Damp under bay window

[alison7148]

Looking for some advice please. We live in a 1930’s detached house. My daughter has the front bedroom with a large bay window (with 20+ year old aluminium framed window). The walls of the bay (and the ceiling of the bay too) are black with mould when she peeled the wallpaper off.
We put extra insulation above the ceiling a year or so ago that hasn’t really helped much.
We don’t have a ‘hot ‘ house, central heating hasn’t been on much yet, and when it does come on the thermostat is usually around 18.5 degrees.
She tends to keep her windows slightly open all year round (but does keep a gecko and fish in her room).
What should we be doing to try and rectify this???
Thanks xx

 

[mrrusty]

99% that's going to be condensation on the cold bay walls. Fish isn't going to help the humidity in the room, so it's probably fairly high - you can get a cheap hygrometer from amazon. Let's guess it's 60%. At 18.5 and 60% RH the dew point is about 10-11 degrees, so if that wall surface ever gets colder than that you'll get liquid water condensate that black mould loves. If the humidity is even higher, so will the dewpoint be (and v.v.)

The remedy is more insulation, (and possibly relocating the fish!) and sealing the room air as far as possible from the cold wall structure. Insulated plasterboard usually has a vapour control layer. Insulation will allow the surface temperature to be nearer to room temperature and above the dew point. The reason to seal the room air, is because if it gets behind the insulation it will still condense inside the fabric of the wall.

The old ally framed windows won't help - they'll be cold and the cool air will be sinking down the wall helping to cool it. Keeping the windows open a bit will be helping, but maybe not enough.

What sort of insulation did you add to the ceiling? If there isn't much space, use PIR rather than rockwool as it performs better. (or just add more rockwool)

There is, of course, always the possibility the water is coming from penetration - round the window, in through the roof etc, but that is usually isolated to a particular area. If the black mould is across the wall, then it points to condensation.

 

[martygturner]

Its amazing how far water vapour travels from source, closed doors unless air tight provide no barrier. Thermodynamics says that a warm mass will seek to heat a cooler mass, as the bedroom wall is quite probably a focus point ie its the coldest point in the house ( eg 0.1 of a degree cooler) insulation will not do much good unless the room is warm and the wall is warm... what keeps things cold and hot - insulation, insulating a cold wall will keep it cold it will not warm it up. See a coolbox.

So identify the sources of moisture in the house, kitchen, bathroom are primary and make sure these a ventilated at source.
Keep the offending wall space ie the bedroom warm and ventilate it.. we humans chuck out half a litre per night just sleeping. So open the window in the morning for an hour to vent out the humid air, then pop the heating on low.

 

[Phil473]

As rusty mentioned. An effective solution could be to have a radiator under the bay window.

We have the same issue; I run a dehumidifier which helps but not completely. I periodically check for mould, will try an anti condensation paint. But for a complete solution, probably a radiator under the bay window.