Is this mold on osb cold roof?

[nordita]

Hello all

Can anyone identify if this is mold or a left over of the manufacturing process of the OSB?

Whilst fitting insulation board between my rafters I noticed my osb has some mold like marks on the under side. The room at the moment isn't heated but has been kept shut most of the time. The windows and doors have trickle vents open all the time and sticking my hand by the soffit vents I can feel a trickle of air come in although it is faint.

Scrubbing the marks with watery tissue paper slightly fades and smudges them but they do not wipe cleanly.

 

[Ivor Windybottom]

Definitely looks like dried mould, probably just the result of warm/damp air in the room meeting a cold surface. Guess the grey parts are the wood that happens to be more absorbent so has suffered more. I doubt the OSB left the factory like that, although it may have happened in a builders yard I suppose. You could spray with bleach-based mould & mildew remover if you don't care about your own health, plus it may damage the wood. Or you could just not worry about it. If it stays dry then it won't do any harm.

Have you got a good through-draught from end to end along each joist gap?

You need to ensure this, along with a 50mm or greater gap between your insulation and the underside of the roof. I used a very thin batten pinned to each side of the joists, with Celotex pushed up against it. Make a simple L shaped jig the same height as the Celotex to set its height before pinning or screwing it. Seal all gaps between the Celotex and joists

Ideally put another thin slab across the entire lot - joists and all, seal/tape joints then plasterboard over. Just remember to mark the joist positions on the Celotex so you can screw the plasterboard up. Alternatively use insulated plasterboard.

 

[nordita]

Thank you kindly for your detailed response.

Mold is fine as long as it doesn't rot the roof away.

It's hard to tell if the airflow is adequate. The other end of the building is against a fence. I also think I installed the soffits the wrong way round ?

I have some leftover primer (mapei primer G, synthetic-based primer in water dispersion). Would it be of benefit going over the wood with this?

I already have 100mm thick celotex so my plan is to stick it between the joists and foam and silver foil any exposed wood or insulation joints.

So far I have been cutting them to a snug fit then using a mallet and scrap wood to fit them in place.

 

[Ivor Windybottom]

The soffits are the wrong way round but it's not important really. As long as there's a vent at the other end they'll be fine.

Your Celotex needs to be against the ceiling, definitely not against the underside of the roof. The cold/dry outside air should vent the bare underside of the OSB.

Paint could be worse than nothing, as it would prevent the surface breathing. So any water finding its way in from anywhere would be trapped behind the paint rather than free to disperse into the ventilated air.

What was there before? Presumably not much vapour barrier and/or insulation? Nothing can rot without water, if you're solving the ventilation issue then the mould will just sit there dead for ever and not do any harm.

 

[nordita]

This is something I built from scratch in the back garden so there was nothing there before.

Is your comment about the celotex in reference to my picture?

I planned to place the celotex in so that the ventilated gap is the top of the celotex and the underside of the osb.

I will staple a green vapour barrier after the insulation.

 

[Ivor Windybottom]

Sounds perfect. You could put plasterboard under the Celotex, but if it's a shed you could use more OSB, might be more suitable for its purpose. You won't get condensation on it as it won't be cold. You could also paint it.