# Shed build vapour barrier help!



## S.Badger (3 Nov 2013)

Hi

I am building a shed/workshop and need some advice on the vapour barrier. I have been doing a lot of reading and realise the importance of a good vapour barrier but I am not sure where to place it in the floor layer. The shed is going to be built as a raised deck over a concrete slab using treated 2x6 joists supported by 4x4 posts. Between the joists will be foam insulation and then an OSB sub floor finished with laminate. My question is, should the vapour barrier (polythene sheet) be placed under the OSB or over. If I place it over the OSB will the OSB be susceptible to damp/rot as its untreated (should I be using treated ply). If I place the barrier under the OSB does it matter that the barrier will not join with the wall barrier as the wall plate is on top of the OSB. Am I worrying a little too much about this?

Any advice greatly appreciated.

Thanks

Stu


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## MMUK (3 Nov 2013)

Under the OSB. Then laminate underlay on top.

What I would do is pin the vapour barrier directly onto the joists then use a tile lathe on top before fitting the OSB to leave an air gap.


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## nev (3 Nov 2013)

imho
I wouldnt have a vapour barrier on the floor. if the floor is raised and there is airflow beneath it then you should have no damp problems.


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## twothumbs (3 Nov 2013)

The simple rule to remember is the vapour barrier is always on the warm side. This avoids condensation forming on the wrong side. Best wishes.


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## PAC1 (4 Nov 2013)

I agree with Nev no need for a vapour barrier. I would not use OSB for flooring personally shuttering ply is what I used in my workshop. if there is a risk the edges will get damp use WBP ply. I then varnished the ply with Ronseal Diamond hard so no need for laminate on top.


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