DIY Doctor UK DIY Forums

Hi,

I am at second fix in an almost completely new build, which has a very large (15mX4m Dormer) and two large flat roofs (cold roofs).

Since booting up the boiler we have started to have problems with condensation in the eve's on the sloping roof side of the house and also having opened up the ceiling in the bathroom in the flat roof.

Initially the vents hadn't been cut outside, but they have been now for 3 weeks, and we also have a blower pushing air into the roof space from the house and we can feel it come out of all the vents in the dormer.

However, when things got colder on Xmas Eve, there were a couple more patches of water literally bleed out of two fo the internal stud walls, and also a few small patches in the eves. I am not sure it added to the problem, that the builders (who are great btw), switched off the 3 dehumidifiers we had in as they didnt want to risk an electrical fault over XMAS.

The house has been plastered over the last two weeks, using a special thin plaster which rolls on.

I am obviously concerned that as the house is really really well insulated (Ecotherm on inside of wood dormer, and then Styrofoam on the outside), that this might become a long term problem and my new roof might rot. Also I am due to have carpets fitted in about a month and although the water wasn't pools, it would have wet a new carpet.

How long should it be before the water from the plaster and next the paint dries out to levels that should need a constant dehumidification?

How long should I leave it before I get concerned that this is more of a chronic problem with not enough ventilation in a too well insulated house.

I have already bought myself a Meaco dehumidifier for when they have gone, and the question is whether I should look at PIV system or anything else to stop a long term problem, or is this just to be expected in the first month or two after plastering, and I should just keep the 3 industrial dehumidifiers going a lot longer!

Also the builders are reluctant to open the windows as its damp and cold outside, but I am thinking that its may not be half as much moisture in the air outside as there is in even during this damp December weather??

One other thing I have done is switch the boiler (underfloor heating) down, as we dont have the thermostats on yet, and the internal temperature was hitting 20-21, so it is now 17.5. I did this on the theory that we may have been pushing the water too quickly out of the walls from the plaster.

Any help or reassurance would be welcome!

Hi daspearce
In any new build there will be a lot of water to evaporate and get out of the building especially once the heating is turned on.
Opening the windows is a good move - it’s the quickest way to get air changes in the property. Run the de-humidifiers when the windows are closed.
Regards S

Thanks, thats reassuring. I have done four things today:

1. Switch the heating down so it is about 16 degrees.
2. Get 3 dehumidifiers on full tilt.
(humidity came down to 49%, although it rose as they finished some plastering upstairs to about 64% again, but I can see it coming down quickly as the dehumidifiers are on.

3. The verges of the sloping roof have not yet been capped, and I think the air gap between tile and felt will create a cold layer at the level of the felt, rather than have that as an air pocket. So the builders are going to use expanding foam to quickly seal that gap, and we can put the trims on later. That way I would hope that the cold surface is the roof tile and not the felt, in which case moisture should condense on that and then run down it, which is fine!

4. I have taken the blower out which they put in the ceiling space. This may have been useful for initially venting the flat roof, but logically all it is doing now is blowing the warm air from the house into the loft and flat roof space, which appears to me to be the opposite of what is suggested, as people say insulate around your loft hatch.

5. I have also got the guys to seal up the inspection hole in the bathroom ceiling as again this just allows the warm (moist) air from the house straight into the flat roof, above the insulation layer and then back into the main roof.

I am not a builder, but understand a bit of physics, and it seams to me that the tactic is to allow the house to get rid of the water, and there must be over 1000 litres if I count the buckets of plaster used in the last two weeks, but keep the Dew point as low as possible (reduced temperature internally and reduced humidity do that I think) and keep it away from the voids in the flat and sloping roof, and keep the space between felt and tiles as warm as possible so the cold point is where it doesn't matter where moisture forms.

Anyway, fingers crossed that it works. I may need some more trickle vents etc, or even a PIV system, as the house is very well insulated, but I dont think it is the time to conclude it definitely does as the water load from the plaster and later this week the pain is huge and I think its premature to panic. The roof and flat roof are well ventilated, and there is definitely a cross flow of air when i put a lighter into the inspection hole we made in the bathroom.

Thanks for your reassurance.

Hi daspearce
Not sure turning down heating is going to help, warmer air holds more moisture and the de-humidifiers will have more to work on.
Has a breathable underfelt been used to allow moisture out?
Better to seal all holes into the loft area and open all windows to reduce the amount of moisture laden air getting into the loft.
Regards S

Thanks,

Yes breathable membrane is in place.

Building inspector was very reassuring today. Said roof is well constructed and just a bad time of year to be plastering!

Closed all the holes into loft and verges today. I’ll crank the heating up a bit more and make sure doors and windows open during the day.

Humidity down to 45-50% now
Thanks.