Flat concrete roof, codensation or a leak?

[roamintherange]

We have just recently bought a new (to us) house which has an outhouse that we have converted ito a utlility room off the kitchen, I have got to the stage where we have plasterboarded and skimmed and it's looking ok.

However I have just started to notice a build up of water on the ceiling which looks like codensation to me although I am not sure.

It only seems to have happened since I skimmed, could it be the plaster drying out? the only ventilation in there is the outlet for the tumble dryer but the weather is very damp at the moment anyway, we have had a bit of rain recently.

The roof felting looks ok to me from above but I am no expert on roof felting, there are a couple of damp areas, none of which are above where the build ups are occurring.

Any ideas?

 

[JohnD]

Is this a concrete roof with no insulation? And the room below is inhabited and next to the kitchen? Then yes you will get condensation (but not in summer when the roof will probably be warm from the sun. Any internal wet then will be rain).

The new plaster will be adding to the humidity as it dries out.

Is there enough headroom to add an insulated lining with vapour barrier?

Even a couple of inches would do, using insulated plasterboard (comes ready made with a polystyrene layer).

An extractor fan in the kitchen will also help keep the humidity down.

An uninsulated ceiling/roof loses a vast amount of heat in winter.

It might be possible to rurn it into a warm roof with insulation on top and then sheet roofing, but this would be harder.

If you ventilate the utility room well and don't heat it then there will be less cause of condensation.

 

[roamintherange]

Thanks John

Yes the room is uninhabbited for now because we havent moved in yet but there will be washer, dryer fridge, freezer as well as a seperate toilet at the end.

I have put a false ceiling in there because we are putting mains fed downlights in, didn't put any insulation in due to the fire risk though, there was jsut enough headroom for this. The elctrician said these roofs do suffer condensation but if anything, the heat from the downlights should help dicipate this.

We have just installed an extractor fan but again aren't using it yet.

The room is much warmer now due to the plasterboarding, it's quite amazing how much difference that has made, maybe this is adding to it?

As long as it's condesation, once the false ceiling is in place, this should be cut down I would have thought?

 

[JohnD]

You need:

Insulation on top of the false ceiling. Even 25mm will help, but the more the better.

A vapour barrier under the insulation. This will stop moisture getting at the cold surface.

as the vapour barrier is unlikely to be perfect, and water cannot escape from the top side of the roof slab, it will help to ventilate the cold air space between the slab and the false ceiling. Even a couple of 25mm holes in the wall would do (opposite sides).

I think the vapour barrier and the ventilation are the most important steps to stopping condensation on the roof slab (which will drip onto your ceiling and lights).

 

[roamintherange]

Thanks John

I will sort that this wekend. What should I do regarding the fire risk from the insulation, is there a safe reccomended distance away from the lights?

Thanks

Jon

 

[JohnD]

You can get a smoke hood for downlighters, or use a terra-cotta flower pot (!) to stop the insulation flopping onto it. The smoke hoods are usually specified to prevent smoke from one room rising into the room above.

If you're using fibreglass and plasterboard, it isn't flammable. I don't know of other fire regulations around downlighters and insulation, except that the wires inside a fitting which gets hot are usually sleeved with heat-resistant material (this is usually supplied, or you can buy it in small packets)

 

[roamintherange]

Thanks John, your a legend.

The downlighters have heat resistant sleaving already on them and I like the sound of the terracotta pots from a safety point of view.

Thanks again mate

Jon