by Perry525 »
Sat May 16, 2009 6:52 pm
The figure of 270mm of insulation, relates to the use of glass fibre insulation only.
Glass fibre is not very good for lofts as it looses its insulation qualities when it gets wet, if you compress it by laying things on top it doesn't work as it relies on the air trapped inside to work. It is transparent to water vapour and the wind blowing over it does suck the trapped warm air out.
For a loft a closed cell insulation like polystyrene is far more practical, it is to all intents water proof, wind proof, you can lay things on top of it, without it compressing and loosing its insulation qualities and it is far more effective in as much as, you get a better result from a thinner layer.
If you pour foam between the joists you can be sure of a tight fit. With glass fibre it is difficult to fit tightly and the result is lots of gaps for the heat to escape through