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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Building Questions >> Osb board water droplets https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1327952604 Message started by del3165 on Jan 30^th, 2012, 7:43pm

Title: Osb board water droplets
Post by del3165 on Jan 30^th, 2012, 7:43pm

Looking for some advice. I had an extension built about 3 years ago, since the extension dampness has been forming on the osb boards causing some mold. It only ever happens when the temperature drops below freezing. I recently insulated the loft ceiling and put plasterboard up, now water droplets are forming on the osb board between the insulation and the board. I have soffit vents, and ridge vents, but no roof tile vents. Would more ventilation ie roof tile vents help.There is now a radiator in the loft.

Title: Re: Osb board water droplets
Post by CWatters on Jan 30^th, 2012, 10:35pm

Check you have enough ventilation.

If this is a flat roof the recommendation for soffit vents is free area equivalent to a 25mm wide slot the whole length. If you do that with round vents you need a lot, possibly even two rows of them. Work the numbers or see here for typical spacing required..

http://www.manthorpe.co.uk/Building/Products/Roof-Ventilation/Circular-Soffit-Vent.html

Quote:

To achieve 25,000mm2/m of free airflow the G700 should be fitted at 86mm staggered centres

If it's a pitched roof I think they recommend soffit vents with an area equivalent to a 10-20mm gap and ridge vents equal to a 5mm gap both sides.

You should also have a vapour barrier (plastic sheet) behind the plasterboard to stop an water vapour getting through gaps in the insulation to the cold areas.

Title: Re: Osb board water droplets
Post by del3165 on Feb 1^st, 2012, 6:16pm

Thanks for the reply. It is a pitched roof, the problem seems worse on the gable end of the house (sw facing), unfortunately there is no soffit on the cable end to put vents in. That's why I thought roof tile vents might help. The insulation between the plaster board and Osb is water proof. What would happen if I painted the Osb board would that help stop the moisture forming on the board. The old part of the roof still has the wooden slats which don't suffer from any moisture build up. I can take more pictures if that helps.

Title: Re: Osb board water droplets
Post by Joiner on Feb 1^st, 2012, 6:31pm

Is it happening when the outside temp warms after a period of very cold weather?

What is directly above the OSB?

Title: Re: Osb board water droplets
Post by del3165 on Feb 1^st, 2012, 6:47pm

It happens when the outside temp drops. Above the Osb board is felt then Marley smooth tiles.

Title: Re: Osb board water droplets
Post by Joiner on Feb 1^st, 2012, 7:11pm

That could be the problem then, either OSB itself is getting so cold that the air within its cavity is condensing on the underside of the board, and/or warm air is finding its way up past the insulation. Either way, you've got warm air meeting a cold surface.

Jeez, you're going to need a hell of a lot of eaves ventilation to stop that happening and, ironically, it could make the situation worse by making the OSB even colder.

Exactly how much insulation have you got in there?

Cwatters will probably work out how much you should have.

I suspect that whatever the diagnosis the cure is not going to be a cheap one and the builder of the extension may have a few questions to answer.

Title: Re: Osb board water droplets
Post by TIMBA-WOLF on Feb 1^st, 2012, 7:33pm

Just a small question del... the picture shows, in the "distance", what looks to me to be a noggin....between the rafters.....with the insulation going, up to it.... if that is the case, then surely, you have BLOCKED OFF any airflow within that section, and any others like it...

normally, if any of my clients are going to insulate, to give a "semi" warm roof, then i'd get the noggin's and either turn them through about 45 degs, or take eack one out, then add 2 set flat but staggered to still allow the air flow up between each rafter set!!!

Title: Re: Osb board water droplets
Post by del3165 on Feb 1^st, 2012, 9:06pm

The noggin is actually the hip pitch running from the ceiling to both corners of the loft.There was never any dampness from the oxters to the top of the roof until the insulation was installed by myself, I did have dampness when cold from the bottom of the raffters to the start of the oxters, but none on the main area of the loft. This is the insulation I used
http://www.thermawrap.co.uk/loft_wrap.html
Should I make sure the insulation is air tight to stop warm air hitting the cold Osb board

Title: Re: Osb board water droplets
Post by Joiner on Feb 2^nd, 2012, 10:32am

"Should I make sure the insulation is air tight to stop warm air hitting the cold Osb board"

Vital!

Title: Re: Osb board water droplets
Post by del3165 on Feb 2^nd, 2012, 9:51pm

Joiner wrote on Feb 2^nd, 2012, 10:32am:

"Should I make sure the insulation is air tight to stop warm air hitting the cold Osb board"

Vital!

Vital!??

Title: Re: Osb board water droplets
Post by CWatters on Feb 2^nd, 2012, 10:23pm

Vital as in...

Quote:

You should also have a vapour barrier (plastic sheet) behind the plasterboard to stop an water vapour getting through gaps in the insulation to the cold areas.

How about fascia ventilation if there aren't soffits?

http://www.carryduffdesigns.co.uk/home/images/eaves-vent.jpg