Vapour barrier over joists - garden room floor

[bendeneo]

Hi there all,

I am building a garden room, and have created the 'raft' of timber joists.

Some places I have researched say to put a vapour barrier over the timber, underneath the OSB that I am going to lay on top, some say not to bother.

What do you guys think?

Thanks!

 

[freddiemercurystwin]

A dpm if anything rather than a vapour barrier, which if you asked for one are quite thin, to prevent the OSB getting damp and going rotten. Marine ply would be a lot better.

 

[big-all]

in my humble opinion use exterior/wbp ply its stronger than osb/sterling board much better on the feet
if you have good ventilation then barriers wont help as they will trap more moisture than they prevent

12/18mm ply will stand greater point loading without problems

what thickness is your board and at what centers are your joists/floor timbers

 

[bendeneo]

Thanks guys,

Essentially because of the gradient of the land, there is loads of ventilation... I.e. One end is ground level and the other is 2.5 concrete blocks (layed flat) high.

I totally agree that a barrier could be a bad thing, and with so much ventilation it's possibly unnecessary, however I was also thinking that if it were a thick barrier then maybe it would stop rodents from gnawing the insulation board (cellotex) between the joists.

What do you reckon based on the gradient/air space?

 

[bendeneo]

Actually forget my last post, of course the barrier will be above th. Insulation.....durrrr

 

[freddiemercurystwin]

If you use Celotex it will have a foil facing so that acts as a vapour barrier anyway.

 

[bendeneo]

True, I will be using QuinnTherm which is the same thing, so perhaps thats the way then..... not bother with the vapour barrier under the floor.

NB - Any thoughts on how best to stop animals from getting under the 'raft'?

I was thinking chicken wire or something....

How much around the bottom edge do you think i can cover up and still achieve a decent amount of ventilation?

Regards,