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Airgap required while insulating a metal profile roof ?

Postby scotty_ian » Thu Nov 26, 2020 2:11 pm

I recently converted my carport which has a metal profile sheeted roof, into a shed, pictures attached. I need to insulate the inside of the roof to stop any condensation in the winter. I've bought some 50mm sheets of foil backed Celotex and my plan was to batten the roof out and either fit the Celotex up tight to the metal roof, or with a 50mm airgap. Either way I would cover the Celotex joints with aluminium tape on both sides.

A few years ago a local firm that makes profile sheeting made me a new insulated garage roof where the insulation was laid on top of the timber purlins and the profile sheets laid on top and screwed down through the insulation into the purlins. I asked them about my idea for the carport/shed ceiling and they only recommended the insulation on top of the purlins, meaning me having to remove the profile sheets and flashings to lay the Celotex on the purlins then refit the sheets and probably buy bigger flashings to account for the increase in roof height.

What would you guys recommend as the easiest solution, with an airgap or without ?


Shed ceiling 2.jpg
Shed ceiling 1.jpg
Shed Outside.jpg
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Postby stoneyboy » Fri Nov 27, 2020 10:19 pm

Hi scotty_ian
You will not be able to stop condensation forming on the underside of the steel sheets in cold/frosty weather.
The advantage of fitting the sheets on top of the celotex is that a dpm can be incorporated.
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Postby B4Less » Thu Dec 03, 2020 2:56 pm

An air gap is recommended and would be best practice. In a normal house/dwelling scenario the minimum recommended depth is 50mm but for a shed you should be able to get away with less if space is a premium, possibly 30 - 40mm. Make sure to tape and seal any gaps/joints in the insulation.
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