DIY Doctor

Condensation and damp smell despite low humidity?

Postby deerhound » Wed Jul 22, 2020 10:19 am

I've had a bad smell in my upstairs front bedroom for perhaps a year.

It comes on overnight. After rain it's a damp, almost mildewy smell. When it's dry it's more musty.

It seems mainly to come from around floor level in the back corner of the room. One wall in that corner is a party wall and the other is internal to my house - i.e. it's nowhere near any possible source of water ingress. The roof is fine. I've had the floorboards up and the subfloor timbers are all bone dry. No plumbing in the area. The neighbours on the other side of the wall don't have any problems. The smell is also also quite strong on the other side of the back wall of the room, from the floor inside a fitted cupboard. I occasionally get a damp smell on my stairs outside the room too.

A protimeter showed some dampness (16%ish) in the bricks behind the skirting board in the smelliest areas. But some bricks in the same areas measure 0%. There's no dampness measurable on the plaster anywhere in the room and the skirting board itself wasn't damp.

It sounds like a straightforward case of condensation. However, a hygrometer left next to the bricks overnight shows maximum humidity is normally 40-50%, occasionally almost 60% but never higher, and the temperature never gets anywhere near the dew point. Several windows are left open in the room every night, year round.

I've had two independent damp people out to look, who thought the damp readings in the bricks were probably caused by salts rather than moisture. One treated one patch of "damp" bricks with a salt treatment. However, the smelliest area is very to access as it's right at the point where one wall meets another, in a small cavity above where a beam sits in the party wall. There's also a joist running 1-2 inches away from the smelly wall which makes most of the bricks in the area hard to reach. I probably can't lift that joist without damaging the old lath ceiling it's attached to.

A structural engineer I had out last week to look at another problem (don't ask) suggested that hygroscopic salts on the bricks were causing condensation to happen at much higher temperatures than the normal dew point. Does that sound right?

He recommended sealing the bricks to stop damp air reaching them.

Specifically:
1. Where the bricks are accessible, seal with a silicone based solution than cover with an anti-salt treatment to be sure. Alternatively, just pin a plastic sheet over them.
2. Where bricks are inaccessible, fill the area with insulating spray-foam.

To treat this I'm going to have to rip out a fitted cupboard. The spray-foam in particular is also hard to undo so I'm keen to get a second opinion on whether this all sounds like a good idea before going ahead.

Any views hugely appreciated!
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Postby stoneyboy » Wed Jul 22, 2020 7:47 pm

Hi deerhound
Your statement that the problem occurs after rain is a good indication that there is an issue linked to the roof. Is the tiling between you and your party wall neighbour continuous? ie is there a join in the tiling?.
Regards S
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Postby deerhound » Thu Jul 23, 2020 2:44 pm

Thanks stoneyboy.

It's a terraced house with a parapet separating the rooves. There's no problem visible with the roof and there's no sign of damp anywhere in the loft or above skirting board height in the bedroom, which makes me think that's not the issue.

My theory on why you get the smell after it rains was just that the humidity goes up then. But like I say, according to my hygrometer the humidity never gets high enough to cause condensation - so I'm confused.
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Postby stoneyboy » Thu Jul 23, 2020 8:39 pm

Hi deerhound
Thanks for responding.
Does the parapet wall have a dpc in it following the line of the roof and is there flashing from this down onto your roof. Do your damp tests at the lowest point of the party wall you can reach from the loft - this may give some indication as to whether the party wall is the cause.
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Postby deerhound » Thu Jul 23, 2020 11:10 pm

There is flashing - no idea about the DPC. Would that be a slate layer in a Victorian parapet? Where would it normally sit?
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Postby deerhound » Fri Jul 24, 2020 11:21 am

https://photos.app.goo.gl/GoK9u3MZssyiBN7L9

Here's the parapet from both sides. The rendered side is my neighbours. The rooves are south facing and the neighbours' west-facing side of the parapet takes most of the weather. The dark vertical lines might suggest some kind of problem in the past? It does look like there might also be a small gap in the coping tiles at the peak. Can't see any problems on my side.

I've also been up in the loft again with my damp meter just now. I can't get a reading off the bricks but the lime mortar does read around 16% damp (or salty) at the peak, underneath the gap in the coping tiles. But that falls to 8-10% halfway down the wall and maybe 3% near loft floor level. So I'm struggling to see that as the clear source of the problems at skirting board level on the floor below.

What do you think?
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Postby stoneyboy » Fri Jul 24, 2020 10:37 pm

Hi deerhound
Thanks for the pictures they do clarify the arrangement.
There probably is no DPC. However it is clear from the staining below the joins in the coping on both sides of the parapet wall that this may be the issue.
Presumably your neighbour has rendered their side of the parapet wall to stop water ingress.
I can only suggest you take damp meter readings again in the loft area when it has been raining for some time.
Presumably your roof is felted below the tiles and it is sound.
Regards S
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Postby deerhound » Tue Jul 28, 2020 3:17 pm

We had some rain over the past few days so I've taken a lot of readings in the loft. This post is quite complex so bear with me.

I've had a lot more success getting damp readings off the mortar than the bricks themselves.

There's definitely some water getting in through the parapet. Right at the roofline in the loft the mortar measures about 23% damp and the top couple of rows of bricks are visibly darker. Towards floor level in the loft, the mortar is much drier though: generally 3-5%. The loft is well ventilated and extremely warm which I guess dries the bricks out pretty well.

Measuring the mortar rather than the bricks has also given me a new picture of where the damp in the first floor bedroom is. The bricks only measure damp in a couple of spots but the mortar behind the skirting board is about 18% damp all the way along the wall.

The plaster on almost all the walls of that room - not just the smelly wall - reads about 12% damp, which is about double the reading I get in the plaster elsewhere in the house. (There's also a high reading from a bit of penetrating damp under one of the windows which takes the full force of most of our weather.)

There's also a lot of water coming down the chimney on the other side of the house - the upper bricks on the flue are saturated. However, the chimney has a big bend in it at loft level where it splits towards two fireplaces and the water doesn't seem to penetrate further down. The wall below that chimney is the driest wall in the bedroom below.

So, taking all the together, I think you're right about the parapet. I guess the next step is the have a chat with the neighbour about getting the flashing checked and the coping stones relaid with a bigger overhang, a good drip channel and possibly a DPC underneath them? Also to get some rain caps on the chimney pots on the other side, even if that doesn't seem to be causing a problem at the moment.

Then wait a few months to see if the damp readings improve.

Does that sound right? Anything else you'd suggest?
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Postby stoneyboy » Tue Jul 28, 2020 10:10 pm

Hi deerhound
Suggest you render your side of the parapet wall like your neighbour has done and repoint the joins in the existing coping. Probably not worth replacing the copings at this stage.
Regards S
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