Why Ventilation goes hand in hand with Insulation
UKCMB, Moisture guidance for existing homeowners
Insulation Makes Your Home Warmer - and Tighter
Before insulation, homes were naturally leaky. Cold walls, draughts, and gaps allowed warm, moist air to escape. That was inefficient - but it also meant humidity was less likely to build up.
Once a home is insulated and made more air tight:
Warm air stays inside, saving you energy.
Surfaces stay warmer, reducing some condensation risks.
Air leakage is reduced, which means moisture no longer escapes on its own.
This is good news for comfort, but without proper ventilation, everyday activities like cooking, showering, and even breathing can quickly raise indoor humidity.
Why Relative Humidity (RH) Matters
Relative humidity is simply the amount of moisture in the air. Keeping this between a range of 40-65% is recommended as a healthy zone for living. Too dry can cause issues as will too much moisture. Typically in the UK we are more at risk with homes with too much moisture because of our cold and wet climate. This can lead to mould causing real health issues as well as damage our buildings themselves.
How to Use the Hydrothermal (Dew-Point) Chart
This chart helps you understand when condensation will occur. Think of it as a guide to how warm the air is versus how much moisture it holds.
Look along the bottom for your room temperature.
Move up until you reach your indoor humidity line.
Move left to find the dew-point temperature.
If any surface in your home drops to that dew-point temperature, condensation will form.
Practical Steps for Homeowners if RH is too high.
A. Consider upgrading the ventilation in your home
Adding a sophisticated bathroom/kitchen fan (dMev) that will run at a low speed, slowing moving air through the house then boost when it senses moisture. These can be installed in kitchens and bathrooms. Alternatively MVHR systems provide really high levels of ventilation and capture the heatloss associated with a heat exchanger. See link to learn more.
B. Open windows strategically
Ventilating your house can be more important after a retrofit as air might not naturally enter as it did in the past. Many windows can be left on a trickle setting or trickle vents installed in windows. Opening windows to purge ventilate a room in one go can also be very affective. If you leave open your windows for 10 minutes this will allow the air to move without hugely impacting the temperature of the house as the building itself doesn't have time to cool in that time.
C. Turn up the heating
One easy way to reduce the RH is to increase the temperature. For example, if your home is at 80% RH at 15°C, simply warming the room to around 18–19°C can reduce the RH to approximately 60–65%, even without removing any moisture.
