. If you can get external shutters, they’re the best option- and worth researching if you have the money and an overheating problem.
Internal shutters look lovely in BBC period dramas, but are equally out of reach both price and opening-space-wise for most of us.
This means that usually internal blinds (or blind-like-things- see the ideas in the linked Activity of the Day posts) are going to be the most practical option to keep the sun out.
If you’re able to get blinds, choose as pale a colour as you can. If it’s practical (for example in bedrooms where you're less likely to want some daylight to come in) choose blackout fabrics or other styles that block a lot of the light (such as reflective metal venetian blinds) to keep as much of the sun’s energy as possible out of the room. Fit blinds as close to the glass as you are able, to keep the “greenhouse” area of the room which will heat up between the blinds and window as small as possible.
Curtains are also good for keeping heat out, provided they’re not really dark coloured on the back as dark colours will absorb more energy and heat the room more: Pale coloured or reflective linings can be fitted to the backs of curtains (you can make them yourself if you have old sheets or similar going spare- or follow the instructions for a simple blind-curtain below). Curtains are usually hung further from the window than blinds, though, so do leave a larger "greenhouse" area between the window and the shading to get heated by the sun than most blinds.
If you’re not able to get blinds, there are other options:
Years ago I worked on the third floor of a 1980s building in a room where the whole South wall was giant single-glazed window panes. To prevent people falling out, only one window opened, and that only a tiny way - but the blinds had broken and couldn’t be closed. Unsurprisingly, the room got very, very hot. For “health and safety” reasons, we weren’t allowed to put in new blinds or fit curtains, but the temperatures were so high the room wasn’t safe to be in anyway!
Sticking scrap printer paper all over the windows (which apparently wasn’t a breach of health and safety) brought the temperature down into the bearable range- though it did cut out the view as well: There’s always a cost!
The next few Activity of the Day posts are going to look at different options for shading windows (and any other glazed areas)- they’re either cheap or free, and some should be suitable for every situation.
The last thing to remember is that shading for windows only works if you use it!
No matter how incredible the window blinds, shutters, curtains or whatever you have are, they won’t work if you don’t use them. They need to be shut before the room starts warming up for the day and stay shut until overheating isn’t a risk any more. During a hot spell or heatwave, that usually means keeping windows shaded most of the day (West facing blinds may be able to be left open in the morning, East facing ones may be OK open in the afternoon).
East and West facing windows are the worst directions for causing overheating as the sun shines straight through them for hours each day in summer, whereas the high South sun doesn’t come in through South facing windows as much as you’d think, and is easier to shade. It’s also easy to forget to close blinds on windows that only have the sun on them for part of the day. But one day of forgetting can cause enough heat buildup to take a few days to reverse…
In places where lots of heat reflects off hard surfaces like tarmac and walls, you may find you need to keep your windows shaded even when the sun has gone, to prevent all the heat from the surfaces coming in through your window. You’ll know whether or not this is an issue for you- if it feels like an oven when you stick your head out of the window even when you’re in shade, there’s a lot of radiant heat coming from hard surfaces and hitting you: Keep that heat out of your room by keeping the windows covered!