Warm roof VPC location?

7 Feb 2012
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United Kingdom
hi there,

I'm putting 50mm of kingspan tp10 between the 8x2 rafters, 18mm ply ontop of the rafters then 100mm of kingspan tr27 on top of that.

Confused as to where the Vapour control barrier should go. I guess on the underside of the tp10 between the joists. The only type of VPC that would work properly in this setup would be the tar backed foil self adhesive stuff!

Any thoughts anyone?
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Hmmm, that insulation set up seems like a mix of cold roof and warm roof..... The VCL should go on the warm side. Having your set up limits this somewhat.

Why not go traditional warm roof setup and have 120mm of Kingspan on top of the deck - nothing in between the joists? That way you can just put the VCL in between the deck and the insulation.
the theory says warm side as a rule of thumb. However for you it's a little harder due to the insulation value of the timber. I'm guessing it will be minimal and won't take it down to the dew point, but the official way to check would be a condensation risk analysis on your buildup..
This is called a hybrid warm roof, LBC do not normally like them as it confuses them.
They do work however you have to be mind full of the condensation risk, your vcl should be a plaster board level however any down lighters, speakers etc will puncture this.
You can use rockwool etc in between the joists.
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Vcl stapled to underside of rafters between plasterboard.
Presuming this is a flat roof, so has joists not rafters, then the vapour barrier goes between the deck and the insulation above it. The insulation below the deck makes no difference to the vapour barrier location in this type of design.
General opinion is that the vcl goes on the warm side of the insulation, but if the insulation is in two layers, the vcl should go under the thicker layer.

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