Dormer cheek and wall insulation

8 Jan 2015
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United Kingdom
Hi, in the middle of doing a loft conversion and had a question about the positioning of the kingspan in the dormer cheeks and main dormer wall. The dormer cheeks are constructed with 4x2s and the main dormer wall is 6x2. Both finished with plywood and breathable membrane with clay tiles plus master board on dormer cheek. Currently in the dormer cheeks I have 70mm insulation with airgap on the cold side and the main dormer wall has 120mm insulation with air gap on cold side.

so the question is do I need to leave an airgap at all? I am thinking if not then I can put 100mm insulation in the dormer cheeks and maybe push the 120mm back against the plywood to allow for cabling rather than cutting channels in the insulation.

Hopefully some can help. Thanks
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You might think of the cheek walls as a flat roof as they are similar in construction? Research "Warm Roof" construction, ie, the stud wall is left free for ventilation and an insulating layer is added to the outside. Maybe clad the walls on the outside with 6mm ply, a vapour barrier and then min 100mm Celotex/Kingspan?
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On my small 1960's Dormer, we removed Tiles then we decided to keep it simple and filled the void in the Studding with Celotex, then OSB, followed by a DPM/Vapour Barrier, then vertical battens and then finished with Marley Boards. We could have re-fitted horizontal battens and Tiles but prefer the look of the Marley Boards.


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