Mould and Mildew Growth

Mould and Mildew in Homes: Causes, Hidden Moisture, and How Ventilation Prevents Regrowth

What the Problem Is

Mould and mildew commonly appear as black, green, or grey patches on ceilings, walls, bathroom grout, window frames, behind furniture, on clothing, and inside wardrobes. They are often accompanied by musty odours and can return repeatedly, even after cleaning.

While mould is sometimes treated as a surface issue, it is almost always a ventilation and moisture problem.

Why Mould and Mildew Exist

Mould requires three things to grow:

  • Moisture
  • Still or stagnant air
  • Time

Modern homes are particularly prone to mould because they are:

  • More airtight
  • Better insulated
  • Less able to release moisture naturally

Everyday activities such as showering, cooking, drying clothes, and breathing increase indoor humidity. Without consistent air movement, moisture settles on cold or poorly ventilated surfaces — creating ideal conditions for mould.

Mould commonly grows:

  • In bathrooms and laundries
  • On external walls and corners
  • Behind wardrobes and furniture
  • In bedrooms with closed doors overnight
  • Low down on walls where air movement is minimal

How Ventilation Fixes Mould and Mildew

Ventilation works by removing moisture before it can settle and linger.

Effective ventilation:

  • Continuously lowers indoor humidity
  • Prevents stagnant air pockets
  • Flushes out moist, contaminated air
  • Helps surfaces dry more quickly

Cleaning mould removes visible growth, but ventilation addresses the cause, making regrowth far less likely.

Recommended Ventilation Systems

1. Home Ventilation Systems

Home ventilation systems provide continuous or regular fresh air throughout the home, helping to control humidity across all rooms.

They are effective for mould prevention because they:

  • Reduce overall moisture levels
  • Improve airflow into bedrooms and living spaces
  • Operate automatically without relying on occupant behaviour

These systems are well suited to homes experiencing mould in multiple rooms.

2. Heat Recovery Ventilation (HRV/ERV) Systems – Airtight Homes

HRV and ERV systems are ideal for modern, airtight homes where natural air leakage is minimal.

By supplying fresh air and extracting stale air at the same time, they:

  • Maintain balanced pressure
  • Remove moisture consistently
  • Ventilate rooms even when doors are closed

HRV/ERV systems are particularly effective where mould occurs in bedrooms, wardrobes, and living areas.

3. Subfloor Ventilation Systems – Low-Level Mould and Wardrobes

Subfloor ventilation can play an important role where mould appears:

  • Low down on walls
  • Behind furniture
  • Inside wardrobes
  • Along skirting boards

Damp subfloors allow moisture to rise into the home, increasing humidity near floors and external walls. Improving subfloor airflow helps reduce this moisture source, supporting healthier indoor conditions.

Subfloor ventilation works best as part of a whole-home moisture strategy, rather than as a standalone solution for widespread mould.

4. Targeted Exhaust Ventilation With Make-Up Air

Bathrooms, laundries, and kitchens produce high levels of moisture. Targeted exhaust ventilation removes moisture at the source.

For best results, exhaust systems should:

  • Be quiet enough to run for extended periods
  • Vent outdoors (not into roof spaces)
  • Be paired with make-up air, so moisture is actually removed rather than redistributed

This approach is especially effective for preventing ceiling mould and moisture build-up in wet areas.

Other Helpful Fixes (Non-Ventilation)

Ventilation is critical, but additional steps can help reduce mould risk:

  • Improve insulation on cold external walls
  • Maintain consistent indoor heating in winter
  • Avoid pushing furniture hard against external walls
  • Repair leaks and plumbing issues promptly
  • Avoid drying clothes indoors without ventilation

Key Takeaway

Mould and mildew are not cleaning problems — they are moisture and airflow problems. Without proper ventilation, humidity builds up, air becomes stagnant, and mould is almost inevitable in modern homes.

Home ventilation systems and HRV/ERV systems provide reliable, whole-home moisture control, while targeted exhaust and subfloor ventilation address problem areas at their source. By combining the right ventilation strategy with sensible home practices, mould growth can be significantly reduced and prevented long term.