Do your window covers block out the cold? It turns out that horizontal blinds work pretty well, especially if you tilt the rails downwards, so that they open towards the floor, a bit like the feathers on a bird.
I used to think that it would be best to have the opening upwards, as I thought it would reduce the coldness falling from the window into the room. But it turns out that the bigger issue is to prevent warm air from inside the room from falling onto the window in the first place. Angling the rails down stops cold air circulating – a bit like feathers on a bird.
I took some thermal images to test this theory. I chose a cold night of about minus four degrees Celsius outside, after our heater had been on for several hours, so it was about 20 degrees inside.
The following pictures show a series of close-ups in the same mid-section of the blinds. The thermal scale is the same in each picture, so you can compare the impact of tilting the rails. In these photos, blue and green are cold, and white is warmer than red.
The rails are tilted down in the first picture, which is the right way to do it. Here are some close ups. When the rails are angled down, there is a lot of white, quite a bit of red, and no green or blue. Rails tilted down stops cold air from circulating out from the centre of the blind.
In the next pictures, the rails are tilted upwards. There is less white and more red here. The blinds are still stopping a lot of the warm air leaking out the house, but not as much as when the rails are pointed down.
And in case you are thinking that maybe the blinds don’t do much at all, have a look at them when they are open. The heat loss is huge, with glaring great blue-cold glass leaking the warmth out right away. I could really feel it too, when I opened the blinds and the room felt colder right away.
It’s also worth thinking about the whole blinds. The photo from further back shows that there is still some coldness down at the bottom. These blinds would work even better if I could seal off the base, but I haven’t worked out how to do that yet.