Damp smell in extension to modern house

[John Kelly]

Our home had an extension built earlier this year. The extension was to the side of the house and involved knocking through. After 7 months the room smells damp or musty and seems particularly bad in damp weather (like we have at the moment). There's no obvious signs of damp around the walls or floor and everything seems dry. The room is shaded with a large hedge running to the side of it (if this makes any difference). What could cause this problem and what can we do about it? It's not very pleasant to be in the room.

Thanks

[Phil L]

Check the ceiling for any slight dampness which may not be visible. Leave the window open slightly for a week or two to allow it to dry out properly. Last resort: get a dehumidifier and leave it running for a few weeks.

[John Kelly]

The window's been open (on the latch) since March. There's no sign of dampness in the plaster or the floor. The air is heavy with damp though. Is it somethig as simple as not yet having laid a floor (we're going to put tiles down) in there?

[Phil Anthropist]

If it smells damp then it's damp. Even if the hedge shields the extension from the sun I doubt that cutting the hedge down will fix the problem. I would start with the floor, which I presume is concrete, and spread some newspaper out and cover with a tarpaulin or plastic sheet. Check after a few days to see if the paper is damp. Obviously it should be dry. The only DIY way of checking the walls for damp I know of is to fix small pieces of glass to the plaster with a ring of plasticine or putty, so that there is a space between the plaster and the glass, and the air is sealed under the glass. Check after a few days - if the glass mists up on the inner surface then that indicates that moisture from the wall is condensing out onto the glass. Have you reported your concerns to the builder? Has it got a flat roof? Is that watertight? You could get a survey/damp report done by a chartered surveyor or structural engineer.

[Phil Anthropist]

I wouldn't lay tiles or anything else on the floor until the cause of the damp smell is found, in case the source of the damp is the floor.

[John Kelly]

Thanks - I'll be very upset if it is damp. We paid the builder off back in march and it'll be difficult to get him back - we has a problem with a badly installed window and he wasn't very good about it. I'll try the paper and the glass bit and see. At the moment I've got a sheet of plastic on the floor which I assume is the same as putting glass on the wall - ie: it may get damp in a few days. Seems odd though that the room only feels damp when damp outside.

[Phil L]

The bare floor shouldn't make any difference. What kind of roof is on it, flat or pitched? Is there insulation in the roofspace? What is the plaster construction? - EG, drylined, render, bonding, browning etc?

[John Kelly]

Sorry - no it hasn't got a flat roof. Forgot to add that bit

[Phil Anthropist]

So it's got a sloping roof with tiles? Can you look into the roof space to check for signs of damp? What are the walls made of, brick outside, then an insulation-filled cavity, then blocks inside, then internally plastered, or some other construction?

[John Kelly]

The roof is a standard clay tiled roof - not flat. There is insulation in the roofsapce. The new walls were built with that insulation board between them and the plasterboard was bonded onto the walls and then plastered over - drylined.

The only thing that concerned me at the time was when they concreted the floor. Where they'd knocked through they put slate on what was left of the old wall and then concreted over that. I asked if it needed anything else and they said no.

[Phil L]

Damp meters can be hired from plant hire shops, although they are fairly expensive.....I'm a bit concerned about the walls being damp in a newbuild, considering the blistering heatwave of a few months back, it /should/ be bone dry by now...

[Tournifreak]

I'm no expert by any means but when we had our extension built last year I had a kind of similar problem. I looked up in the loft about 8 weeks after the build was complete only to find huge amounts of moisture up there. It's a sealed loft (as is usual these days) with a breathable membrane under the slates. But the membrane was obviously nowhere near breathable enough to cope with the vast quantity of water that has to dry out of a new build.

I put a dehumidifier in the upstairs room with the loft hatch (open) and collected about 4 litres per day for 3-4 weeks. By then the loft was dry, and it's never beena problem since.

I wonder if its possible that your loftspace could be harbouring a lot of dampness? Do you have a loft hatch? Might be worth getting a dehumidifier in anyway. Have you done your Building Control final sign-off yet? The BCO might have some advice?

Jon.

[Dave Fawthrop]

|Our home had an extension built earlier this year. The extension was to |the side of the house and involved knocking through. After 7 months the |room smells damp or musty and seems particularly bad in damp weather |(like we have at the moment). There's no obvious signs of damp around |the walls or floor and everything seems dry. The room is shaded with a |large hedge running to the side of it (if this makes any difference). |What could cause this problem and what can we do about it? It's not very |pleasant to be in the room.

Suggest you get the room as hot as you can for as long as you can. say

25-30 deg C for a week, with thermostated heaters. That should dry out the walls, floor and roof. Do it before the warm weather ends and makes the method *still* more costly.

Also put some extra insulation anywhere you can.

Brute force and ignorance sometimes works where other things have failed.

I have read the rest of the thread.

[John Kelly]

Yep - also it has plasterboard bonded to the breezeblock and then plastered.

Unfortunately we can't look into the roofspace but the ceiling is very dry.

[John Kelly]

I'm good at brute force and ignorance :-)

[John Kelly]

It's all been signed off and the builder's long gone - they finished in early March. I may open up the roofspace anyway. I do feel a little uncomfortable having a space in a house which I can't get to. The builders seemed to think I was being silly suggesting such a thing telling me it would spoil the look of the ceiling. By this time (4 months down the line) I was just wanting them to finish and go so left it.

[Guy King]

The message from John Kelly contains these words:

I can do both at once!

[Phil L]

Newbuild should also have crossflow ventilation in roof spaces, usually a soffit vent running the full length of the fascia board and airbricks in the walls or ridge vents on the roof..what may have happened here is that they have blocked the effectiveness of the soffit vent with insulation pushed too tightly against the joist ends.

[John Kelly]

The fascia board is solid. No airbricks either from what I can make out. The building inspector person seemed perfectly happy with it all when he came round.

[Phil L]

'Breathable' undefelt is *not* sufficient on it's own, I'm surprised the plans were passed or the BCO let this go like this. I meant the soffit vents are behind the fascia, is there a gap between fascia and wall? - if so is there a plastic slotted vent, sometimes only an inch across? - roof vents? (like raised tiles with a slot in them?)

Aslo in one of your earlier posts, you mentioned that you had polythene down on the floor in this room? - if you lift it up, does the smell get worse and is there any moisture underneath it?...either way I would leave this off the floor for a while, in case it's trapping moisture in.

If you have no luck, you may have to cut an access hatch into the ceiling and inspect the insulation for condensation, but if it were mine, the next course of action would be a dehumidifier.