Cold roof ventilation needed?

[ivixor]

So I have a garden room with cold roof design. I'm aware of the guidance requiring ventilation for a cold roof as per: http://www.classicroofing.ie/pictures/insulationLittle.jpg


My question is whether this is really needed if there is a full vapour membrane on the warm side of the insulation. It seems a bit redundant to have ventilation in that case.

 

[John D v2.0]

My question is whether this is really needed if there is a full vapour membrane on the warm side of the insulation. It seems a bit redundant to have ventilation in that case.

If it really is a perfect membrane with no penetrations, and the unvented area really is completely dry, then you don't need.
But as the temperatete drops the humidity rises, also there's always a chance that it can get damp later

 

[^woody^]

Is this a shed or a proper masonry building?

 

[ivixor]

It's a timber framed garden building.

 

[^woody^]

Timber frame? If you just mean a timber shed type frame with T&G on the outside then it wont need any mebrane or vapour control or anything specific.

If you mean a proper timber frame with different material external rainscreen and ventilated cavity and all that, then that's a bit different.

 

[ivixor]

Yes it's proper timber frame. From inside to outside - Plasterboard, VPC membrane, Insulation between studs, OSB sheathing, Tyvek housewrap, timber cladding.

 

[^woody^]

Yes it's proper timber frame. From inside to outside - Plasterboard, VPC membrane, Insulation between studs, OSB sheathing, Tyvek housewrap, timber cladding.

If you have a cold roof with an air void in it, then air will condense in the void on the underside of the cold deck and it wont be able to evaporate through the roof as the cover is impermeable. So it needs ventilating.

It's a little different for walls because moisture can pass through the structure and membrane and evaporate away.