Condensation and mould

When you insulate and draught-proof your home, you are making it more airtight. While this is great for keeping heat in, it also means that moisture generated inside the home (from cooking, showering, and breathing) can become trapped if the ventilation is not properly managed.

If moisture-laden air hits a cold surface, it turns into liquid water (condensation), which can lead to the growth of black mould.

The “Insulate, Ventilate, Heat” rule

To prevent damp issues after a retrofit, you must follow these three principles together:

  1. Insulate: Create a continuous thermal barrier to remove cold spots.
  2. Ventilate: Provide a controlled way for moist air to escape and fresh air to enter.
  3. Heat: Maintain a consistent, low level of heat to keep the internal surfaces of your walls above the “dew point.”

Common risk areas

1. Thermal bridging

A thermal bridge is a gap in your insulation (e.g., where a wall meets a floor or a window frame). These remain cold, even when the rest of the wall is warm, making them magnets for condensation.

2. Behind furniture

Placing large wardrobes or sofas against an external insulated wall can block airflow. If the wall behind the furniture stays cold, mould can grow unseen.

3. Lofts

Topping up loft insulation makes the roof space above it much colder. If moist air leaks through the loft hatch or around pipes, it can condense on the underside of the roof tiles, causing rot.

How to manage ventilation

You should never block intentional ventilation. Ensure the following are clear and functional:

  • Trickle vents: The small vents at the top of your window frames. These should be left open most of the time.
  • Extractor fans: Use them every time you cook or shower, and ensure they run for at least 15 minutes after you’ve finished.
  • Air bricks: These provide essential airflow under suspended floors.
  • PIV systems: Positive Input Ventilation (PIV) is a common retrofit solution that gently pushes filtered air into the home to prevent stagnation.

Practical tips for residents

  • Dry clothes outside: Or use a tumble dryer that vents to the outside. Drying clothes on radiators is one of the biggest causes of indoor moisture.
  • Keep lids on pans: This reduces the amount of steam released into the kitchen.
  • Wipe down surfaces: If you see condensation on windows in the morning, wipe it off with a cloth to prevent it from soaking into the frames or walls.
  • Maintain a steady heat: Using a smart thermostat to keep a minimum background temperature (e.g. 15°C) even when you’re out can help prevent surfaces from reaching the dew point.

Professional Advice: If you are planning major insulation works, such as solid wall insulation, we strongly recommend using a Retrofit Coordinator. They will design a ventilation strategy that meets the PAS 2035 standard, ensuring your home stays healthy as well as warm.

When to worry

Surface condensation is common, but if you see damp patches on the middle of a wall, or if the mould has a “musty” smell, it may indicate a technical failure in the insulation or a structural leak. Contact your installer or a building surveyor for an inspection.