mold/damp coming through wall

marsaday · Posted: Wed Oct 14, 2009 6:04 pm

I have a damp area in my loft which is developing black mold.

This section of the wall was re-rendered on the outside a year ago as it was obviously letting water in.

I didnt remove the damp plaster on the inside at the time and just reskimmed over this.

Should i have removed this plaster? do you think the damp coming though again is trapped moisture?

if it is i will take down this section of plaster work and let it breath for a bit. any idea how long i need to let it do this?

cant think of any other reason why the wall would be wet. the rendering was done spot on and so i know this will not be leaking.

JohnD · Posted: Wed Oct 14, 2009 6:09 pm

in a loft? are you sure the roof is not leaking? Why is a loft plastered?

tell us how the loft is ventilated.

marsaday · Posted: Wed Oct 14, 2009 9:23 pm

its the 2nd story. an old attic, not a conversion. it has its own gable window.

^woody^ · Posted: Wed Oct 14, 2009 10:04 pm

Black mould is generally condensation related. In this case a possible cold spot

Now, water penetration may cause localised cooling, and thus cause condensation, but there would be other signs too - such as water staining

Moisture would dry out of a wall and plaster would not trap it in, nor would it trap mould in. If the plaster was damp or hygroscopic, then there would be white salts on the surface

marsaday · Posted: Thu Oct 15, 2009 12:16 pm

this wall affected is the party wall, so has a terrace next to it. however, we are on a hill and so the terrace is lower than mine.

the top section of this party wall is rendered and then about 2m lower down the neighbours roof starts. it is this section which is getting the mould.

If this is a cold spot, what can i do to prevent the mold developing?

JohnD · Posted: Thu Oct 15, 2009 1:10 pm

marsaday wrote:

the top section of this party wall is rendered and then about 2m lower down the neighbours roof starts. it is this section which is getting the mould.

Aha! Might be flashing or something. Can you post some pics please of the outside
http://www.diynot.com/forums/viewtopic.php?t=129539

marsaday · Posted: Thu Oct 15, 2009 2:02 pm

unlikley as the flashings were done at the same time. myself and my mate did the work. he is a quality builder so i doubt that is the problem.

if i can get some pics next week i will.

marsaday · Posted: Tue Oct 20, 2009 6:44 pm

Hello, i now have some pics. You can see the new rendering done about a year ago. The inside wall where the mold is where the rendering has been done. any ideas what the problem is?

if it is a cold spot will a tanking membrane lined with battons and plasterboard help?

^woody^ · Posted: Tue Oct 20, 2009 7:11 pm

Either

the render is trapping water - there is staining externally from the verge. This will make the wall cooler, but will be distinct from penetrating moisture

or

you have a general condensation issue in the room - there seems to be the main mould patch and other areas of blackness to walls and ceiling.

If you are satisfied that all is well externally, the one thing to do will be to install a 215x140 airbrick in that gable - 300mm down and in from the top corner. This will vent the room and also vent the wall

Or thermal board the gable wall.

But you may still need to ventilate

marsaday · Posted: Tue Oct 20, 2009 7:34 pm

ok thanks for info woody.

we did use water proofer in the render mix if i remember rightly. Was this the wrong thing to do?

The wall pic shows most of the mold in the top right area, but i noticed there is mold lower down on the wall which you cannot see.

i will go with the air brick idea i think initially. I will drill a hole and put a shower vent cap over it.

^woody^ · Posted: Tue Oct 20, 2009 8:02 pm

With the air brick, don't use a plastic liner.

The idea is to have bare brickwork to allow any moisture in the wall to evaporate off. So I don't know how you would fit the shower vent cap

marsaday · Posted: Tue Oct 20, 2009 9:14 pm

ok will do, thanks

JohnD · Posted: Wed Oct 21, 2009 3:10 pm

the cap is surely to keep the rain out?

I like the cowl vent with a slope to cast the rain off (the ones like a little venetian blind rattle annoyingly). If you want you can use non-drip gloss on the plastic to tone it with your brickwork. There are screw-holes in the corners to fix it to the wall. Preferably use round-headed brass screws as there is little load but considerable risk of rust otherwise

The mildew near the top of the internal wall does suggest condensation, as water vapour is lighter than air it rises. Condensation also occurs a lot on the wall behind furniture that is against the wall

marsaday · Posted: Wed Oct 21, 2009 8:24 pm

Just a thought....

Would drill holes in the window frame in the room increase the ventilation enough to get rid of the vapour? Or would this be a minor improvement.

JohnD · Posted: Wed Oct 21, 2009 8:29 pm

http://www.diynot.com/wiki/building:condensation_in_houses