Shed roof insulation

[ojustaboo]

Hi all.

About 10 years ago I built a smallish workshop. I used tanalised cladding for the walls and OSB for the roof.

At the time I insulated the walls with polystyrene and then a vapor barrier, and ply/mdf on the inside (had a some ply offcuts and some MDF, hence some parts of the inside wall are ply, some mdf).

I didn't leave any gap,and put the vapour barrier on the inside (outside wall, polystyrene, vapour barrier, ply/mdf) so far there's zero evidence of rot/mold (I presume due to not using T&G hence the walls can breath through cladding gaps)

I've decided to insulate the roof.

I was thinking of gluing polystyrene straight to the OSB. I know everywhere I read, says I must leave a gap to prevent moister building between the roof and the insulation, but if I'm gluing the insulation to the roof, will this not stop the possibility of moisture as there will be zero gap?

If not, and I have to batten my roof and leave a gap, do I have to drill a few holes through the shed at each end at the gap height, to allow air to flow right across it?

I read this post from 2010

https://www.ukworkshop.co.uk/forums/post449627.html#p449627

where the poster says

And by the way OSB is so full of epoxy that it is an effective membrane in it's own right.

Do I take it that I don't need to worry about a vapour barrier as my roof is OSB?

many thanks

 

[TomGW]

Essentially what you are proposing is a 'warm roof' construction, where there is no air gap above the insulation. Google "warm roof construction detail" for info. It's a perfectly legitimate approach, providing that there is no potential for even a minor leak from above.
However, regardless of whether it's a 'warm or 'cold' roof, the vapour barrier must be on the warm side of the insulation, below the insulation / rafters and totally sealed. I didn't / couldn't read the post you've referred to but I assume that the writer was using OSB under the insulation, as a ceiling, and was relying that it would act as a vapour barrier. For the sake of the few pounds that a vapour barrier would cost I wouldn't risk it.