If you remember only one thing from this article, it should be this:
“If your heating is on, keep your bathroom window closed”
Why? Well, this article is going to explain this simple rule and why it is so important to remember by explaining the relationships between heating, insulation and ventilation, and why these three factors need to be addressed together to prevent damp and mould issues, especially in bathrooms.
Damp and mould problems often stem from a combination of three factors:
- Lack of proper ventilation
- Inadequate insulation
- Insufficient or inconsistent heating
Think of them as the legs of a stool - remove one, and the whole structure becomes unstable. By addressing all three, not only can you prevent damp issues and mould growth from developing, but you can also help to stop issues returning to rooms that are already prone to damp and mould. Let’s break them down.
1. Ventilation: It’s Not About Opening Windows
Many people believe that cracking open a window in the bathroom during or after a shower is enough to remove steam and reduce moisture. Unfortunately, it’s not that simple, especially in colder months.
Why? Because effective ventilation isn’t just about letting moist air out, it’s also about how new air comes in - and where it's coming from! Opening a window will let out steam, but it also allows outside air to enter, and there is no control over the flow of air in either direction. However, once mechanical ventilation is installed, the open window becomes a one-way street inwards only, which brings a new set of problems.
Mechanical ventilation systems, like extractor fans, work by creating negative pressure: they pull air out, but that air has to be replaced. Opening the bathroom window to allow for fresh air to come back in has its own pitfalls. As air is removed from the bathroom, a supply of new air to fill the void will come from a source of least resistance; in this case, an open window becomes the primary source of return air. In colder months, this means the fan is not only exhausting the humid air to the outside, but it is also drawing in cold air back through the open window. This cold air then cools the internal humid air, causing the moisture content to condense faster, making the airborne humidity heavier and slower to move, while also cooling surfaces. This, in turn, moves the surfaces closer to the dew point, condensing the humidity as it comes into contact with the surfaces. All of this means more moisture is retained in the room and making the fan's job that much harder to efficiently remove the moisture.
Essentially, we want to stop cold air from entering the bathroom and maintain higher temperatures of both the air and surfaces. Therefore, the ideal source for the return air is from the rest of the house, allowing air to be drawn in passed the closed bathroom door, either under the door or through an internal passive door grille to improve flow. This is usually a simple fix for older builds, which “breathe” through the fabric of the building, but return air can be problematic in newer homes, which we will look at in the next section on insulation.
Bottom line: Opening the bathroom window doesn’t solve the issue of removing moisture and can cause more problematic issues than it fixes. You also need to address the operational balance of your mechanical ventilation between removing moisture and a suitable source of return air, while retaining heat and keeping cold air out.
2. Insulation: Seal the Warmth, Not the Moisture
Modern homes are sealed tight to improve energy efficiency; double-glazing, UPVC doors, cavity wall insulation, and loft insulation all help keep heating bills down. But here’s the catch - insulation reduces passive airflow, which used to help carry moisture out of the house as well as unwanted heat loss and also supply the house with fresh air, albeit in line with the outside air temperature.
Insulating a home without addressing ventilation traps heat and also moist air inside, especially in rooms like bathrooms and kitchens. That’s why ventilation and insulation must work hand-in-hand. A well-insulated home stays warm and energy-efficient and mould-free only if there's a proper route for moist air to escape and a sensible return for fresh air into the home.
So, don’t skimp on insulation - but don’t forget to pair it with mechanical ventilation that can work in a sealed environment:
You can install MVHR systems during build or renovation to both extract air from the home while also supplying filtered fresh air that is warmed by harvesting the warmth of the outgoing stale air. This allows for a balanced circulation of air throughout the home while also retaining the insulated properties of the house and minimising heat loss.
Single-room dMVHR units do the same job as MVHR but are easily retrofitted to existing homes. These can be very useful to help control individual rooms with damp and mould issues by preventing moisture build-up whilst sorting out the return of fresh filtered air and maintaining minimal heat loss
Standard extract-only systems such as MEV, dMEV and intermittent fans will still need a return supply of air into a bathroom when running. In these situations, we suggest allowing passive trickle vents in UPVC window frames to be opened elsewhere in the house - not in the bathroom - to allow this air to be warmed as it is drawn through the home before reaching the bathroom, rather than being drawn directly into the bathroom causing the same issues with cold air we talked about above!
3. Heating: The Dew Point Danger
If you’ve ever seen water droplets form on a cold can of drink on a warm day, you’ve witnessed condensation. This happens because the cold surface of the can cools the air as it touches it, and the moisture in that air condenses back to liquid form and settles on the surface. This is known as the dew point and explains the early morning dew, where the temperatures overnight have dropped sufficiently to allow the moisture vapour in the air to condense and drop onto the ground. The same thing happens in your home when warm, humid air meets cold surfaces, like bathroom tiles, windows, or uninsulated external walls.
This is why consistent background heating is key. Maintaining a steady indoor temperature (ideally 17–19°C), especially in bathrooms, helps keep surfaces warm enough to reduce condensation. Even with great ventilation, if your walls are cold, moisture will still condense, and damp and mould will follow - and the colder the surface, the faster the condensation will form. You may always have some condensation forming - it's almost impossible to avoid it completely, but the warmer the surfaces, the slower the build-up will collect, meaning less retained surface moisture and more moisture held in the air, which can be dealt with more easily via extraction.
Yes, energy prices are high. But a well-insulated home can retain heat more efficiently, reducing the cost of maintaining that consistent temperature. It’s a long-term win that saves on both utility bills and mould-related repairs.
Why You Need All Three: The Ecosystem Analogy
Think of your home as a mini-ecosystem. Just like nature, balance is everything. Here’s what happens when the triangle breaks down:
- Ventilation and heating without insulation: Airflow increases, but so does heat loss and energy use, causing faster condensation and higher bills.
- Insulation and heating without ventilation: Heat stays in - but so does the moisture, creating the perfect environment for mould to thrive in.
- Ventilation and insulation without heating: Cold surfaces cause condensation to speed up, and damp can take hold, causing structural damage and the perfect breeding ground for mould
Only by combining proper ventilation, adequate insulation, and stable heating can you create a space that resists damp and mould effectively and supports healthy living.
Bonus Tip: If You’re a Landlord or Tenant...
Understanding these principles is just the start. If you rent or manage properties, educating tenants or discussing improvements with landlords can go a long way. The Housing Ombudsman Service has excellent resources to help landlords and tenants navigate damp and mould responsibilities.
Final Thoughts: A Healthier Home Starts with Balance
There’s no magic product, but there is a smart strategy. By treating your home as a closed environment that needs balance, like a greenhouse or a planet, you can create conditions that work with nature, not against it.
The Holy Trinity Strategy for Controlling Damp and Mould:
Let the air flow (but control it)
Keep the heat in (but don’t trap moisture)
Maintain warmth (avoid the dew point)
Follow these three principles, and you’ll stop damp and mould in their tracks - and as mentioned before, remember this - if nothing else:
“If your heating is on, keep your bathroom window closed”