Insulating both joists and rafters?

16 Feb 2022
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United Kingdom
Given the massive rise in energy bills coming, insulation is going to lead to much bigger savings on energy bills. Yet our loft is low and so I have a rather unusual issue - I can't insulate it to 270mm.

I have just a 100mm cavity between the joists in our loft and insufficient height to raise the floor. Given there is also a 100mm cavity in-between the rafters, I could add a further 50mm there with sufficient ventilation above.

In terms of heat loss - is 100mm of joist insulation + 50mm of rafter insulation equivalent to 150mm of joist insulation, or basically the same as 100mm of joist insulation?
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It's pointless insulating the roof if the loft cannot be used, get the most efficient insulation for the joists.


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