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Pitched Roof Insulation


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#1 boisdevie

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Posted 30 April 2016 - 09:15 PM

Single story building with ceiling up to the underside of the roof. I'm going to insulation the inside of the roof. To get a U of 0.18 as per regulations I'll need 120mm of something like Celotex FR5000. BUT my rafters are only 70mm deep so my idea is to use 70mm between the rafters then a further 50mm across the rafters thus reducing the loss of cold bridge effect of the rafters. Obviously will then need longer fixings to pierce this 50mm insulation to fix the plasterboard into the rafters. Am I missing anything here?

#2 Crofter

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Posted 30 April 2016 - 10:24 PM

Do you need to leave a gap for ventilation on the cold side of the insulation?
You will indeed need long fixings. Remember to mark the position of each rafter before offering up the insulation, or you will struggle to hit them.
When I come to do this I will have battens fixed over the insulation, much easier to screw down tight. Then the plasterboard fixes to the battens. It is very easy for screw heads to pull straight through plasterboard so I would be wary of trying to hang 50mm of insulation plus plasterboard off the same screw.

Edited by Crofter, 30 April 2016 - 10:24 PM.