Insulation

Black mould around windows

Warning Do not attempt any electrical or gas work yourself. Always use a qualified, MCS-certified, or Gas Safe registered professional for repairs.

What you’re seeing (symptoms)

Small black or dark green spots appearing on window frames, sealant, or the surrounding plasterwork (reveals).

What’s normal vs not normal

Normal:

  • A tiny bit of mildew on silicone sealant in a bathroom that can be wiped away during regular cleaning.

Not normal:

  • Extensive fuzzy or “sooty” black patches on the walls around the window.
  • Mould that returns within days of being cleaned off.
  • A strong, persistent musty smell.

Likely causes (ranked)

  1. Condensation: Warm indoor air hits the cold window surface and turns into water, providing a breeding ground for mould.
  2. Poor ventilation: Cooking, showering, and breathing create moisture that isn’t being vented out.
  3. Inefficient glazing: Single-pane or failing double-glazing is very cold, encouraging condensation.
  4. Cold reveals: The walls around the window may be poorly insulated, making them “cold spots.”
  5. Blocked trickle vents: People often close window vents to stop drafts, which traps moisture.

Quick checks you can do safely

  • Open your window trickle vents and leave them open 24/7.
  • Wipe away condensation from windows every morning with a cloth or window vac.
  • Ensure you are using extractor fans when cooking or showering.

When to call an installer

  • If the mould is spreading despite your efforts to ventilate and clean.
  • If you suspect your double glazing has “blown” (foggy inside the glass).
  • If you are considering upgrading to high-performance windows or adding reveal insulation.

Questions to ask your installer

  • “Are my windows fitted with adequate trickle vents for this room size?”
  • “Can you check if the window reveals are insulated?”
  • “Is the sealant around the outside of the window intact and weather-tight?”

Related NetZeroNow tools & guides

FAQs

Is black mould dangerous? It can be, especially for people with respiratory issues, allergies, or weakened immune systems. It’s important to address the root cause, not just the symptoms.

Can I just paint over it? No. You must kill the mould first with a fungicidal wash. Painting over it will just mask it, and it will grow back through the paint.

Why is it only around the windows? Windows and the surrounding walls are usually the coldest parts of a room’s “envelope,” so that’s where moisture settles first.

Will new windows fix the problem? Higher quality windows (triple glazing) stay warmer, which reduces condensation, but you still need proper ventilation to manage indoor humidity.

How do I clean it safely? Wear a mask and gloves. Use a dedicated mould-killing spray or a mixture of water and mild detergent. Avoid dry-brushing as it releases spores.

Written by NetZeroNow | Last updated on 2026-03-02