DIY Doctor

Installation of Kingspan between rafters

Postby ElToten » Wed May 13, 2020 5:11 pm

Hi,

we currently getting our roof reslated. We have a loft conversion in place, so parts of the underside of the roof cannot be accessed from the inside. We contracted them to remove the old insulation and install Kingspan and replace the old tiles with slate. After they removed everything they started installing the membrane without putting down the insulation first. When I asked them why they are not installing any insulation they claimed they will do it after they reslated the roof by sliding the boards into place where the ceiling is installed against the underside of the rafters. Is this common practice? I cannot see them being able to properly slide the Kingspan flush against the plasterboard without leaving any gaps.
Also, they're only installing 30mm Kingspan onto 110mm rafters. Surely, they could have used 60mm but they claimed 30mm will be fine. Is 30mm really enough? It seems awfully thin to me.
ElToten
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Postby stoneyboy » Wed May 13, 2020 10:28 pm

Hi eltoten
If the slide in method is used the insulation board must be smaller than the gap to allow for clearance. The result will be gaps with cold bridging. Insulation should be tightly fitted between the rafters as the work progresses.
Current regs require something like 75mm of celotex between the rafters and 50mm under the rafters. Even allowing for kingspan to have a better U value than celotex, it seems daft not to make maximum use of the depth available.
I assume you are not involving your local BR department in this work.
Regards S
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Postby ElToten » Thu May 14, 2020 5:08 am

Thanks for the reply. I am involving them because I want it done right. But because if current lockdown BR do not do site visits. I have spoken to them and was told to go ahead with the work and inspection will be done retrospectively.
I have written assurance from the contractor that everything will be done to “building specs”. The roof is now nearly fully slated. Can this still be fixed? I have not made any payments yet.
ElToten
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Postby stoneyboy » Thu May 14, 2020 9:22 am

Hi eltoten.
Suggest you speak to BR and ask that if 30mm Kingspan is being installed above the plasterboard and nothing below rafters, whether this is likely to be approved if there is a room in the roof.
Regards S.
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