So you’ve upgraded your insulation, draught-proofed your house and installed a shiny new renewable heating system, but now you’re finding black mildew growing in some of your rooms – WTF is going on, and how do you fix it?
Before you go any further, get a damp survey done by a local specialist. It’s important to rule out penetration or rising damp. A specialist will use a moisture meter to assess your property and advise on how to fix any underlying issues. If you’ve already done this and they’ve confirmed you have a condensation problem, read on…
The key to fixing condensation-related mould and mildew problems in your home boils down to three things:
- Heat
- Humidity Control
- Ventilation
Heat
It doesn’t matter how you heat your home – whether it’s with a heat pump, gas boiler or even buckets of burning coal (please don’t) – if you want to prevent mould and mildew, you need to ensure that all living spaces stay above 14°C. This is the critical temperature below which condensation begins to form on surfaces, leading to mould growth.
Many modern heat pump systems are designed to keep your entire house at a steady temperature, around 21°C, so if that’s the case, ‘heat’ may not be your problem. However, if you’re someone who likes to micro-manage their heating – for example, keeping only your study warm during the day while leaving other rooms cooler – then you might have an issue.
Heat pumps are efficient when set up properly and left to run continuously. They’re not designed to be switched off and on, allowing your house to cool down before being reheated. This can not only reduce efficiency but also increase the risk of condensation as rooms drop below 14°C.
To avoid this, consider using smart heating controls, such as the Tado V3+ Smart Thermostat and Smart TRVs. These devices allow you to manage the temperature in each room while monitoring humidity levels to identify potential problem areas.
If certain rooms in your home struggle to maintain 14°C during particularly cold weather, it could be a sign that your heating system needs attention. A power flush might be required, or your radiators may need to be upgraded to larger units. Remember, heat pumps operate at lower temperatures than traditional boilers, so larger radiators are needed to achieve the same comfort levels.
Humidity Control
Even if your home is warm, you won’t stop mould and mildew if you’re introducing excess moisture into the air without a strategy for controlling it. Kitchens, bathrooms, and shower rooms are the main culprits when it comes to generating steam, so it’s vital that you fit extractor fans in these areas.
Ensure you have an extractor hood over your hob, and always switch it on when cooking or even when boiling the kettle. Leave it running for a few minutes after you’re done to clear all the excess humidity.
In the bathroom, basic extractor fans are better than nothing, but ideally, you want humidistat-controlled extractor fans that switch on automatically when they detect high humidity levels and run until the air is sufficiently dry.
Drying clothes indoors can also contribute to humidity. If you hang washing over radiators, you’re adding moisture to the air and extending the drying time. Instead, designate a room for drying laundry, use a clothes rack, and run a dehumidifier in that room. A dehumidifier with a capacity of 12 to 20 litres is usually sufficient for an average-sized room, and it’s much more eco-friendly than a tumble dryer.
Ventilation
If you’ve got your home warm and are controlling humidity but are still seeing mould, the issue may be ventilation. Airflow is essential for preventing condensation, so make sure trickle vents are open in each main area of the house. This allows for continuous air changes without causing unnecessary heat loss.
If all else fails, you may need to install a Positive Input Ventilation (PIV) system. A PIV system works by introducing fresh, filtered air into the home while extracting stale, humid air, significantly reducing condensation and mould. Companies like Envirovent offer free, no-obligation surveys for homeowners interested in this solution.
However, increased air circulation can lead to higher heat losses, so you may need to reassess your home’s heat loss calculations and possibly upgrade radiators or your heat pump to compensate for the increased demand.
Summary
- Ensure your heating system keeps all rooms at a minimum of 14°C, and make sure it’s properly maintained.
- Fit extractor fans in bathrooms and an extractor hood in the kitchen to control humidity.
- Use a dehumidifier in a designated room for drying laundry.
- Keep trickle vents open in main areas to allow airflow.
- Consider a PIV system if mould persists, but be mindful of potential heat losses and the need for system upgrades.
Written by David Stanford from Genous.
Very useful, @David Stanford. Whilst this should normally not be an issue for new builds or retro fits for relatively new houses, there have been several discussions on the forums recently which are centred around heat pumps and insulation for older houses, so this is a very timely article.
What would be particularly useful (follow-up article?) would be tackling the subject of how to insulate older houses – especially listed or period buildings – whilst still ticking the boxes you’ve outlined in your advice.
@Majordennisbloodnok I cheekily started a topic about Listed Buildings and insulation of same – perhaps readers who are living ing Grade 2’s would care to pick up on it?
Exactly one of the topics I was referring to, in fact. An industry professional’s take on how to balance the insulation and reduction of draughts in an older property against the prevention of condensation, damp and mould is an incredibly useful resource given the high proportion of old properties in the UK’s housing stock.
@Majordennisbloodnok @Mars, do you have any contacts? Toodles.
Indeed, and it is a complex area. My primary observation (I have an old leaky grade II listed building) is that installing a heat pump appears to have reduced condensation significantly, especially on windows at this time of the year. One complexity is that some of my windows now have secondary glazing, and that also helps to reduce condensation (and reduces draughts, another variable changed…), but even the windows that haven’t yet got secondary glazing have less condensation, presumably due to the heat pump, or more accurately, the heat pump is on most or all of the time. Another welcome effect is the house feels more comfortable, less damp.
After that, it get complicated. Old buildings like mine are supposed to ‘breathe’, which I have previously mentioned is a misnomer, because the ability of an old building to ‘breathe’ is not about how air gets in and out (that’s permeability) but rather about how the construction materials absorb (rather than condense, thereby reducing condensation) and release moisture. But old buildings get worked on over the years, not always sympathetically. My ground floor has damp proof rendering part of the way up the walls, on one level a pretty dumb idea if you want the wall to ‘breathe’. But in practice it doesn’t appear to have caused major problems. The walls don’t feel damp, standard emulsion paint stays put, and wall mould is not a problem.
I suspect the building still has multiple in/out air leaks, some of which I know about eg windows yet to be secondary glazed and ye olde doors which don’t fit well, and I suspect (again, no evidence) they reduce condensation by removing humid air. They also mean more air changes per hour, which increases my heat loss, and so heating costs. And therein lies the problem: if we didn’t have to worry about heating costs, then we would simply heat the house and ensure it is well ventilated. But if we do worry about heating costs, as we do, then the two objectives (heat for comfort, ventilate for comfort) are at odds. I for one don’t know the answer.
The answer if you’re doing a major renovation is a MVHR, mechanical ventilation with heat recovery. No need for trickle vents, cooker extracts can be re-circulated versions and hence no heat is lost through ventilation.
But to be effective the pipes through buildings via loft space or between floors need to be a decent cross-sectional area of 4 or 5inch in diameter. Always better to fit these types of infrastructure when doing other work like re-wiring or re-plumbing.
We routinely dry clothes in the house and have never had any condensation issues not even in the cooler corners. We’ve distributed small temperature and humidity sensors just to check too.
‘Curates Egg’ this one @David Stanford. Good in parts, and a good discussion topic. Thank you.
14degC dew point approximates to 20degC DB at 50% RH (14degC WB). Keeping rooms above the dew point (which varies dependent on absolute moisture content in the air, not always 14degC) is generally effective at combating condensation or mould. But for older solid wall properties, or properties with damp in the fabric causing thermal bridge to outside, the wall surface temperature can be several degC lower than the surrounding room air temperature. That can lead to condensation and mould even if the room is heated and reasonably well ventilated. All it takes is for the area where the cold patch is on the wall to have little or no air movement past it, and a boundary layer of cooler damp air will form around it, often reaching dew point temperature on the wall surface. A wardrobe on an external wall in an older house can easily and unwittingly create damp and mould behind it.
Radiators and heat emitters create convection currents in the room, and it the movement of air caused by convection coupled with mechanical ventilation or passive air change [infiltration & exfiltration] that is effective at avoiding condensation and mould. mechanical ventilation, such as local extract in wet rooms and over cooking appliances, MVHR and dehumidifiers are all effective at preventing build up of excess moisture content in the air.
I would not recommend positive input ventilation (PIV) without heat recovery under any circumstances to any property, PIV is a product developed by global companies [like Vent Axia, et al] to sell product, using very inefficient corrugated flexible ducting that isn’t fit for purpose. Introducing cold air with direct electric heating to temper the supply air temperature isn’t energy efficient. MVHR = ‘yes’, but PIV = ‘no’. A dehumidifier or two, for less than the cost of PIV, would be far more effective at controlling indoor air humidity levels.
I agree with @Judith – MVHR is the way forward.
A slow trickle of pre-warmed air is great at discouraging mould and condensation.
I have self-installed MVHR in my renovation property, which is a Devon farmhouse (1937).
I have photos showing the various types of (100mm) pipework, and how I threaded this through the ceiling voids.
I’ve also installed a 2-room mini-MVHR in a new-build garden-pod.
Again, I have photos showing how I placed the heat-exchanger unit in a space above a door-frame.
If there are renovators who need this info, I’m happy to respond to posts here, and I can suggest suppliers for the required parts.
You need to ensure your house is pretty airtight ideally 3ACH or better at 50Pa. Otherwise you are just adding additional ventilation to and from the property. For little or no no energy gains as you would expect. MEV and dMEV with self regulation trickle vents and ventilation on demand (humidity activated) is generally better for a lot of homes. No need to replace filters every six months, only extract fans need to run, only room requiring ventilation are vented. You need the same door undercuts as MVHR to give adequate cross ventilation. Several studies have shown the energy input to the house is very similar with demand MEV and MVHR.
Ventilation strategy is a must for all houses, overhauled or otherwise.
@Johnmo – you’re providing excellent insights into the options and requirements…
… if only I understood all those Three Letter Abbreviations (TLAs).
Could you consider giving us the benefit of the expanded terminology when it’s first used in a topic?
I’ve met terms like these before, and can appreciate that 3ACH refers to Air Changes per Hour in this context,
but it’s also:
😉
MEV – Mechanical Extract Ventilation. A central extract unit, that takes damp air out of wet room (bathrooms etc). Demand flow rate, varies flow based on humidity with extract terminals closing and opening as required. The central extract unit fan speed chances in response to demand requirements.
dMEV – decentralised Mechanical Extract Ventilation. Non central extract with small individual fans in each wet room. Run at a very low rate 24/7, should be silent in operation, with automatic boost based humidity levels.
MVHR – Mechanical Ventilation with Heat Recovery. Mentioned above by others
Pa – Pascal. A pressure, in this case used to define the depressurisation of the house when tested for air tightness. Air testing requires a fan in the doorway and air is sucked out the house until the house is depressurised to -50Pa. The flow through the fan is measured to identify the extract leakage rate of the house on the equivalent of a very windy day.