Why 100mm then 50mm insulation in loft rooms?

14 Sep 2010
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United Kingdom
I'm relatively new to the site and a novice DIY'er.

I seem to read a lot about condensation due to lack of ventilation between the insulation and roof. 100mm insulation between the rafters with 50mm insulation covering the rafters and 50mm air gap seems the norm.

Would 75mm between, 75mm covering and 50mm air gap insulate the room better?


50mm between, 100mm covering and 75mm air gap prevent condensation and ventilate better?
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the bottom line is put in the most you can practically based upon cost and keep your air gap as big as you can. Consider headroom as that will have an effect on how much you have under the rafters. you are looking for a u-value of 0.18 for your roof. have a look on the celotex website they have a calculator where you can fiddle about with the various options
What sort of covering do you have under the tiles, is there a breathable membrane, felt, close boarded or nothing. This will determine the amount of airgap you need, but 25mm is normally a good rule of thumb. This would mean either 75mm insulation to the top of the rafters, or add a 25mm batten to give you 100 insulation. Then you'd use 50mm insulated plasterboard placed across the rafters. Fill any gaps in the first layer with expanding foam, and them use aluminium tape on any joints on the top layer.
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You'd get a slightly better u-value if you put the 100mm over the joists as you minimise the thermal bridging from the wood but then you'd loose 100mm of headroom rather than 50mm..

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