Spoke to my BCO, who has said that he will not accept taping the joints of my foil backed PIR in my sloping roof as an acceptable vapour control layer. He has asked for an additional VCL. 🙄

 

I have 60mm over the top of the rafters which I will go ahead and tape the joins anyway, but is there any point in me buying a VCL or should I just put polythene (a vapour barrier) over the top? Typical SD value of foil tape is 200m so a "controlling" layer will be of no use anyway? The controlled release will just be sealed by the foil anyway?

 

Thanks

You won’t need the extra barrier for your pre plaster inspection 

So it’s up to you if you add an extra barrier But your boarders  will only take a matter of minutes  to do each ceiling 

It is worth remembering why a VCL is fitted.

It is to stop humidity condensing on hidden cold surfaces.

Cheers guys, my question was more towards the theory, if you end up taping the joints of the aluminium foil back you essentially seal the envelope due to the high SD value of aluminium foil? So putting a controlling/slow release VCL with an SD value of say 10m in would be deemed pointless, am I right in saying that?

4 minutes ago, CH_18 said:

my question was more towards the theory

Basically yes.

Really comes down to what happens when something fails.

Foam sheets shrink over time, that stresses anything covering the joints.

Polyethylene sheet has a plasticiser incorporated into it, so it can move due to thermal and mechanical expansion.

Just about to purchase either VCL or a Vapour barrier (polythene). Anymore thought on this before I order? If I'm foil taping anyway shall I just stick some polythene up instead of a controlling layer?