Damp in just one spot

Connacht

I was in a house the other day that has damp in just one place.
Upstairs in the semi-d at the corner of the external wall, there's damp high up in the bathroom.
The bathroom has a window which is always left open when people are showering and there's an extractor fan with 4 inch (maybe 6 inch) hole thru wall which comes on automatically with the light switch. I went up into the attic above and there's no leak - felt dry, fibreglass insulation dry, plasterboard dry, wood dry.
Right next to the bathroom along the same wall, there is another bathroom. There is not damp in there, even tho it has no extractor fan fitted. Further along same exterior wall, there's a landing and then a bedroom - no damp.
How can just one 4 foot stretch be damp and only high up on th wall ?
As always, thanks for your help.

mickdw

Is it gable wall or eaves wall? There are numerous reasons for this.

Wall ties falling inwards at this location,
Cavity crossed due to dirty cavity or faulty cavity construction
Wrong damp proof detailing
Soffit & fasia not covering cavity closer at top of wall (eaves)
Wall vent slightly sloped inwards allowing water to flow to inside leaf.

Possible condensation due to cold wall meeting humid bathroom air.

If you give as much detail as possible, I might have some ideas.

Is it a cavity wall construction?
Is there anything adjoining the house outside the damn spot?

If its a major area of damp that just stops when the bathroom stops, it may well be down to condensation

muffler

In a word - condensation. That is the most likely cause.

Connacht

Thanks lads, sorry for delay in getting back.
The wall is a gable wall, in a typical semi-d built in 1990's.
There is nothing on the outside of the wall. The wall that is damp is only in the en-suite bathroom. But I can't understand why it would be condensation, when the extractor fan kicks in automatically when light is switched on and the window is left open.
It's cavity wall built.

MicktheMan

Whats the heat source in the ensuite?

phelixoflaherty

Damp in one spot. Pwah

Connacht

Mick,
The en suite is around 3 m2 and has one single bar OFCH radiator, under the window (not the best place for it, but typical).
Thanks.

MicktheMan

Is the wall north or east facing?
Is there insulation in the cavity wall?
My guess is that it is condensation from the moisture generated in the shower hitting the colder wall (below the dew point of the moisture laden air). You mention the extract fan. Is there a time delay (say 20 mins) on the fan after the light is switched off? There should be!
Also working against you here is the size of the ensuite at 3m2. This is quite small but typical.
Solution is to:
- increase core temperature of ensuite before taking a shower,
- Open window of ensuite and bedroom (leaving the ensuite door open) thus creating cross ventilation.
- Run the extract fan longer or replace fan with a larger capacity fan.