Loft insulation - What thickness to insulate rafters?

9 Sep 2008
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United Kingdom
I have a Victorian mid-terrace house with a large loft. So I could use it for storage, I had a loft conversion company install two steel beams between the party walls and then strap up a floor between them. This is great but the temperature fluctuates massively at different times of the year and even in the day/night. I would like to insulate the rafters to try and stabilise the temperature a bit so the stuff I'm storing doesn't deteriorate over time.

I have read that for a full blown loft conversion, the regs require 200mm of insulation slab with a 50mm gap between the insulation and the roof felt for ventilation. This seems excessive for my needs but what thickness should I go to?

I can see that insulation slab is available in 50mm and 100mm thickness. My rafters are approximately 70mm deep so if I do 100mm and leave the 50mm for ventilation then I will need to add 80mm battons all the way around.
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To comply with regs, you wouldn't need 200mm., more like 125 -150. Whatever thickness you use, you must leave a reasonable gap above for ventilation.
Why not; 1. fix 2x1 battens underneath the rafters to increase the depth to about 95mm
2; fit 50mm board between the rafters (this will give you a gap of 45 or so above the board which should be OK for vent.)
3. put 25 or 50 board underneath (across) the rafters. This won't comply fully with regs but would go a long way to improve matters.
Thanks - that sounds like a good way to go.

With regard to the electrics, I have 4 x 36W fluoresent tubes up there. Would it be OK to cover the wiring with the insulation? It would only be using 0.6A.

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