Hybrid flat roof Vapour barrier?

[firapara]

Hi Guys,

Having a dormer built in the loft. Due to the height limitations need to go with a Hybrid roof.

Where does the Vapour Control Layer (vapour barrier) go in this scenario?

 

[Ivor Windybottom]

The purpose of the VCL is to stop air-borne moisture from the room condensing within the void. So it needs to go on the warm side of any insulation. Therefore the right one is definitely wrong, as you could potentially have condensation within that void around the celotex, where the moist air from the room cools. But I don't know whether the left one is a valid method of building.

 

[stevie888]

You might want to ask your BCO: it doesn't matter what we thinkif he doesn't like what you've done.

Time was insulation over and between joists was frowned on: you might want to check his views on that too.

 

[^woody^]

The BCO should ask for a condensation risk analysis, as that's not a recommended design. Bear in mind that rot occurs long after the builder (and any warranty) are gone. And pattern staining from the joists will be a problem too.

Whay can't that design be flipped and have the insulation below the joists?

 

[Ivor Windybottom]

The websites I glanced around seemed to say it needs a very special kind of breathable membrane or something. Sounds a bit iffy.

I'd agree with the above - put the insulation between the bottom halves of the joists, another layer between joists and plasterboard. It all ends up the same thickness either way. Then you can ventilate the void. Which is in fact a cold roof.

With a warm roof, any air movement within the void behind the plasterboard will make it cold. But it seems that a warm roof is the fashion these days for many.

 

[Scally-Ho]

If it was up to me, I'd use foil backed plasterboard and the skim it with board finish. Nothing in the form of vapour will permeate the foil.

https://www.buildershoponline.co.uk/news/Foil-Backed-Plasterboard-All-you-need-to-know

 

[firapara]

Thank you guys,

I was under an impression that this Hybrid Roof setup is "acceptable" although not ideal?

 

[^woody^]

Hybrid roofs can be acceptable if the entire roof is filled with insulation, no air gaps or voids, and then the vapour check is on the room side. Even with this, a layer of insulation should be on the underside of the joists even if it's a thin layer.

 

[Ivor Windybottom]

With any type of warm roof I think I'm right in saying the void has to be sealed. Otherwise you're going to get draughts against the back of the plasterboard which is going to negate all insulation. This would be reasonably easy to achieve in newbuild or a standalone flat roof, much more difficult with a loft conversion where you're probably adjoining draughty loft voids.