Cold, damp corner.

Hi All

I posted about this last year, but the problem, sadly, hasn't gone away.

I discovered in December 2018 that in one corner of our bungalow's main bedroom, the floor was damp under the carpet, to the extent that the carpet had to be binned and replaced. After taking advice on this forum, I opted to lay a DPM sheet over the bitumen painted floor, polystyrene insulation over that, then rubber underlay, then a new carpet. Since then it appears that the damp has continued to occur, and after moving some furniture and noticing that the new carpet smelled damp in that same corner, I lifted it and found the carpet gripper covered in black mould and the rubber underlay pretty damp. The problem appears to be confined to the corner of the floor in an area about a metre square, so with luck it is just condensation forming on a cold external corner.

The polystyrene insulation appears unaffected by the damp, so I'm assuming that condensation is forming between the insulation and the rubber underlay, then seeping through the underlay to the carpet. I think I need a localised solution using some breathable material to replace the rubber underlay so that the condensation can escape to air, along with measures to reduce the condensation such as a dehumidifier and ventilation. Could anyone suggest such a material? I did read that wool or felt underlay might be able to perform this function, but I'm unsure.

Would putting a vent in the wall near the corner be likely to have any positive effect?

Thanks in advance.

It's suffering from cold bridging. I assume it's a concrete floor that touches the wall.

No, don't fit an airbrick, airbricks are to vent the area under wooden floors.

Yes, it is a concrete floor that touches a wall. I've heard the term cold bridging before, but never troubled to find out what it is.

It seems unlikely that I can do anything about the conditions in the room other than improving ventilation, but I'm hoping there's a way of allowing the moisture to escape to air rather than remaining under the carpet and causing damage.

What’s immediately outside wall? Pics please ( near and far).

Some pics have duplicated, but I think I've got all angles. The injected dampcourse only runs along the rear wall of the property and a few feet around the corner, and was presumably a response by the previous owners to the same problem.

Thanks in advance.

Ok, I'll rabbit off some information on construction and building regs.

1) Drilling and interjecting a DPC is for solid walls.
2) You have a cavity wall, the outer leaf should not be touching the inner leaf, ie. at least a 50mm clear cavity.
3) Outside ground level should be a minimum 150mm (two course of brick) below DPC. Not really relevant to your problem.
4) Between paving and brickwork, you should install a 6 inch gap filled with gravel to reduce rain splash onto brickwork. Not really relevant to your problem.
5) The cavity below DPC should be clear by 225mm (three course of brick). This is so any debris/mortar snots do not build up when the wall is constructed and thus bridge the cavity.
6) I construct concrete floors as follows, 4 inch hardcore, 1 inch building sand, 1200g DPM sheeting, 150mm kingspan laid flat, 25mm Kingspan upstand against outside walls and then 4 inch concrete.
7) No airbricks. Houses are vented by window vents, brick ones are no longer used. Airbricks are used below wooden floors to allow the wood to breath so fungus doesn't rot the wood.

So with the above, the previous householder was conned into an injected DPC. Is there debris in the cavity? Remove a brick and look.

If it was mine, I would dig the floor up, dig down and install a correct floor.

On a budget, remove the skirting board. Chisel out the floor around the walls that face the outside. Standup the 25mm kingspan to stop cold bridging and put DPC between the wall and Kingspan, then fill with an ST4 concrete (usually specified for when rebar is installed but it contains more cement. Saves trying to mess on with a DPM if the floor doesn't have any.

Many thanks for that detailed and very interesting reply. I'll certainly check for debris in the cavity: I know the previous owners also paid for cavity wall insulation, and I did read about the problems such insulation can cause if it gets wet.

I'm sure you're right that the only effective long term solution would be installation of a properly insulated and dampproofed floor, but given that we don't intend to stay here for more than another 3 or 4 years,I don't think I can justify the cost and disturbance that would entail. I was hoping there might be a quick fix for this, but if not then I guess I'm committed to laying polystyrene sheeting or some other suitable material over the floor and hoping that keeps the damp to a minimum in the meantime.

Edit: your "on a budget" option might not cause too much upset to the household, so I could well consider going for that if its likely to have medium term benefits.

no matter what precautions you take for "sealing off" that solid floor they wont work.
Why, because of Gravity. Why, because you are on a steepish hillside, and the water table is so high its forcing high pressure ground water under your cavity wall and solid floor.

As above, you could dig out and create a membrane wrapped new solid floor but that might create the possibility of the pressured ground water moving further into the house beyond your membrane or finding a way up your internal walls?

fwiw: i wonder if the cracked concrete path was due to incorrectly placed concrete or ground pressure causing the cracks?
A previous remedial channel has been dug at the base of the retaining wall?
the lower courses of exterior brickwork are bleached.

It’s an interesting thesis. We are on a hillside, and there’s considerable ground water runoff from a field above us. The pathway alongside the house was flooded a few years back, about a year after we moved in, and it transpired that the drainage for that area was totally inadequate. Thats since been remedied, but the grassed clay slope above the house does hold a great deal of water after prolonged heavy rain. The channel at the base of the retaining wall takes runoff from the slope via weep holes, but yes, some water could seep below the concrete path and there could be something in what you suggest.

"there could be something" etc - then where do you think the moisture that persists under your floor is coming from?

bobasd said:

"there could be something" etc - then where do you think the moisture that persists under your floor is coming from?

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At first I thought it was condensation, but the weather has been mild recently up here and we've had the windows open at night.

I've accepted the possibility of rising damp through the floor, although once you remove the saturated coverings the floor surface itself feels dry.

As for groundwater under pressure, that's not something I'd heard before, hence the restrained nature of my response.

It's a real poser.

amfisted said:

As for groundwater under pressure, that's not something I'd heard before, hence the restrained nature of my response.

It's a real poser.

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No need to be restrained with bob, he's not and he's right.......most of the time. You'll learn a lot from his recent posts, drains, water etc.

Nige F said:

No need to be restrained with bob, he's not and he's right.......most of the time. You'll learn a lot from his recent posts, drains, water etc.

Click to expand...

Thanks for that.

Question is, if this is a result of groundwater under pressure, is it possible to find a solution that doesn't break the bank, or move the problem further into the house?

Incidentally, the cracks in the concrete weren't picked up by the surveyor when we bought the house, and I didn't even notice them until I came to replace some of the path with flagstones. There must have been a lot of concrete left over when the floors were done, because that path is about five inches thick, and breaking it up nearly killed me.

What does bleaching of the brickwork indicate?

Can’t see evidence of original dpc so may be below the ground , and injection Damp course ineffective .
Have you measured down from windows to floor inside and out , would be interesting to know the result .

The original dpc is there, yes. In fact coincidentally, I drilled into it the other day when I was fitting kitchen waste pipes. It’s in the joint above the injected dpc, but you’re right, not obvious in the pics.

I’ll take those measurements you ask for in daylight, tomorrow.