Loft Room Insulation = Condensation

[markasread]

Over the last couple of months, I have removed a filthy old bedroom ceiling installed thick mineral wool insulation, covered it with plasterboards, and last weekend had the whole room skimmed. A lot of plaster was needed and it's still drying out after three days. I have got eight downlighters installed and through one of these holes in the plaster noticed that the roofing felt and a good deal of the insulation is soaking wet. I think this was caused by insulation making the outer surface quite cold. Due to the fact that there wasn't much space, there really isn't any space for airflow. Currently I've got the heating on full and have just bought some BIN primer/sealer which I'm intending to cover the plaster with once (and if) this ever dries out.

I realise that it would have been better to have used something like polystyrene rather than mineral wool, but a) am I kidding myself that this will ever dry and and be sorted out by painting a moisture barrier and b) what are the chances that I will have to tear the whole lot down and start from scratch?

Advice much appreciated!

 

[foxhole]

Sounds fine the loft could do with more ventilation but with recent weather plaster is going to take it's time drying out and heating only increases the amount of water vapour the air can support leading to condensation .

 

[Chukka63]

lack of airflow sounds like a condensation potential to me.

 

[Alastairreid]

vapour barrier and ventilation? glasswool between rafters and no airgap

 

[Steve]

downlights wont help either.

 

[IJWS15]

Not helpful now but you need a moisture barrier between the plasterboard and the insulation. Other wise any moisture in the room goes through the plasterboard and condenses on the cold felt - as you have found.

Ideally there should also be a ventilated space between the insulation and the felt to deal with any moisture that does get through.

 

[markasread]

Thanks.

It would have been tricky to put a moisture barrier beneath the plasterboard due to the recessed lights - I have now removed all the rockwool insulation from the sloping roof and after letting it dry out replaced it with 50mm polystyrene which allows for a 50mm gap between the felt and the insulation - this seems to be doing the job. I'm going to paint over it with moisture barrier paint to finish and reckon that will be enough. I've noticed the problem isn't nearly so bad on the flat sections, which may be because there is a layer of wooden boards beyond the felt, which I presume is providing further insulation and reducing the condensation - I'm still not quite sure whether to re-do this or leave it be - it isn't totally dry so maybe a risk that it will rot?

 

[Chukka63]

celotex or similar would have been better but more expensive than the polystyrene.

 

[Magic Man]

A dessicant dehumidifier will dry it out.

 

[Wickedkitten]

Did you actually put the insulation all the way up to the eaves without leaving a gap?

 

[markasread]

Just to update on what happened. I decided to bite the bullet and by removing some sections of the plastered boards was able to remove all the insulation and gave it away through Freecycle. I replaced it will 50mm polystyrene which has left a gap of about 50mm for moisture to escape. When the room has been replastered I'll paint vapour barrier and at some point in the new year get a roofer in to install venting roof tiles to increase air flow. To replace some of the insulation I was able to access the insulation from the adjacent rooms - pulling out the old stuff with a hooked pole and replacing it with loose-fill insulation.

In reply to the last reply, the old insulation left no space for air at all.

Thanks for the helpful comments.