Cold roof, can some explain please.....

[Post and beam]

Our build is a 'cold roof' according to the TF provider.

The airtight boundary is at the first floor ceiling level between the plasterboard and the rafters ( not yet installed). With 400mm rockwool due to be placed on the loft decking.

So the loft will be unheated, and presumably cold in the winter because the insulation should make this so.

My confusion is therefore..... If the airtight membrane allows vapour to pass upwards into the loft will it not condense out any moisture contained within ?

 

Have i totally misunderstood the role of the various membranes?  Highly likely i think.

[ToughButterCup]

Cold roof design

Cold vs Warm roof

[lookseehear]

Everything you say is correct other than the vapour won't condense because a cold roof space must be ventilated. Assuming this is a pitched roof the eaves of the roof typically have 50mm clear of any insulation to allow free air flow, and it's this ventilation which prevents moisture buildup.

 

In terms of different types of membranes there are a lot! Typically you'll have a VCL on the warm side of the insulation which can also be an airtightness layer, but its primary purpose is to reduce moist air from living spaces moving into cold spaces. VCLs can permit a small amount of vapour through, but not enough to cause a build up of moisture in a properly ventilated loft. VCLs are not the same as breather membranes which are designed to allow vapour through but not liquid water, and are normally placed on the cold side of the insulation to allow any excess water vapour within insulation to pass through.

[Iceverge]

Forget about vapor diffusion. 

 

It's absolutely miniscule. Completely insignificant compared to moisture carried by air leaks. Like thousands of time less moisture gets transferred this way. It all gets carried by drafts. 

 

I would aim for a complete sealed airtight layer. "Reduced" or "good enough" is like "kinda" fixing your puncture on a car. 

 

If you do this you can implement a sealed attic too with ventilation above the rafters and no ventilation through the eaves. It'll prevent wind washing of your insulation. 

 

 

 

 

[Post and beam]

Thanks for the responses yesterday guys, i feel more comfortable about this now. I intend to use the Pro Clima Intello Intelligent Airtightness Membrane at the ceiling ( warm side) Yes it is a pitched roof.

I think there is a lot of work in making this Airtight boundary as good as it needs to be and still have access for the lights etc from the loft space cabling down through the membrane. Whilst also trying to make sure the plasterboard installers dont abuse my efforts.