Controlling damp and mould - to vent or not to vent?

[GSte]

We live in a ground floor North facing flat and have a problem with mold forming on cold external walls and the nearby areas of ceiling. My understanding is that the way to beat mold is to keep moisture out of the air by either ventilating rooms regularly or using dehumidifiers, and to try to keep rooms warm.

We've been trying to do this but it hasn't solved the problem yet, with mold still growing in cold spots. The flat has six ventilation bricks which were part of the original build on external walls with corresponding interior vents. We've purchased two dehumidifiers which we use daily to keep the air dry, so I'm wondering if it would be okay to get the vents bricked up.... They seem to add no value but create extremely cold areas around and above them which is where the mold now grows.

Anyone have any experience of these or have any thoughts?

 

[noseall]

Anyone have any experience of these or have any thoughts?

Yes. Your walls need insulating from the inside.

We had the same trouble with a ground floor flat, yet none of those in subsequent floors above (three floors in total) had the same issues as the GF flats. It must be symptomatic of the design. The flats I lived in were probably late 1970's.

 

[GSte]

Thanks for the reply. Do you mean the cavities need insulating, or we should paint them with insulating paint, or something else?

 

[noseall]

Thanks for the reply. Do you mean the cavities need insulating, or we should paint them with insulating paint, or something else?

I would say that yours need Celotex type insulation fixing to the internal face of the exposed walls.

 

[GSte]

Have just had a read about internal wall insulation - that's not really an option unfortunately, it would be a huge and costly job, though I'm sure it would solve the problem.

 

[noseall]

Have just had a read about internal wall insulation - that's not really an option unfortunately, it would be a huge and costly job, though I'm sure it would solve the problem.

It is disruptive - but so is the mould. You need to weigh up your options regards whether it will be a selling point issue in the future etc as well as the here and now and how it affects your living standards.

 

[endecotp]

My understanding is that the way to beat mold is to keep moisture out of the air by either ventilating rooms regularly or using dehumidifiers, and to try to keep rooms warm.

The first thing is to reduce the amount of moisture going into the air by fitting and using good extractors in the bathroom and kitchen.

I'm wondering if it would be okay to get the vents bricked up....

No.

 

[Scarlet Pimpernel]

If you are using dehumidifiers, I would block up the air vents. (Mold spores will enter via these air vents, along with cold and moist air)
It's pointless removing moisture from the air, only to have it replaced with cold, moist air form outside.

If your house has had air vents fitted from the outset, I suspect that it is a very leaky (air wise) house anyway.

 

[^woody^]

The mould triangle is heating, ventilation and insulation. Miss one out and you'll be going around in circles.

With north facing buildings, the walls are invariably colder, so moisture is more prone to settle on them - and it will do this at higher temperatures than the same flat facing south.

The problem with relying on heating and ventilation is that it's not only a fine balance, but also very impractical and hard to do.

Insulation would be beneficial. But have you also got extract fans in the kitchen and bathroom?

Also, if controlling condensation and mould is a problem, rather than mess about with dehumidifiers and the other rigmarole, consider having a positive pressure system fitted. This is simply a single quiet fan that moves air around the home and prevents mould forming. These are very effective for difficult flats.

 

[GSte]

The first thing is to reduce the amount of moisture going into the air by fitting and using good extractors in the bathroom and kitchen.

Thankfully the kitchen is unaffected as we have a vented extractor hood and always have the window open when cooking or boiling the kettle.

The bathroom's been another story, it's a small room but the extractor is at the other side of the room from the bath/shower combo and we had a big problem with black mold on the ceiling above them. I've fitted plastic cladding above the bath and we always shower with the window open and leave it open for a good while afterwards which has largely resolved the problem. We're also getting a smart TRV on the rad in there to keep the room warmer once the lockdown has eased a little which should also help. The extractor wasn't working when we moved in so I replaced it like-for-like with a 4" Manrose low-voltage unit - does that sound adequate or do you have recommendations for anything better?

No.

Care to elaborate?

 

[GSte]

If you are using dehumidifiers, I would block up the air vents. (Mold spores will enter via these air vents, along with cold and moist air)
It's pointless removing moisture from the air, only to have it replaced with cold, moist air form outside.

If your house has had air vents fitted from the outset, I suspect that it is a very leaky (air wise) house anyway.

The flat used to haemorrhage heat as it had poorly fitting large single glazed windows and back door when we moved in, which we've had replaced.

That was my thinking re the vents, they seem to be adding nothing and we'd still have the option of venting the room by putting the new windows on the notches/security hooks morning and evening. I might block the ducts temporarily with some chunks of 100mm Kingspan and see what effect it has.

 

[GSte]

The mould triangle is heating, ventilation and insulation. Miss one out and you'll be going around in circles.

With north facing buildings, the walls are invariably colder, so moisture is more prone to settle on them - and it will do this at higher temperatures than the same flat facing south.

The problem with relying on heating and ventilation is that it's not only a fine balance, but also very impractical and hard to do.

Insulation would be beneficial. But have you also got extract fans in the kitchen and bathroom?

Also, if controlling condensation and mould is a problem, rather than mess about with dehumidifiers and the other rigmarole, consider having a positive pressure system fitted. This is simply a single quiet fan that moves air around the home and prevents mould forming. These are very effective for difficult flats.

Thanks for the recommendation, I'd heard of those systems before but had forgotten about them.

Room temperature has improved due to the double glazing but the two rooms that are mainly affected - the lounge and bedroom - have heating problems. In the lounge we've blocked the radiator with a huge sofa which can't help and need to rearrange the furniture. The bedroom is very cold due to a single radiator that's too small, we're getting this replaced with a double with a smart TRV to keep the room at a higher temp and hoping this will improve things. We're also repainting and I plan to add both anti-mould and insulating additives to the paint. Where we've painted one wall with this combo it's been unaffected for the past year which is positive...

 

[endecotp]

It's pointless removing moisture from the air, only to have it replaced with cold, moist air form outside.

It’s important to understand that COLD air from OUTSIDE is NOT MOIST once it is INSIDE.

The amount of moisture that air can carry depends on its temperature; warmer air can carry much more water than colder air. Here’s an example. Say it is 15 C inside , and 5 C outside and raining, so 100% humid. At 5 C, each cubic metre of air can carry about 7 g of water, but at 15 C it can carry about 13 g. When that air comes inside and warms up, it is only about 50% humid. If it’s not raining outside and you open a window, the fresh air will be even drier when it’s inside. This is why ventilation plus heating reduces humidity.

 

[foxhole]

You get mould with poor ventilation if the spores cannot settle it cannot grow.

 

[Scarlet Pimpernel]

It’s important to understand that COLD air from OUTSIDE is NOT MOIST once it is INSIDE.

The amount of moisture that air can carry depends on its temperature; warmer air can carry much more water than colder air. Here’s an example. Say it is 15 C inside , and 5 C outside and raining, so 100% humid. At 5 C, each cubic metre of air can carry about 7 g of water, but at 15 C it can carry about 13 g. When that air comes inside and warms up, it is only about 50% humid. If it’s not raining outside and you open a window, the fresh air will be even drier when it’s inside. This is why ventilation plus heating reduces humidity.

Most of what you say is perfectly correct. But it is important to remember, that the cold air entering a building will retain exactly the same amount of water vapour when it is warmed. Yes, I do appreciate that warmer air can support more water vapour than colder air. But the air does not magically lose the water vapour it is supporting. When that warmer air meets cold surfaces, it loses its heat, and when it loses sufficient heat to approach the temperature that the water vapour condenses, it has reached the dew point. Hence water condenses on the cold surfaces. Hence, you will (almost undoubtedly) get mold forming.
The more water vapour that the air supports, the higher the temperature that is the dew point.

To remove water vapour, you have to let in cold air (and its water vapour), heat it, and let out that warmer air and (hopefully) even more water vapour contained in that air. Only that way can you reduce the RH inside the building. But it is not an energy efficient process. You are warming air to dump outside continuously. Whereas, dehumidifiers dump the water into a container, or down the drain, and reuse the warm air.

We could go into more detail about letting in the coldest air that you possibly can, so that the air contains the least amount of water vapour possible, heating it to a more than normally comfortable temperature, then dumping it outside (hopefully ) along with even much more water vapour. But I rather think we are going way beyond the remit of advising someone on a DIY issue.