Vapour confusion

[jaybeex2]

Im renovating a mid terrace house and am drywalling the exterior walls using 50mm polystyrene and studs. Ive spent a very very confusing weekend trying to decide on where to fit (or IF to fit) a vapour barrier. Some say vapour barrier against the brick the the studs, others say ooo no bad idea barrier between the studs and the drywall. Both make a compelling case but but are adamant the other is woefully wrong. So, who is right as right now im thinking leave the damn thing out?

Very confusing
Regards
John

 

[Alastairreid]

Vcl goes to the warm side..ie behind the gyproc.

 

[jaybeex2]

Marvellous thank you for that

 

[SimonH2]

To add to alastairreid's reply, the reason is : you need to prevent moisture reaching cold areas.

If you fit the VCL anywhere but on the warm side (or don't fit one at all) then moisture will migrate through the structure. At some point, when the temperature is lower, this will condense out and make the structure damp.

So if you fitted the VCL on the wall and then added the battens and insulation, moisture would diffuse through until the temperature gets cold enough for it to condense out. At this point, the insulation and timber will become damp (which considerably reduces it's insulation qualities) - and the timber will rot.

 

[ban-all-sheds]

If you fit the VCL anywhere but on the warm side (or don't fit one at all) then moisture will migrate through the structure. At some point, when the temperature is lower, this will condense out and make the structure damp.

jaybeex2 - bear in mind you risk the same thing happening wherever you penetrate or remove parts of the VCL, e.g. electrical cables, accessory back boxes, CH pipes....

 

[Alastairreid]

http://www.constructionstudies.ie/practical_guide_to_building.pdf