Vapour Barrier?

phil p · 18 May 2022

Hi,

I have a sectioned built garage that has corrogated cement fibreboard sheets for the roof which I thought was leaking, however I'm not so sure it actually is?

I insulated and boarded the full garage out quite a few years ago, however a few years back I noticed the floor was wet after long periods of rain, I also emptied about a third of a bucket of water (still not sure how that happened as I'm sure it was a sealed unit) and last year I removed the ply and Celotex to see if I could find out where the water was getting in, and of course it's been bone dry!

It's not that big of a garage and I'm sick of tripping over the boards so I have decided to put them back on after I reinstall the Celotex, however I wonder whether the water was caused by not originally adding a vapour barrier?

I initially built a 2 x 2 frame for the Celotex, however there was still a 2-3 inch air gap between the roof and Celotex, also the underside of the fibreboards, which I gave a coat of bitumen on them.

Could the reason for the ingress of water be due to not having initially adding a vapour barrier of some kind?

Any opinions/thoughts would be appreciated.

Bingy man · 18 May 2022

I’m no expert on this issue but is it in anyway due to condensation forming in between the insulation and the outer walls, I would expect to find evidence of damp, mould or mildew etc behind the insulation. Secondly you mention the floor is wet after several days of rain- it’s amazing how rain driven by the wind will find it’s way into a building or via a roof , bit daft ( maybe) but have you inspected the roof during heavy rain -from inside. If the roof is sound and watertight what about doors and windows etc . Finally ( sectional concrete ) are the joints sound in between the sections/if applicable. Personally I’d start with the roof and illuminate that then move onto any other openings especially the door. With regards the roof is it connected with the main house ( lean to type structure ) if so check any lead flashing for damage. One thing I’m sure is if water is getting in it may not be obvious and also water can travel a long distance before actually entering a building. I’m sure there will be members on this forum with the same type of garage that have also had this issue and may be able to give you more specific advice.

mikej460 · 18 May 2022

I have a sectional garage and water gets in via condensation on the inside of the corrugated roof but this isn't the main cause, it just created annoying spots of water all over my bench. The main culprit is water ingress under the bottom wall sections after heavy rain. I have never fixed it as I always planned to build a new workshop and this begins in a couple of weeks (well as soon as I've sold the garage anyway). My tip would be to check for ingress under the bottom section and if it is the culprit seal it with OB1 from Screwfix. If you have insulation and a vapour barrier under the roof I doubt the problem is condensation - however this assumes you've installed it as follows

Corrugated ceiling - air gap - insulation - vapour barrier - then some sort of sheeting preferably OSB3.

baldkev · 18 May 2022

If the roof is condensating in the corrugations, a vapour varrier wont stop it, but will help keep it from dripping on the floor, but it would travel to the edge of the vapour barrier and then come down. A better option would be to fill the voids in the corrugations or stick a thermal roof quilt to the corrugated panels, then vapour varrier, then boarding. Often the moisture comes from inside the building, either from people breathing for instance, or in a garage, a 'damp' floor ( i.e not got a dpm )

Molynoox · 19 May 2022

Is the roof vented? If not then you shouldn't have an air gap between insulation and roof. An air gap there allows your moist air to come into contact with the cold external roof surface and potentially condense.
Martin

Lard · 19 May 2022

Molynoox said:
Is the roof vented? If not then you shouldn't have an air gap between insulation and roof. An air gap there allows your moist air to come into contact with the cold external roof surface and potentially condense.
Martin

Yep, I’d agree with that.

When I built mine I used metal profiled sheets that I knew would condensate but I wrapped the whole shed (walls and roof) with a breathable membrane. I also built-in continuous ventilation that runs from front back. This construction doesn’t stop the problem but DEALS with it as any water droplets that happen to appear, fall off the roof, hit the barrier and run out into the external air.

There’s only 2 ways to manage condensation, either increase the air temperature or ventilate (better still, utilise both). In your situation the simplest option could be to add some/more ventilation.

Jones · 19 May 2022

I would suspect water coming under walls rather than through the roof particularly if there's a problem with gutters or puddles form outside. If there's no broken bits of roofing or open fixing holes it's unlikely to be a roof leak.