My sweaty garage roof

[JesusNinja]

So, I've had this nice big prefab garage put up, and I'm insulating it. The walls are being done with 2 layers of 25mm polystyrene panelled over with 9mm ply. This all seems to be going to plan.

Now, the roof....

I had entertained the idea of using various off cuts and left overs of rockwool etc, but after cursing the damn stuff and pinning it up there with staples and gaffer tape, I abandoned it, in favour of spending some hardearned on more polystyrene for the roof (OK, I know foil backed PIR would be even better, but I can't justify the expense for a workshop)

However, and here is the point of my post, the small area I had done with glass fibre, I had also covered with a layer of DPM I had left over to stop bits of it dropping down. The plan had been to take the DPM off and then panel over with 3.6mm ply, but then, after much thought, I decided upon my new course of action.

It had been like that for some weeks whilst I got on with other jobs, but yesterday I decided to whip it all down, ready for doing "properly". It was all dripping wet! Water has been condensing (the fibre cement panels get very hot, and of course it's cooler in the garage).

So - what's the polystyrene going to be like - am I going to create the same problem? How can I solve it?

Thanks!

 

[spidiespark]

as a person with an old victorian house i have also suffered with condensation problems. from what i understand about the issue even if you insulate you still have to let the area breathe. ie you have to leave an air gap between your insulation and your garage roof. if you leave say a 100mm or 4 inch gap obove your insulation you should be fine

 

[JesusNinja]

Hmm, OK. The roof sheets are corrugated and sit on 50mm purlins. I'd planned to fill the gaps between the purlins with the 50mm polystyrene, but clearly that might not help the problem.

I don't really want to come down any furth and lose valuable headroom though.

I wonder if I were to go for something 25mm thick (perhaps back to PIR) leaving a 25mm gap between the top of the insulation and the underside of the roof?

 

[JesusNinja]

Oh, one other thought... At the ridge and eaves, where the corrugated panels meet a flat surface (at the eaves they sit on horizontal timbers which sit atop the walls, at the ridge a length of aluminium flashing laps over both sides) I have filled the gaps with expanding foams, as the damn garage kept filling with leaves and beech mast (much to my annoyance when I painted the floor!)

I suspect this won't help the situation.

Whip it out?