condensation on inside of shed roof

magicbastarder

just wondering what the best solution for this is - i have a block shed out the back with a flat roof; the roof is 3/4 inch WBP with torch on felt over it (built on a frame made of 2x4s).
there was a bit of damp in the shed (water was getting in through one of the walls in the recent wet weather, since been fixed (hopefully)) but for the past few days, there's been condensation buildup on the inside of the WBP. i assume the roof is cooling more quickly than the air in the shed, so it's condensing there first.

i was wondering how best to deal with the condensation issue? i am dubious about packing the joists with insulation and lining the roof with foil backed plasterboard, as has been suggested to me, as it might seal in moisture in future - easier to let moisture in than it is to get rid of it.

given the damp, there is an unusual amount of moisture in the shed i reckon, but the condensation might be indicative of a longer term issue, unless i fix it.

jack of all

Water vapour will condense on the coolest surface within your shed- most likely to be the underside of the roof decking. You are right to be hesitant to insulate this as you can have interstitial condensation, UNLESS you incorporate adequate ventilation (around the eaves, roof vents etc) to prevent vapour build-up within the construction. This is a cold roof construction. A more common approach on falt roofs nowadays is to make the roof a Warm Roof Construction- with the roof membrane laid directly on an insulant layer, on vapour check, decking and roof joists etc. This type of construction eliminates any risk of condensation and is more efficient in terms of heat retention in winter and cooling in summer.

As this is only a shed I would suggest that the only solution is to provide a good vapour check, insulate and ventilate (cold roof construction) but only do so if you can get sufficient ventilation otherwise you're wasting your time (insulation gets wet with interstitial condensation, reducing its effectiveness and leady to problemswith mould and mildew down the line).

magicbastarder

cheers - would warmboard insulation (i.e. usually used on walls) screwed to the joists - thus preserving the air gap between the insulation and ply, and providing a vapour barrier to prevent more moisture getting in from the main space in the shed - sound like a solution? I can readily easily allow the venting of the air space between.

jack of all

Unless you are using a foil backed plasterboard or a vapour barrier beneath the warmboard you would need to tape the joints to prevent moisture-laden air from getting up into the void via the gaps. Depending on the use of the shed you might want to have some kind of a finished ceiling on the underside (warmboard or equivalent won't look great), if you don't want to go down the route of plasterboard you could sheet out with 1/2" OSB which would paint up well and provide a durable surface.

magicbastarder

i'm not worried about aesthetics, i'd be happy with warmboard with a foil layer, and i can tape the joints.
thanks for the feedback!