condensation in outbuilding

[trowelandhawk]

Hi all.
Straight to the point...
I had a 7x4m workshop built in the summer and the inside ceiling is now covered n moisture. The construction is dense block on outside 70mm colotex insulation then thermalite block for inner skin. The base is concrete and had a dpm underneath it before being poured. The roof is a flat roof which has got three layers of torch in felt. This was all done by a pro builder/ carpenter and roofer. I spoke to builder today who says it’s condesation due to ventilation and he may have to put soffit vents in rear to allow air to circulate. Apparently this may not work either. The OSB board ceiling has droplets forming everywhere and the timbers have damp and mould forming. See photos for explanation please.
Apparently this is my fault as I asked for the building to be sealed from draughts!.any help would be appreciated.
Many thanks

 

[JohnD]

you mention the wall insulation, but how is the roof insulated?

 

[trowelandhawk]

Sorry... the roof is not insulated as yet.. I was going to run the wire for electrics then put 100mm loft insulation in the gaps of the rafters hence I looked up and saw the state of the ceiling!.

 

[JohnD]

you will need a vapour barrier between the room and the cold part of the roof. With luck someone will add some diagrams of alternative flat roof designs.

I would be thinking of a foil-skinned rigid foam, and plasterboard for fire resistance.

I'd suggest taping clear plastic tightly to the floor to see if water vapour is rising through or off it. Also the walls if you have any suspicion of damp.

Water vapour is lighter than air so will naturally rise towards the ceiling. As yours is uninsulated it will be very cold so condensation will occur.

The water of construction will take a long time to dry out of the walls and floor. Ventilation is the cheapest way to get rid of it.

 

[^woody^]

The OSB board ceiling has droplets forming everywhere and the timbers have damp and mould forming.

That's because its not insulated and is a cold surface!

You need insulate the roof as either a vented or unvented roof.

 

[trowelandhawk]

Thanks guys.
So would I be doing the right thing by putting loft roll insulation in between the rafters or by the sound of it cut some sheets of leftover colotex down to fit between the run light and socket electrics after that. ?.

 

[John D v2.0]

No
Firstly since your roof covering is there by insulating inside you'll have a cold roof so you need lots of ventilation between your insulation cold side and the impermeable weatherproof layer ie where the condensation is now. So sort that out before you insert the insulation and make sure there's a continuous space for air flow.
The other option is to use the OSB/ply layer as the vapour control layer and put your insulation on the outside over the roof and then another roof covering on top of that, which means warm roof and no need for ventilation.

 

[^woody^]

You can vent it with an air gap below the deck, or you can completely insulate and seal it leaving no gaps.

 

[John D v2.0]

as long as it's a non vapour permeable insulation, yes, but if the OP uses rockwool it's not going to end well.

 

[trowelandhawk]

sorry... bit thick... what is ‘the deck’. Is it the underside of the osb where the droplets are forming, ie where the osb is screwed to the rafters on the inside of building. (Just been told it needs straps too!)
I have just paid good money to have a roof done and putting insulation over it won’t work so it will have to be inside.. I get the jist that if I get a load of Celotex sheets cut them and stuff them into the gaps so there is no gap then that would work??? Or would I still need a gap of 50mm as someone has told me.. my builder say he will install circular soffit vents too... I’m just losing faith in what he says though.

 

[John D v2.0]

Yes that's the deck.
For a standard cold roof leave 25mm minimum at the top of the insulation between the OSB and insulation and ensure there's plenty of ventilation across.

 

[geraldthehamster]

Or as Woody suggests, pack the gaps between the joists with insulation and fit a vapour barrier below to seal it completely. Easier than trying to introduce an air gap.