Depth of Loft insulation

15 Feb 2010
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United Kingdom
I am reinsulating my loft and at the same time boarding it to create storage, the depth of joist is approx 100mm, is 100mm insulation thick enough? and/or does squashing 170mm insulation under boards do more harm than good?
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Squashing insulation justs wastes money. The isulation depends on holding air to insualte and when it is squashed there is less.

I have read - don't know myself - that you will only get the value of the finished depth i.e. if you squash 170 mm down to 100 mm , you will have the value for 100 mm.
in your case, I suggest counter-battening over the existing timbers, at right-angles to them, with 70mm or 100mm timber. This will enable you to lay an extra layer of insulation between the new battens, and because it is at right-angles to the existing insulation, it will reduce the risk of thin patches or gaps. Lay your flooring onto the battens. They will also help spread your weight while you are moving about up there

Alternatively, lay rigid insulating foam boards (the PU is better than polystyrene) on the timbers, and the floring on top of that. PU boards provide more insulation for the same thickness than loft quilt does, but is more expensive.

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