Dry out floor slab

Builderfromhell

Have 75mm screed on 75mm KIngpan insulation. Walls are Cavity with Platinum insulation. Walls are also dry lined with KIngspan insulation/plasterboard panels bonded to wall.
PROBLEM
Floors got a lot of rain before flat roof went on house (Contemporary design-long story). NOw I need to dry them out as mould is showing on newly painted plasterboard at skirting level.
Not sure whether to ;
1. Wait for Underfloor heating to be commisioned and turned on and then leave heating on with all winodws open.
2. Get a few De-humidifiers in now and leave for a week or 10 days - €300.

My concern about the 1st option is that I will create a high level of humidity and where the water evaporates from the slab at the edges it may migrate behind the KIngspan dry-lining and condense on blockwork. It may then migrtae through blockwork to outside or cause mould growth throughout building - Warm moist air trapped behind plasterboard sounds like perfect conditions for mould.

Any comments, suggestions appreciated.

sinnerboy

Dehumidifiers can be quite aggresive . Option 1 is better if you can afford the time . Best in long run to spend time on drying out , if you can .

Open widows now to dry out - don't wait for UFH .

When UFH is complete , bring on heat slowly over 7 days . Rule of thumb says 1mm per day drying out time . UFH will speeed this up a lot . Hire a concrete floor moisture meter and wait till % reading drops to 4% . ( 3% if you want to use timber flooring over ) Dont rely on readings from wall type moisture meter

I would judge that the amount of moisture to migrate into the wall from the screed depth would not be so great - the fabric of the wall will absorb it safely .
As a precation i would remove the skirting and discard - I would think they will warp in time now sadly .

Builderfromhell

appreciate advice Sinnerboy. Skirtings not yet fitted - 'damp at skirting level' may have been mis-leading. Curious to know how de-humidifiers could be too agressive. Do you maen they dry out the fabric too quickly and maybe cause timbers to warp?

sinnerboy

Yeah BFH - if internal joinery is fitted - dehumidiiers can warp it badly .

If not - well that changes the picture . You may get a fair few more poppers ( plaster board nail heads bursting through skim ) but that about it .

Sorry for the apparent u-turn , but if no 2nd fix to worry about then DH's maybe the way to go , if time is pressing .

If you are not really stuck for time , my pref. still would be to dry out as naturally as possible .

Builderfromhell

I'm inclined to dry out screed slowly with under floor heating and loads of ventilation throughout house. It has since occured to me that while the screed will dry out any moisture trapped below the insulation and above the DPM will not escape so easily. Even if the slab gets very warm the insulation would prevent the water under the insulation evaporating. Even if it did evaporate where would it go. It can't condense on and be absorbed by the floor slab as this is warmer. My only guess is that this vapour would filter out through the edges of the slab eventually.
I wouldn't be surprised if this is a common problem with installing Under floor heating systems in rainy weather. My floor was open to the elements for about a month of rain, due to specialist roofers from UK not turning up. But it could and probaly has happened to many houses.

sinnerboy

BFH I wouldn't worry over much about water retained in the sub floor . Like in the case of ICF walls ( where the polystyrene effectively holds the moisture in ) the concrete will continue to "process/absorb" the water - I think the correct term is " hydration " . Concrete loves water - ( test cubes are kept submerged )

Any vapor release paths will be very small ( if indeed any exist with the insulation and screed over ) and vapor releasing into a warm internal space will not cause much harm . The only side effect you are likely to get is an extra tough sub floor