Vapour barrier?

[skipdiver]

Related to my other post on insulation, i've decided to put omnifit earthwool batts in my rafters and leave a 2 inch air gap above as per Knauf recommendations. They also recommend to fit insulated plasterboard beneath this to bring up the u value but i'm not concerned with that as it is only a workshop. Their diagram doesn't show a vapour check barrier but i imagine the insulated boarding will be doing that role. So do peeps think i should polythene sheet over the rafters before i board them with OSB or not, given it's not a habitable space like a house. I'm buying some polythene anyway to put under the floating floor, so i will have some but there are conflicting ideas about their use on the net, regarding where they are in a building, what the building is used for, and the local climate. What's the prevailing opinion; have you done it or left it out?

Cheers.

 

[stuartpaul]

Steve,

I'm about to do the same as you and I won't be using any form of vapour barrier for the roof.

If you think about it we don't use them for insulating in houses so my guess is there won't be an issue. My previous workshop/garage didn't have one in the roof and showed no signs of any problems after 15 years.

Stuart

 

[skipdiver]

Steve,

I'm about to do the same as you and I won't be using any form of vapour barrier for the roof.

If you think about it we don't use them for insulating in houses so my guess is there won't be an issue. My previous workshop/garage didn't have one in the roof and showed no signs of any problems after 15 years.

Stuart

That's my thinking also. I've just paid £40 for a 25mt roll of 1000g Polythene to use under my floating floor and i only need 5mts, so have plenty of stuff to do it. Was just wondering if it was worth it as some specs i've seen show it and some don't. I think it also provides air tightness to buildings if done correctly. I think it's probably overkill for a workshop but i have it to hand and would hate to not put it in while i'm doing it if i later have a problem.

 

[Fitzroy]

My understanding is OSB is a much better vapour barrier than plasterboard, which is the normal reference point for the need for a vapour barrier in a wall/ceiling.