Got Moisture between Foil Backed Board and roof board - Help

[meb97me]

I've been building my wooden garage and its almost finished the last little bit left to to is to finish installing the roof insulation and board over it.

the roof stucture is as follows from outside in
1) asphalt shingles
2) NonBreathable Roofing underlay (http://www.screwfix.com/prods/31795/Building/Roofing-Insulation/Protect-A1-Roofing-Underlay-1m-x-45m)
3) 18mm OSB Board
4) 60mm foilbacked insualtion board
3) 4mm ply boards

i had half installed some of the boards ie put a length of the insulation boards between the roof rafters and ply boarded over them, but i needed to add some extra bits up to roof ridge ridge. However as i came to fit the extra insulation i noticed that there was moisture between the back of the insulation boards and the underside of the OSB roof panels. The roof isn't leaking so i think its some sort of condensation issue. The garage door has been open so it has been pretty cold in there of late.

Will it be sorted once the rest of the boards are in place and everything plyed over? or do i have some other problem?

also do the insulation boards need taping round the edges to the rafters before panelling? as there may be some small airgaps round the edges in a few places between the insultion boards and the rafters, or where the insulation boards meet each other

thanks for any advice

cheers

 

[jeds]

You are right. You have condensation above the insulation boards. Unfortunately the only way to resolve this is ventilate above the boards; either by stripping the roof and re-covering with a breathable membrane or ventilating the existing structure. I don't think the latter is really possible in your case because of the OSB sarking board and the small gap between insulation and that board.

The situation will be improved by adding a vapour barrier to the underside before plasterboarding but this is no guarantee of eliminating condensation and it wouldn't comply with building regulations.