Localised damp above air brick

[geraintf]

hi there, first post, hope someone can advise..

I moved into my mid-terrace, c.1900 house in North London about six months ago. Standard construction for the age: solid walls with suspended (& exposed) timber floor, the sub-floor void ventilated front and back with an air brick at external ground level. Outside is a cementicious plinth about 15cm high coated in a bituminous product. I assume some form of DPC (?slate course) between that and the brick course above.

When I got round to painting the ground floor skirting board I noticed that part of it was pretty rotten under the old paint with accompanying damp smell. When I removed said skirting the cement render over the brickwork had completely deteriorated and the bricks and mortar behind were damp.

Only a c.60cm length of wall is affected. To either side it's more or less dry as far as I can see. Funny thing is that c.60cm happens to be located over the air brick. The air brick is not blocked, and I've not noticed any buildup of standing water after rain. There may be a bit of 'splashback' in heavy rain, as there is a projecting window sill above and there is hardstanding outside. Could this be causing it?

I find it strange that a damp problem has developed in exactly the place it shouldn't: where there is good ventilation. Could it be that somehow there is water ingress through the air-brick?

How can I cure this prob? If I dry out the damp area with a portable radiator or similar and then reinstall the skirting will the damp just come back eventually?

Thanks for reading this lengthy post! If anything's not clear please let me know. Any advice welcome!

Untitled

• geraintf
• 24 Mar 2010
• 2

Untitled

• geraintf
• 24 Mar 2010
• 2

Untitled

• geraintf
• 24 Mar 2010
• 2

 

[Burnerman]

I can't actually see any sign of a DPC just above the airbrick - although there is a small piece of mortar missing.
Do you reckon the outside plinth is original, or has it been added later to combat this problem, maybe? Likewise the concrete yard - the original depth could have been much lower than it is now.
Is that a sign of woodworm on the exposed laths inside?
This problem may have manifested itself many years ago, and maybe the outer wall has been pointed since.
Presumably you have lifted the floorboards in proximity to this, and not noticed anything untoward?
Anyway, leave the area exposed to allow ventilation, and check with a damp detector to see that the wall is actually damp, and not just cold.
It will help if any dripping water is sorted, just in case water is splashing up the wall.
John

 

[Nige F]

I don`t want to be alarmist - but I really don`t like the look of that white furry stuff on the wood

 

[geraintf]

thanks for the speedy response John!

I reckon the plinth is original cos I can see it on the other houses. As you say, it's quite likely that the external ground level has risen a bit, although I don't think there's bridging of any DPC going on. Well spotted on the woodworm it was treated about five years ago by the previous owner.

I'll keep an eye on the situation as you suggest, and have a look at the exterior wall next time it's pouring down (normally I'm up in the roof with a bucket at that point but that's another story )

 

[geraintf]

hi nige f,

yep that battern is rotten on one side but ok on the dry side (to the left in the pic)..

 

[brig001]

I have seen something similar where damp was caused by by ventilation cooling a wall - condensation will form on the coldest part of a room. Do the wall and floor feel colder in that area? If so, could a deflector be fitted under the floor to stop the cold air from outside blowing directly on the floor, but without reducing the ventilation under the floor itself?

Note: not qualified in this area, so check with someone with more experienced first.

 

[Nige F]

Exellent point And one for the " don`t fill the cavity brigade" . If you want to live in a house with efectively one brick thick wall- get that insulation out and ventilate the cavities - like in the 1920`s