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How much insulation is required over joists to prevent thermal bridging? Presumably this is a question of the difference in insulative value of the wooden joist and the insulation between it, and then adding insulation to make up? Similarly, if you have boxed off recessed lighting, is it necessary to put more insulation over the boxed area to restore the u-value?
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Thermal bridging is where a (often structural) member with poorer thermal performance has a lower thermal resistance. IT is a matter of degree and cannot be avoided just minimised. Therefore you would need to compensate by increasing the level of general insulation.
With recessed lighting you would need to allow extra insulation and also be aware of airtightness around recesssed lights as this will often be a bigger factor than the lack of insulation.
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any tips for avoiding airtightness losses? Just using sealant around the boxed off bit I suppose?
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David
The link should help you:
http://www.est.org.uk/housingbuildings/publications/?mode=search&keyword=CE137
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How much insulation is required over joists....
Some is the answer how thick are the joists and are they wood?
Say 100mm joists then 100mm quilt between the joists and 150mm over the lot. There will still be thermal bridging through the joists but it is very much reduced. There will even be thermal bridging through the metal plasterboard fixings which is possibly a bigger problem. This is why I only use plastic nails to fix plasterboard ;-)
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thanks tony, do i need a plastic hammer for the nails
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