New Garage - condensation issues

[100K+]

I have been lucky enough to replace my old wooden garage with a new 20ft x 12 ft detached single skinned brick garage, to work inside on my V70 AWD R. My 60yr old bones had enough of lying outside.
The roof has11inch deep roof joists supporting osb roofing sheets covered with a rubber membrane. However, the underside ( the osb) is suffering badly from condensation.
I have tried to read up/seek a solution, but the more I read the more contradictory the offered solutions become and so the deeper my confusion.
My garage has a roller door, which has an approx 1" gap at the top which allows air into the garage and I have added two 6 x 4 " ventilation panels to a side door near the opposite end to the roller door. There is no additional ventilation within the structure or heating.
Something I feel must be done as if left unchecked won't the roof rot away in double quick time??
My thinking is thus :
Leave/don't leave a 50mm air gap between the lower osb face and insulation placed between the joists.
Use Rockwool as insulation between the joists as its a "vapour permiable insulation" where as "Kingspan" is not, but possibly easier to fit.
Place/don't place a "Vapour control layer" on the underside of the insulation selected.
Ensure this VCL is sealed/not sealed to the joists to ensure a "full layer" of protection.
This work can/cannot be undertaken with the roof in its current situation ie damp.
Thoughts/ suggestions welcomed.

Cheers

Bob

[green van man]

When I had a garage I built it with cavity walls and a tiled roof, even so I got condensation on the underside of the felt under the tiles.

Being a ridged roof the condensation was not really a problem as it ran to the eves.

You get condensation because it's hotter inside than out, there are 2 cures I'm told, either heat the garage hot enough to dry the condensation, not really an option unless you are a Russian oliogark. Or increase the ventilation such that the inside temp is more equal to the outside temp.

Perhaps if you have power in the garage an extractor fan would help increase ventilation.

Paul.

[galloot633]

Not sure if this will help your situation, but I bought a 10 x 8 steel shed for my bikes , shocking condensation , I bought cheap packing expanded polystyrene and lined the underside of the roof , perfect , now dry as a bone
Dave

[Simmy]

Originally Posted by 100K+

I have been lucky enough to replace my old wooden garage with a new 20ft x 12 ft detached single skinned brick garage, to work inside on my V70 AWD R. My 60yr old bones had enough of lying outside.
The roof has11inch deep roof joists supporting osb roofing sheets covered with a rubber membrane. However, the underside ( the osb) is suffering badly from condensation.
I have tried to read up/seek a solution, but the more I read the more contradictory the offered solutions become and so the deeper my confusion.
My garage has a roller door, which has an approx 1" gap at the top which allows air into the garage and I have added two 6 x 4 " ventilation panels to a side door near the opposite end to the roller door. There is no additional ventilation within the structure or heating.
Something I feel must be done as if left unchecked won't the roof rot away in double quick time??
My thinking is thus :
Leave/don't leave a 50mm air gap between the lower osb face and insulation placed between the joists.
Use Rockwool as insulation between the joists as its a "vapour permiable insulation" where as "Kingspan" is not, but possibly easier to fit.
Place/don't place a "Vapour control layer" on the underside of the insulation selected.
Ensure this VCL is sealed/not sealed to the joists to ensure a "full layer" of protection.
This work can/cannot be undertaken with the roof in its current situation ie damp.
Thoughts/ suggestions welcomed.

Cheers

Bob

75mm kingspan cut to fit tight between the bottom of rafters leaveing air flow above then underline over the bottom of rafters with 25mm kingspan held up with laths/battens nailed through to underside of rafters, do it in summer to avoid traping any existing moisture this will also keep your garage cool in summer

[DaveNP]

If you're insulating the roof you want an impermeable type or you need a moisture blocking membrane on the internal side of the insulation. If you use permeable insulation the moist air can still travel through the insulation and the space between it and the roof sheets will now be even colder so you'll get worse condensation.
Basic rule, keep warm moist air away from cold surfaces, either by insulation or ventilation.

[kenny]

You need more ventilation. I built a 6m X 4m concrete block garage. like your it has a gap above the roller door 4 air bricks each side two low and two up high. There is a gap at the top of the pitch of the roof to allow it to vent under the roof tiles.

I get no condensation at all, I store a load of wet gear like wet suits, life jackets vet in there and it dries really quickly with no heating.

[100K+]

Originally Posted by Simmy

75mm kingspan cut to fit tight between the bottom of rafters leaveing air flow above then underline over the bottom of rafters with 25mm kingspan held up with laths/battens nailed through to underside of rafters, do it in summer to avoid traping any existing moisture this will also keep your garage cool in summer

This was one of my considerations, but I was going to use Rockwool, as I read I needed a breathable insulation. Others have said they used Kingspan with no issues and others that it only hides the condensation problem. - hence the need for a breathable insulation. - rockwool.

Originally Posted by kenny

You need more ventilation. I built a 6m X 4m concrete block garage. like your it has a gap above the roller door 4 air bricks each side two low and two up high. There is a gap at the top of the pitch of the roof to allow it to vent under the roof tiles.

I sort of understand what you say. I have thought of adding air bricks, but having just got the thing built, I'm not too happy about knocking bits out. Additionally, I read that allowing MORE outside air in allows more moist air to condense on the internal face of the OSB, and I should be trying to SEAL the garage from the outside world. A pitched roof also lends itself better to ventilation rather than a flat roof, as each section of the flat roof (area between the joists) would need ( I think) independent ventilation.

As I originally said, the more I read the more conflicting the advice/suggestions appear.


Cheers
Bob

[Das boot]

Half inch plasterboard on the underside of the beams mate , it absorbs moisture and dries it away when the weather is dry .

Container storage places use it for the same problem .

[kenny]

My roof is OSB as well And honestly I get no condensation at all.

[100K+]

Originally Posted by Das boot

Half inch plasterboard on the underside of the beams mate , it absorbs moisture and dries it away when the weather is dry .

Container storage places use it for the same problem .

Oh!! can sort of see the logic behind that...

Originally Posted by kenny

My roof is OSB as well And honestly I get no condensation at all.

Don't doubt what you say for one minute. Your advantage I think is the apex roof which is vented, allowing the damp air to escape. My roof is flat with 11" ribs every 15" down the garage length, which capture the damp air and hold it. Only if I drilled the suffit boards at each side of the garage to allow some ventilation would I get each "bay" ventilated.

This image is 99% like my garage, except the "bracing" battens in my garage are vertical.

https://www.google.co.uk/url?sa=i&rc...15834083769550
It eventually links to pistonheads..


Cheers

Bob