Yes.
Yes.
However its normal to put a vapour barrier on the inside of the insulation in this sort of situation.
Ventilation then gets the moist air out before it can condense on that.
On outside walls you generally have plasterboard with foil backing, then insulation, then stone..
The trick is to have the vapour barrier where its too warm for condensation.
And allow the permeable outer walls to breathe outwards.
Controlling vapour is crucial when insulatiing.
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Indeed.
What is the intended finished floor surface? If carpet or wood based, I wouldn't use UFH at all.
Certainly. Personally, if I really wanted UFH heating in there and there really was only a choice between 6mm and 0mm insulation, I would still go for the 6mm. Note that 10mm boards are the same price and the additional 4mm is worthwhile.
What you haven't considered is that if the insulation is adequete, then the wall temperature will be much higher. Provided the ventilation is up to spec, which seems to be very much the case with heat recovery ventilation mentioned, then the relative humidity at the vapour barrier will be much less than 100% and no condensation at all will occur. As the temperature drops through the insulation, what little air (with closed cell insulation) that gets through will be very low in absolute humidity due to the vapour barrier, so no interstitial condensation will occur there either.
Christian.
Thanks again, I'm signing it off now. I would put the 6mm in if it weren't so damn expensive but 10mm is really pushing the headroom issue (partly because the guys who poured the floor were worried about not having enough concrete so ordered the extra 0.5m3 which meant the slab ended up about 10mm thicker than planned so I had less to play with than I thought. BCO is very understanding about stair headroom and stuff like that but I don't want to push him too hard. The problem with the condensation is that the ventilation is inside the room (for the occupants) and doesn't ventilate as well the space behind the insulation and in front of the membrane. Essentially there is likely to be a humidity gradient through the wall.
Anyway the arrangement of the membrane should ensure that condensation on that interface isn't an issue so I will be insulating between the studs for the dry-lining. Before anyone comments on bridging by the studs and using over stud insulation, I know, I know but I don't want to keep bringing the walls in. Anyway U-value for the wall construction even accounting for the bridging is pretty good (I'm going for ~0.3 although heat-loss calculation was based on 0.6). I'm really not going to notice this room on the heating bill.
Thankyou to everyone.
Fash
Can this be sorted with a few airbricks?
The vapour barrier and insulation will prevent condensation coming from the inside.
U-values are not the only reason to avoid cold bridging. One of the main problems is that you will get localised cold lines, which will lead to condensation where the studs are. The condensation might not be excessive, but can often lead to obvious staining from moulds. Can you do the final "plasterboarding" with something like Kingspan K17/K18? This is most typically done using a 37.5m board (25mm insulation+ 12.5mm plasterboard). However, thinner boards are available and will make a significant difference to the cold bridging situation.
Christian.
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