Mould appearing under vapour barrier...?

[The Z'eer]

In the process of constructing the small shed I applied a vapour barrier (about three months back) around the stud frame work and over the OSB plywood roof and eaves (Cromar Vent 3 Light) as pictured below...

The other day I had to remove some of the vapour barrier and to my horror the plywood underneath looked something like this...

I've never dealt with mould before but I'm assuming this is what is? I can't quite understand why the surface of the plywood is now covered in this? Surely the vapour barrier lets out moisture not retains it? Also the vapour barrier is water proof?

The underside of the plywood...

...feels slightly damp but not no black spots.

A metal roof is will finally cover the plywood. What worries me is this mould, if it is mould and that the plywood could rot away?

Any ideas about this?

Thanks

 

[gaz1]

for start off your wood is way to close to the floor you can tell just by looking at your corners alone its drawing water up

by another thing you havent packed your woods out even if you had of used latts that alone would allow your shed to breathe

so yes i had expected some

as goes for outside its natural some wood will draw moisture due to being dried

 

[AndersK]

If you're not putting in insulation, don't use vapour barrier.

 

[The Z'eer]

for start off your wood is way to close to the floor you can tell just by looking at your corners alone its drawing water up

by another thing you havent packed your woods out even if you had of used latts that alone would allow your shed to breathe

so yes i had expected some

as goes for outside its natural some wood will draw moisture due to being dried

The corners may draw a little but I've removed the corner battens and the moisture doesn't draw up past the second layer of feather edge.

This is something to do with the vapour barrier and the plywood. Only the roofed plywood is covered in black spots. It's as if the vapour barrier is trapping moisture directly beneath it.

 

[Ian James]

It may not be the whole of the problem a lot of plywoods these days are totally rubbish we could by plywood years ago that was brilliant stuff it would not grow mold if stored undercover now I have offcuts from previouse jobs that have been store undercover and when you go to them they they are covered in mould dots I purchased one lot from one of the national DIY stores for external use 12 months later a couple of the sheets have totally delaminated

 

[The Z'eer]

It may not be the whole of the problem a lot of plywoods these days are totally rubbish we could by plywood years ago that was brilliant stuff it would not grow mold if stored undercover now I have offcuts from previouse jobs that have been store undercover and when you go to them they they are covered in mould dots I purchased one lot from one of the national DIY stores for external use 12 months later a couple of the sheets have totally delaminated

If that is so then possibly the reason behind the mould appearing. I wonder why mould spores are growing on them?

I'm not too keen to remove the plywood, I'd rather fix the problem with it in situe if possible.

 

[The Z'eer]

If you're not putting in insulation, don't use vapour barrier.

I haven't completely decided on the insulation side of things, hence I went with a vapour just in case.

 

[Wendelspanswick]

First thing, just to correct your terminology, what your using is a Breather Membrane, not a vapour barrier, a vapour barrier would be something like polythene.
I wouldn't worry about the mould you have, I imagine that the plywood was pretty wet when the breather membrane went on, the membrane can only pass so much vapour at any time, the recent heat has driven the moisture from the ply and it's not passing through the membrane quick enough so mould has been forming.
Once the ply is dry, the mould will die before it gets chance to degrade the structure of it.
If your really worried you can strip the membrane off, bleach it to kill the mould, then reapply the membrane but I personally wouldn't bother.

 

[The Z'eer]

First thing, just to correct your terminology, what your using is a Breather Membrane, not a vapour barrier, a vapour barrier would be something like polythene.
I wouldn't worry about the mould you have, I imagine that the plywood was pretty wet when the breather membrane went on, the membrane can only pass so much vapour at any time, the recent heat has driven the moisture from the ply and it's not passing through the membrane quick enough so mould has been forming.
Once the ply is dry, the mould will die before it gets chance to degrade the structure of it.
If your really worried you can strip the membrane off, bleach it to kill the mould, then reapply the membrane but I personally wouldn't bother.

The plywood appeared dry to the touch when handling and initial fitting, it had been stored in a spacious well ventilated warehouse. I decided to phone Cromar the manufacturers of the membrane, their response was that the plywood should've been covered with the final roofing medium ASAP. They also said people confuse it with the ability that tarpoling has.

This is an actual photo taken just now of the problem...

I did notice that after prolonged periods of heavy rain, the topside of the membrane appeared to be saturated (I couldn't check the underside due to staples fixing it to the wood). I didn't give it much thought at the time but I wonder "if" rain water did pass through, and as you mention the membrane has been slow in allowing evaporation?

I'd like to think it is of no worry, and that the mould spores will die off soon enough but if not further issues could arise.

 

[brightspark]

paint all the wood with a decent preserver. job done

 

[The Z'eer]

paint all the wood with a decent preserver. job done

I might do with the next one, I don't fancy unscrewing this one though.