Condensation and black mold

Hello fellow landlords I am a first time landlord.
My tenants are reporting issues with condensation and I am needing advice re my responsibilities.
Water drips were evident above the window which was felt to be an issue te water leakage into the flat. The management agency have investigated and cant find any cause but have sealed railings on the exterior walkway above and loaned the tenant a dehumidifier.
The issue if ‘damp’ and evidence of black mold continues and the builder has suggested it’s more likely to be condensation.
I appreciate any water entering through an external leak may have compounded the issue and now am confused as to my course of action. I have adviced ventilation ( the uvpc window locks in an open position to allow flow of air which I thought negated the need for trickle vents) however since the builder suggested this my tenant may consider the issue of condensation to be my problem.
Please advise if any of you have experience of this and have useful tips.
To note my son lived in the flat for 5 years before I let it out without issue.
TIA Carole

Put in trickle vents and a fan that reacts automatically to humidity Saying leave the window open is not enough. Read all the posts on this site. .Previously covered in detail nsulate outside wall. Do they dry clothes indoors?

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Thanks we have extractor fans in bathroom and kitchen but are considering further fans inbuilt the issue is where to place it. There is a laundry facility with washer and driers that we pay for that is 6 steps away from their front door and they assure us they use that rather than drying washing indoors.
I will look again at trickle vents.
Thanks again

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I recommend buying & installing a Humidity sensor on wifi then you can monitor to confirm exactly what the issue is, i have loads of them, battery & wifi is the only thing needed & £70
Also check theres a extractor humidity fan installed etc

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Hi Carole2
I had a lovely young couple complain of a similar state in one of my properties, it turned out the culprit was them drying clothes on just about every radiator with the heating on full blast. Compound this with modern central heating systems/Double glazing and an old house that does have air bricks but ultimately not enough air flow through the house and sure enough condensation led to some mould. I bought them a condensing dryer, removed/treated the mould and no more problems.


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Thanks Dean
My tenants are lovely too so I am keen to sort this for them.

I am sure ventilation and airflo through the flat is the issue so need to sort it out.


Buy a internet sensor & be sure first

Hi Andrew
How does that work?

It works like a dream to be honest, you buy the item for about 60 or £70 put in a couple of batteries hook it up to the Wi-Fi download the app and it will notify you when the levels of humidity or temperature or whatever father think your sensing goes beyond the realms of which you programmed it to do so and it will give you an email if you wish so you have tangible evidence in case your tenants are doing something silly like keeping the bathroom door shot and keep in the bathroom window shut your humidity alarm will go off at about85% or whatever you set it to and it will alert you when the humidity levels have come down to an acceptable level and without these devices I would have some real big issues.
I just go to Amazon to make my purchases but first go to the manufacturers website and examine all of their products to calculate which product it is that you want I myself just buy the battery operated humidity sensor on Wi-Fi I think some of them can’t even accept a Sim card for 4G or 3G for instance.
Do you let me know if you need any more help and how are you get on


an eternal problem with flats!

Freeholders turn up days after a leak is visible -always video (most phones offer this) show it to FH rep
Who will attempt to pass it off as condensation -be legal

Where is clearly condensation;

  1. prepare your introductory paperwork to indicate importance of venting -especially during bathing or cooking

list things to avoid
3) need for prompt action (fung icide spray) when early signs

Particularly obvious with foreign tenants from hot countries during a uk winter…or… a tenant late with rent or looking to terminate early!!

Dr David Noble

Tenants always have problems with damp, mould and condensation - it doesn’t matter what you advise regarding ventilation etc they will always accuse you of renting a damp property.

My flat is rented periodically - it’s absolutely fine when I am living there but a damp infested mould fest within a week or so of a tenant moving in.

My advice is to come down hard and quick with a precise list of dos and donts explaining about damp and condensation in words of one syllable.

just buy a humidity sensor

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Suggest you educate yourself (and then your tenants) on the causes of damp because the remedies vary.

If you have penetrating damp, where water is coming from the outside then you are responsible for fixing it, perhaps via a freeholder if there is one. Things to look out for are blocked or leaking gutters, badly fitting window frames, cracks in render that allow water to penetrate but not evaporate.

If it is caused by condensation, as it generally is, then it is fir your tenant to remedy, perhaps with some changes by you to ventilation and heating so it is reasonably practical to avoid it. It is basic physics…the air can hold a certain amount of moisture depending on the temperature, excess water vapour will condense on cold surfaces making them damp, and after a while mold will start to grow on those surfaces. So properties need to be kept warm and ventilated. You need to provide the means to do that and tenants need to use them appropriately.

So drying clothes inside and keeping the windows closed will lead to condensation. Having showers and not getting rid of the moisture will cause condensation. Not heating the flat during the autumn danger time will lead to condensation on external walls as they get colder with falling external temperatures.

Solutions include ventilation as mentioned above. Also simple things like opening the windows on breezy days to change the air. Also arranging furniture to keep external walls clear and to allow airflow across them where practical. Ut is airflow that helps moisture evaporate, so if rooms are stuffed with belongings they stand better chance of getting damp.

Incidentally, condensation can seep to the bottom corners of rooms and lead to misdiagnosis of the non existent rising damp…withs lots of useless but disruptive treatments. Google it to understand. So airflow is critical.

I’ve had the same flat have periods of mouldiness and long periods when it gas been clear. Same flat, same heating, different tenants with different lifestyles. I start with a conversation about condensation when they move in, keep an eye on things during the first year and try to work with them to reach a happy balance. On some occasions ive prodided dehumidifiers.

Hope this helps and good luck.



No need to be a damp expert all you need to do is hire an expensive specialist and damp to come and quote you on any damp issues and they will do a full report and give you a free quote with no times I think I last used a big company called Kenwood they charge A lot but it doesn’t matter if you don’t intend pay them.

You may also want to consider Envirovent. In one of my older stone built properties, there was only a slight problem with only one person living there, but once a family moved in condensation became a problem with black mould forming on regular basis. We had various work done on the advice of ‘specialists’ but the problem continued. After having the EnviroVent system installed the problem was completely solved. This was five years ago, even with 2 adults and 4 children living there, it’s not been an issue. It wasn’t cheap but less decorating costs and tenants have been longer term

Thanks Alison
I am thinking along the lines of venting to aid ventilation.I am going to pursue all options the first being a damp specialist for advice, a humidity sensor and venting.

I dont relish the thought of continuing condensation issues if I am to carry on letting the property.

This forum and the people who have responded is such a good support. Thank you everyone.



Hello Carole,

I too have just used Envirovent to have an air system put into my flat. The bedroom affected has 3 exterior walls and having just replaced the windows and door in that room it was very snug but ideal conditions for the growth of mould. It was even on the back of furniture so very unpleasant. The system has only been in for 2 weeks and no reports of any problems at the moment. I do have to redecorate now because the mould has stained the walls but once that is done I’m hoping it will be an end to it. Envirovent also offer a bit of a discount to NLA members so worth mentioning if you use them!

Hi carol2

We now this last week have exactly the same issue.
Have rented the house for the past 9 years (3 previous tenants) without issues but latest tenants (a young couple who have left their parents to set up a new home together) are now experiencing condensation and mould problems.

Following investigations, cleaning the black mould and discussions with them we have forwarded a copy of the information below on to them via e-mail (proof that we are trying to sort things out) :-

Condensation and Mould problems during Winter Weather.

“ Condensation and Mould problems are caused by excess water vapour (Humidity) in the air which then condenses back to water on cold surfaces making them damp and after a while black mould will start to grow on these surfaces”

Do :-

  • Open windows on breezy days to help change the air inside the house.
  • Keep the Bathroom door closed at all times; and after Showering/Bathing, as above, if the weather allows, leave the window slightly open for at least an hour.
  • Use our supplied Window Vac to remove condensation on windows and window cills on a regular basis.
  • Close the Lounge to Kitchen door and use the fan when cooking.

Dont :-

  • Dry or air clothes in the house.
  • Allow any room of the House (at any time of the day or night during winter) to drop below 16/17deg C (60/63 deg Fahrenheit)
  • Allow black mould to build up; use our supplied “DETTOL Mould and Mildew Remover” to clear it.

We on our part will also check the external fabric of the house just in case there is a problem with the rendering; window reveals or guttering.

We have also purchased/received and will fit a special room Thermometer/Digital Indoor Hygrometer (£15) which will monitor and record the Humidity and Temperature in their bedroom.

If this problem persists we intend to fit a Humidistat fan in the bathroom. This switches itself on and off automatically depending on the % Humidity of the atmosphere in the room.

Hope this helps.

Thanks Jane
This is really useful


Please can you recommend a wifi sensor? Thanks