Garage Conversion - Hybrid Flat Roof?

28 Sep 2021
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United Kingdom
I have seen a few posts on this but never seen this design as a valid design on any website so just wanted to gauge opinion on whether this is valid or not.

I am insulating a detached garage to use as a gym. The roof is a flat felt roof. The ceiling joists are 300mm apart and the fascia board is nailed directly to the joists with no overhang. If I install Celotex between the joists I can only use 70mm to leave a 50mm ventilation gap and then I have the problem of venting through the fascia.

I have seen the mention of completely filling the void with 120mm Celotex so there is no air gap. The vapour barrier on the Celotex and tape would prevent air ingress. Is this still a valid design as I cannot see this anywhere on the web? I am fitting downlights as well, so would that breach the vapour barrier and make this design invalid?
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If you have no way of venting then full fill might be the way to go. If you wanted belt and braces, you could then add 50mm (or thinner) celotex over the 120mm Infill, under the joists, (Check headroom) this would stop cold bridging and any risk of condensation...
the plasterboard etc.
So is it more of a you can do it there is no other option approach? I’m thinking now I will go with 90mm celotex leaving a 30mm ventilation gap and then work out what I do with the fascia later. Even if I have to plunge cut a channel all the way along the fascia at the top and add an above fascia vent kit that should only be a day’s work and at least it will be done properly (we’ll kind of)
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It's like when your pocket square does not match your shirt. There are those who may frown, and those who know how to pull it off.

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