Ventilating house in winter to stop mould?

[Pegs11]

Hi, I’ve been living in my current house for 1.5 years. Last winter I had a big problem with mould on the walls upstairs, in the bedrooms. I managed to get rid of it in the spring but this year, with the recent drop in temperature, it’s come back. It’s mostly in the upper corners of the inside of the exterior walls upstairs, but I’ve just noticed that one entire wall now has a thin spread of mould beginning to appear. I can clean it off, but not sure how to stop it from coming back. I keep hearing people say I need to keep the house warm BUT also ventilate it by leaving windows open. I don’t understand how I can keep my house warm when I’ve got windows open! And I’m worried about energy bills. Also, I’m often away for several days at a time, and can’t just leave the heating on when I’m not here.

Is there anything else I can do? Would a dehumidifier help? As for the heating/ventilation thing, can people tell me what they do in this regard? How often do you ventilate, and for how long? Do you open one window a tiny bit for an hour or two a day, or do you fully open lots of windows for just a few minutes? How do you stop heat loss?

I'd check your guttering is clear and all connected as a start.

We find our bathroom gets it in the winter though and the only way to keep it at bay is having the window open most of the day.

Dehumidifier is a good solution though and it also warms the room.

I agree that you should get someone to check that there is no external cause of damp.

I do agree that some of the advice about ventilation and keeping your house warm can be confusing. I personally don’t really open windows very much in winter EXCEPT for the bathroom windows which get opened (with door closed) after anyone has a bath or shower, to dry everything out completely.

And I use a dehumidifier whenever I am drying clothes inside.

Either dehumidifier (one powerful enough for a whole house is quite expensive) or ventilation.

We open our upstairs windows most days (not if it's pishing down) for about half an hour. The heat does rush out, but the heaters replace it. It is wasteful obviously, so in that regard a dehumidifier would be better, but a bigger initial outlay.
Downstairs we don't get mould so it's obviously showering and all of us breathing in our sleep!

Hi you mention the mould is in bedrooms are these ones that are used overnight or left empty? Ideally you'll need to wipe down the walls and remove as much mould as possible. Then if you can leave the windows open for as long as possible first thing in the morning and have no washing on the radiators; then look to heat the rooms later that day to benefit from the heat yourselves.

Then if you have windows that can have open lock ie just to leave some air through it maybe an idea to have them on the lock overnight or during the day when unoccupied and then closed and the room heated when occupied. We have small disposable dehumidifiers on the windows sills in our rooms and use a Meaco dehumidifier overnight leaving all the doors open to allow the air to circulate. Our main issue has been mould in the bathrooms so we cleaned and washed down our walls and repainted them with Zissner mould paint.

I'm disabled, pretty much housebound and have problems regulating my body heat. So I normally keep the thermostat set to 21 degrees during the daytime all year round.

When energy prices went through the roof I turned the thermostat down to 18 and used a heated throw. I live in an old house built from solid stone blocks that are half a metre thick, it holds on to heat in winter, and takes ages to get uncomfortably warm during heat waves. I've lived here for 20 years and that winter, after lowering the temperature to 18 degrees was the only time black mould appeared indoors. Having been hospitalised with fungal pneumonia previously I decided to turn the heating back up to 21 after spraying the walls with mould killer and wiping them clean as best I could (being unable to stand for longer than a couple of minutes max makes wiping down walls a challenge).

I open the bathroom window after showering, open the back door and switch on the extractor fan when the kitchen starts to steam up, and open all the windows for 5 minutes to ventilate the house regularly. So thought my indoor air quality was pretty good.

Until I got a smart thermostat and it started to nag me about the humidity levels indoors. Anything over 70% humidity made it complain, and at 77% it direly predicts mould and mildew proliferation and demands that I open a window.

That's all well and good if you live somewhere arid, but what good does opening a window do if the outdoor humidity is at 98%? I live in Wales, outdoor humidity far above 70% is pretty standard for most of the year.

I got so tired of being nagged by the app that I rolled out the dehumidifier, previously only used to dry laundry, and set it up in my bedroom, with the door open so that it dehumidifies the entire house. I switch it off when I go to bed, and each morning I empty 3-4 litres of water out of the storage tank before switching it back on again.

I can see on the thermostat app that the humidity climbs during the night, then drops again once the dehumidifier is switched on in the morning. You can see that it gradually starts to stay a bit drier during the night, perhaps because the soft furnishings have been dried out a bit? I left it switched off (in the interests of science) and the indoor humidity took 3 days to reach same level as the outdoor humidity. So I suppose it could be run every other day, but with the dehumidifier running in "smart" mode the energy usage is negligible.

I could turn the thermostat down to 18 degrees again, because the dehumidifier would prevent that black mould from reappearing, but being cold when you can't move about to keep warm is miserable, so I'm economising in other areas to keep the heating at a comfortable level.

It’s unlikely to be penetrating damp. Mould is almost always caused by persistent condensation, caused by moisture in the air coming into contact with a temperature below ‘dew point’ temperature. The coldest areas used to be single glazing panes, but since double glazing, condensation occurs on walls where it can’t be seen and wiped off. I have seen most success by slightly raising the temperature of the inner face of the external walls, by either insulated wallpaper (which is quite thin) or insulated plasterboard (thicker).

I leave my upstairs windows open all the time for ventilation. Even in the depths of winter they're all open a little bit, house gets warm no problem and my heating bills appear to be average. I only close them if I go away. I do think you need to keep your heating on if you go away, obviously not all day long but a couple of hours to keep the house from getting completely cold.

There are so many people who go on about opening windows and "lovely fresh air". This is advice out of the 1960s.

Get a decent dehumidifier and use it. It will pull litres of water out of the air, and they are not too expensive to run either. Do not bother with those pot things they are a waste of time.

The other thing you can do that is cheap is to reduce the living humidity in your house. This is normally free, but involves a bit of effort, eg scraping down the shower to put all the water down the plug hole after you have used it and always making sure you don't boil pans for ages with the lids off.

Thank you all for the helpful advice and shared experiences, and sorry to those who have suffered when trying to manage this issue in the wake of rising energy bill costs.

At the moment I can’t keep the heating on when I’m away because there is no timer or remote control, it’s either “on” or “off”, which means it would need to be left on for the entire time I’m away from the house, as opposed to just a couple of hours a day or whatever. I need to get that sorted because it’s rather inconvenient!

I noticed yesterday that one entire wall in my spare room is covered in mould, albeit a faint layer at the moment. But I will need to clean that off pronto, before it gets worse. Also, every item of wooden / wicker furniture upstairs is covered in a layer of mildew. So I will have another job trying to get rid of that. I’ve bought a load of white vinegar! My house is going to smell lovely, haha.

I live on a development and I’ve discovered that several of my neighbours have had similar issues with their houses - it seems they were all built without adequate ventilation. Some of my neighbours have had airbricks installed, others have had extractor fans put in. I’m going to try the dehumidifier/more ventilation route first (and see if I can get a remotely controlled heating system) and see if that fixes it. If not, I’ll have to copy what my neighbours have done. Argh… money!!!

Also worth using white vinegar to clean it. Bleach doesn't kill the mold but vinegar does.

You have 2 options:

You either open your windows, preferably every morning. Not every window but definitely your bedroom windows. Open them fully for about half an hour or so. Longer on warmer or dry days. Also, heat the property, you need to heat it for as much as you can afford. If you go away keep your heating on but turn your thermostat down. When cooking, open a window and preferably close your kitchen door if you can. When you shower keep the bathroom door closed afterwards and the window open (or fan on) for a good hour or so afterwards. Do not air the steam into the house.

Or
Ignore all that and use a good dehumidifier 24 hours a day.

I do option 1 and don't suffer with mould. Although this house would be prone if measures not in place.

All windows opened first thing in the morning to clear out stale air and help clear condensation. Try and leave open for a min of 20 mins if you can. You can boost the heating when all the windows are closed to heat the house.

When home and sun shining try and open windows as often as you can. We keep windows open all day if possible, we’re lucky we can leave our upstairs windows open when out. Close windows before the sun goes down and boost the heating.

I use disposable dehumidifiers in cold spots that are at risk of damp. Also in all wardrobes, especially ones next to external walls ( end terrace). If you have shelfs stick them behind a photo or something.

We are lucky as we get the sun all day, but in previous houses we have needed to use dehumidifiers. They aren’t cheap to run but do reduce heating cost when the damp is under control. Also once damp is under control we reduced the need to use it.

Wipe key area with white vinegar to help kill off the mould.

If you have bricked up chimney breasts (very common in Victorian/Edwardian properties) I would suggest fitting an air brick into each one that is closed up.

again, if a Victorian/edwardian property do check the floor vents to make sure they are not clogged up with leaves/mud/ detritus.

If it’s cold it’s still worth having windows on the latch but not fully open when you are indoors

Is this a newer or older house? That might change advice a lot

Re ventilation - I just open the windows for ten mins or so to get air flow then shut them again. Often just after I’ve had a shower. So they aren’t open all the time letter heat escape but I give them a blast if that makes sense.

and invested in a decent dehumidifier for drying clothes

A heated clothes drier in addition to dehumidifier, to reduce moisture in the air, venting rooms and airing unused rooms to maintain air flow