Cavity Wall Insulation

[age]

How does cavity wal insulation work?

[Hamish]

To answer this first we need to know why there is a cavity in the first place. Houses were built with cavity walls so that a layer of poorly conducting air is trapped between the two walls, in the cavity. This reduces the amount of heat lost directly through the walls. But the cavity is big and the air can move about. Filling the cap with a foam still leaves a gap filled maily with air (or a gas which is a poor conductor) but stops the air moving about so reducing further the lass of heat through the wall. Hope this answers you question. Hamish

[Haitch]

So why does blowing on my drink cool it? Actually air is pretty good at shifting heat through convection (it always manages to shift all the warmth in my house to the ceiling of the room). It's difficult to conduct heat through a gas because as soon as you heat a molecule the energy you give it makes it space itself out more so it starts to rise and you get all the warm molecules congregating together and not passing on their heat to cooler ones.The idea of a cavity wall is that the 2 layers of brick conduct heat and the gap provides a break so to heat the outer layer it has to heat the air in the gap which then has to pass the heat energy over to the outer layer by convection. In a foam you multiply the number of times heat energy would have to pass from solid to gas and back again. With such tiny air pockets you reduce the ability to convect the heat to the other side.