Black mold and condensation

It seems the condensation issues in my flat continue with the tenants reporting persistent black mould even with good weather and windows open.
I am about to consult damp specialists and am thinking positive invasive ventilation (PIV) may be the way forward. I wonder does anyone on here have experience of installing PIV and can advise whether it’s been successful and fixed the issue.
Thanks for your help/advice.

Hi Carole,
Contact a company called MOULD GROWTH CONSULTANTS 01372-743 334 they are the best in this field and have been going for over 50 years.
They do a Mould eradication kit which kills off the source of the problem.
Have a good chat to them and they will give you any advice needed.

Thanks Tony I’ll contact them.

There’s usually a reason for the black mould. I was told over & over again it was ventilations when actually there was a major leak in the ventilation pipe from waste stack. Once that was fixed which was major works taking tiles off and wall etc, all was fine and no more problems anymore. It needs to be checked thoroughly. It must have been leaking fir at least 2 years


Windows open: Is that your policy or is that what the tenants actually do. Most tenants on a tight budget keep the windows firmly closed.

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Is it mould or mildew, the past I’ve had mildew in property’s were the tenant is drying clothes in the property on radiators etc. i.e. saving on the electricity bill making the air extremely humid. Have you got extract fans fitted in both the kitchen and bathroom venting out to the outside? Is the growth were there is insufficient insulation for example in the roof where a hot / cold spot can occur causing mildew. Is the growth in a North facing wall. Personally I would eradicate all this before paying out including installing a dehumidifier to see whether excess humidity is the issue. Just my opinion hope it helps.

Thanks Alan
I beleive it is a combination of behaviour and ventilation.
We have damp specialist coming next week.
We are also going to put vents in doors of the fitted wardrobes and trickle vents in the window and considering an air vent ( air brick) in the bedroom. It would seem that a PIV sited in the bedroom would be too noisy at night but we’ll see.
Thank for your response.

Unfortunately you’ll probably find the vents will get closed and the air brick will get covered as has happened to me in the past (even when it was legally required due to an older back boiler) due to the fact they go straight to the outside. Good luck, I hope you get it sorted.

I installed Envirovent PIV units and humidity sensor fans across the portfolio
They should it as a 100% mould free
It’s not
We had severe mould despite the units and fans
The main problem was there incorrect installation of the units and incorrect surveys
They require a lot of maintenance
The PiV units require filter changes regularly ( the company lied and said every 5 years)
The fans were installed incorrectly by the franchisee so they failed every few months in one instance
The franchisee ironically is not working for the company anymore …,
In my experience I would look firstly very carefully at the fabric of the building
Minor faults eg loose silicone around windows, gaps left by builders around windows
Leaking taps etc
The right btu radiator for the size of the room ( calculate it yourself don’t trust what a third party tells you)
We were surprised at how large rads were required to get the room warm enough
Towel rads don’t heat bathrooms properly

Providing hooks so they don’t cover rads with clothes and block heat
Roof insulation
A rated glazing
Cavity wall insulation
Ceiling is well ventilated and not one of many !!
We have a clause in contract telling tenants how to heat the house using thermostat, use of trickle vents on windows
Leaving space around furniture
Where to dry clothes

Smart meters are installed so we can see if the house is heated properly
We provide tumble dryers so they don’t need to dry clothes around the house
It’s not just about opening the windows

Mould is a fabric and lifestyle issue
You have to deal with the structural stuff first
You’ll be surprised when you look closely how much is your responsibility
Don’t depend on ventilation systems
It’s very expensive, requires a lot of maintenance. It’s not all it’s cracked up to be
And don’t use Envirovent
I spent £10k with them and it made no difference
I was always cleaning mould

Be careful of damp experts they are good at selling products and independent surveyors
They are usually not that independent

I found dealing with the fabric of the building a more effective solution and avoids the need
If you have mould in this heat you need to look at drainage etc first


Thank you for your very honest and concise response.
I am concerned that the piv unit if not in the bedroom will not deal with the issue.
I am having a trickle vent put in but even that may not be enough.
It is frustrating that we didnt have problems when my son lived there. My next action is venting to complement the window vent.
Thanks again


Trickle vents are only ok if the tenant leaves them open
We have them at home
The wind howls through them. Which results in one closing them. So compliance will be an issue ( I end up closing them if there is inclement weather as the room gets cold!!)
PIV units are installed either in a loft space and the duct in a communal area ceiling so the air flows into rooms
It’s not installed in the bedroom
[In flats , where there is no loft, it’s a wall unit ( but these are really not that effective )]

Envirovent advises taking 10 mm off the bottom of the doors so if the door is closed air will flow under a closed door ( ideally doors should be left open)
Speak to the local authority as taking 10mm off the bottom of doors affects fire regs ( I think only 4 mm is allowed) so you end up being stuck between a rock and a hard place

The filters in PIV units are expensive and need changing every 1-2 years depending on the location of the property
I forgot to mention earlier, I use Pawandeep from Rentolkil for damp surveys. He has been honest.
We have used him for several reports
We called him before Christmas for what I thought was rising damp. He said the DPC was intact but we needed to point the external wall
When we pulled off the plaster internally, as it was blown, we found the builder had built up the window reveal internally with cardboard which was sopping wet.
We were not sold a product under false pretences.
We ended up doing a lot of structural work. We did pay for the survey but his advice was accurate. The tenant has commented how warm the house, the humidity is less and mould issue gone

If you haven’t had mould before the tenant do check there is not a structural issue.
If I could have a do-over I would have looked at structure
The Envirovent franchisee sell their product, despite structural issues, as they are on commission. They always tell you the property is ok

In one house we had bathroom mould for over 20 years, even after the fans and PIV were installed.
I asked the joiner to replace the window silicone at the end of a tenancy
When it was pulled back there was a one inch gap above the window. After we filled it with expansion foam we haven’t had mould
That was a lot cheaper than £1.2K for a PIV and bathroom and kitchen fan !!!
Speak to the tenant about heating and do an inspection so you can see where clothes are dried
Try and work with the tenant

Good luck

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I haad a top floor flat that became empty so I took the opporunity to insulate it, re plasterboard and skim up. There was no mould there anyway. Ordinary fan in kitchen and bathroom EPC at C… Trickle vents already in . Put in a new boiler .Was there other day and the tenant does not use the central heating its so warm . Only the boiler for the hot water. In fact none of my properties have mould as I have well insulated them( it does help that i buy and sell insulation) The only place that has a bit of mould ,irronically is my stone house.!!

Well we had our damp specialist visit and after examination ( there is nil mould visible) but tenant reporting mould on items in the fitted wardrobe the specialist has advised it is condensation which needs to be managed.He did check all areas for damp using a meter and provided us with written advice post visit.
We have taken his advice which was to

  1. treat the outside walls of the wardrobe
    ( cold spots) with:anti mould paint
  2. provide hygrometers in order that the tenant can manage humidity levels by opening windows and using dehumidifier.
  3. Vent wardrobe doors
    To note:
    He didnt suggest trickle vents to window but we’ve done that.
    He didnt advise a PIV system because of the layout of the flat.
    His advice is that condensation is manageable and has to be managed in this context and this is as much as we as landlords can do, we will monitor the issue as best we can to keep our tenants safe and our property in optimum condition.
    Thanks everyone for you advice.
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Wow that was an honest surveyor! Beware of damp companies!!! They will try to sell you stuff you don’t need & many don’t even know how to to damp works properly. Moving air around is of utmost importance. Laminate flooring over old Victorian flooring not good as the boards allow air to circulate. Good tip about taking off some of door st bottom but if renting to multiple occupiers, that will be less private.

The vents in the doors were in the body of the door we didnt take any off the bottom. We put round chrome vents in the fitted wardrobe doors which look good ( we did this at home in a cupboard door where the boiler was housed when 3xtra ventilation was advised.

Very true about ventilation. I went round 5 of my flats yesterday. They were all very humid, windows all closed and streaming with condensation, no vents open. This creates black mold. You can stop it with bleach but it will not remove the black stain. You will get the mold on the bottom of the windows, on the walls if it is very humid, round the silicon sealant, on the grouting of tiles.
This all very green, they are saving on CO2 emissions by keeping windows shut and minimal heating.
Unfortunately you have to redecorate to make the mold disappear, there is no product that removes the black mold colouration.

The wardrobe gets mildew because it does not get it’s share of heating because of the doors. It is colder relative to the room. The wardrobe is completely chilled out even though the air from the room gets in and out quite a bit.The dampish air from the room does circulate in the wardrobe a bit. The walls of the wardrobe (which are probably plastered exterior walls) are cold and below the dew point of the air. Hence you get damp walls inside the wardrobe. You can get damp other things as most things in the wardrobe will be cold. The other walls in the room do not have this problem anything like as much as they are in contact with warmer air. Those walls have a temperature gradient that results in a warmer temperature of the inner part of the wall compared with the wardrobe. (if you don’t believe it point a contactless thermometer at the wardrobe wall vs. the room wall).
Solution: not really to put more vents in the wardrobe walls, but maybe try lining the plaster walls of the wardrobe with 10mm Celotex - cheap and it should work)
Voila, a degree in Theoretical Physics from the days when they taught thermodynamics can sometimes explain what the damp specialists were never taught at school.

Lining walls with an insulation board in bathrooms ,utility rooms and kitchens is always the key to helping reduce mould along with trickle ventilation, I have renovated property for 50 years and also am a landlord ,so I know it works. The hardest part is stopping the tenant turning off the automatic humidity sensitive fans

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