wall questions (timber, insulation, breathable membrane)

[Chief26]

Hi... Got a question... Just built a small garden shed with vertical cladding on the outside horizontal Batens, breathable membrane... Ply...6x2 wall with full insilation then vapour barrier... Is this the correct way of doing it? With the ceiling I had 8x2 joist on a slight fall to the back.... With full insilation... And vapour barrier. I'm worried now tho that I will have condensation happening in the joist space because my roof is Not vented with it being a rubber felted roof with full insulation.... Should I be worried?

 

[tony1851]

With the roof, as long as the vapour control layer is on the warm side of the insulation and pretty-well imperforate, you should be OK.
But if it's a small garden shed, why the concern? Surely there won't be too much water vapour in there, and sheds usually end up being well-ventilated anyway.

 

[Chief26]

Thanks for the reply... When I say a shed is a glorified shed.... Plasterboarded and plastered.... The only reason I was concerned was because I have 6 holes in the ceiling for spot lights witch in my mind has now punctured the vapour barrier and it seems to be sweating between the insilation and the vapour barrier, any suggestions as to why that would be? Cold air coming in from the holes of the spot lights and condensation with the warmth of the roof perhaps?

 

[Metal Mike]

This area is being discussed where I have just starting working: as breathable membrane (such as vent 3) has been used as building paper (such as tyvek house wrap).
As much as I can see BM should be used only on roofing and should not be used against a solid boarding such as Ply or OSB, where as BP (which is also breathable) can be used against a solid boarding on walls and roofing externally (with proper ventilation in the next external layer).
A new built single story timber frame extension which is being retrospectively treated with catastrophic damp problems (last builder to look at job suggested to knock down, an option not entirely ruled out) has, among other problems, vent 3 (same stuff, different name) on the timber frame with vented cladding on top. As the timber frame is at ground level (with dpc) we have noticed that the BM sucks up water and brings in into the timber frame. This is also being caused by bad workmanship, but I have a strong suspicion that we can see that vent 3 will draw and hold on to ground water though capillary action, where Tyvek is less sponge-like.
Hence vent 3 should not be used esp. on anything below 1m up from finished ground level. Also it should be 'hung' between beams, or with support with an insulation such as Celletex. It has insulating property's to help reduce condensation under slate, as well as being a barrier to reduce weather problems.
To save £50 buying a smaller roll of material (with the same meterage cost) against possible £1,000's costs altering work, is not worth it.