Vapour layer, Dormer knee walls and increasing knee-wall insulation

ravendude

Hi there,

I have knee walls on my dormer. They are chalk board backed with fibreglass roll (70mm approx).
The chalk board has a fairly thick foil like back to it and seems to be taped with at the joints again with foil tape. Would this act as a vapour control layer (to some extent)?
Surely vapour cannot permeate this foil backing from within the dormer rooms?

The reason I ask is that I recently added to the knee wall insulation by stapling 200mm Knauf space blankets (designed for attic ceiling level) to the backs of the knee wall, over the existing fibreglass roll. This is the only way I can think of to improve matters without major rework. It seems to be making quite a noticeable difference already, though the real test will come in a few months time.

Obviously, - condensation is the concern...

The space blanket is mineral wool, encased in a perforated foil plastic membrane covering. The Knauf product datasheet ( http://www.space-insulation.com/wp-content/uploads/2012/12/Space_Blanket-Product_Datasheet.pdf ) states that the cover adds "negligable resistance" to vapour, - which I take to mean it is basically breathable. I intend to monitor carefully for condensation over the forthcoming weeks/months/years, - I have seen no sign (yet) in the last few weeks anyway. Is there any danger from something I can't see (eg. dry rot?) ?

I'm thinking if the knee wall boards are backed with some kind of vapour control (the foil I refer to above?), - it might be fine anyway?

Thanks!

Andrew_Doran

ravendude wrote: »

Hi there,

I have knee walls on my dormer. They are chalk board backed with fibreglass roll (70mm approx).
The chalk board has a fairly thick foil like back to it and seems to be taped with at the joints again with foil tape. Would this act as a vapour control layer (to some extent)?

That's what the foil is likely there for but it seems that people are realising that air tightness is far more important when it comes to vapour. Air leaks from the heated space into the cold attic space can transfer huge amounts of water. Hopefully the image below displays.

Andrew_Doran

http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/54110/You-Don-t-Need-a-Vapor-Barrier-Probably

Short article about the above.

ravendude

Thanks for that, - very useful info

Ush1

Would the likes of a siga skin on the warm side of the insulation be recommended for air tightness?

I was at the self build show this weekend speaking to guys from Xtratherm and Kingspan, while they'd be biased they said when insulating the attic there is no need for an air tight membrane.

Andrew_Doran

Ush1 wrote: »

Would the likes of a siga skin on the warm side of the insulation be recommended for air tightness?

I was at the self build show this weekend speaking to guys from Xtratherm and Kingspan, while they'd be biased they said when insulating the attic there is no need for an air tight membrane.

Is there a habitable room up there or are you just doing the ceilings or something else?

Ush1

Andrew_Doran wrote: »

Is there a habitable room up there or are you just doing the ceilings or something else?

Yep, converted attic, insulating at rafter level.

Andrew_Doran

Ush1 wrote: »

Yep, converted attic, insulating at rafter level.

Reading between the lines I'm guessing that the Xtratherm and Kingspan guys assume you'd be taping the joints between their boards with aluminium foil tape. In that case the foil surface of the boards & the foil tape would be the airtight layer so no need for another. The difficulty is in where you'd have those boards joining another surface.. They might have been thinking of something else but that's my guess.