Wednesday, 21 September 2016

Energy efficient homes will 'boost economy'

News from the BBC here about Scottish investment in housing energy efficiency, which will pay itself off for years.

They include this stock photo to symbolise an energy efficient house.



What does the photo tell us?

On a superficial level, it's a thermograph, which tells us that houses are giving off heat, and the fact we've taken a picture of the house means that we care about heat. So, it says low energy building.

A brief analysis of the photo tells a different story. The different colours indicated different temperatures, going from black for the coldest part of the picture (deep space high in the sky) to white for the hottest part of the picture, which is the parts of the upstairs wall away from the lintels.

The windows are colder than the walls.

Does this mean that the windows are doing a better job at insulating than the walls? I guess this is possible for an old house where nice new double-glazed windows have been installed, but no effort has been made to insulate the walls. Or are we seeing through the windows into the room inside, which is colder than the outside walls and the roof?

The gable end is cooler too.

Does that mean the gable end is better insulated than the front of the house? This would be a good idea as end terraces have a lot more external surface area, and need more insulation to reach the same energy efficiency as the rest of the terrace. But I thought there was no insulation in the front wall?
The front gate looks pretty warm too. Interesting. Is it heated?

The house next door seems to be equally red along the wall, and along the roof, except for an area going up into a point on the roof. Could that be the shadow of a tree?

Just a guess, but this picture was probably taken on a sunny afternoon. All the heat it shows has coming from the sun, hence the warm gate and south-facing walls. The east-facing gable end has been in the shade for a while, as has the neighbour's wall and roof in the shade of the tree. The windows are cooler because a lot of the heat is going through them into the house rather than warming them up or reflecting into the camera. The bushes and trees in the garden are cooler still, because they do an even better job at absorbing the heat. Also the trees, and probably the windows too, have lower emissivity, so even if they are hotter, they'll radiate less and the thermograph won't know about it.

A thermograph taken in the day time will tell you almost nothing about the energy efficiency of a house. You need to take the picture on a cold night, when the heating inside is turned up high. Even then it's not obvious what the picture is telling you.