Mould on Windows and Window Frames: Causes and Fixes

Every morning you wipe the condensation from your windows, and every winter the black mould creeps back along the frames, sills, and seals. Window mould is one of the most visible and frustrating mould problems in Australian homes, and in Hobart, it affects nearly every home during the cooler months. But the mould you see on your windows is usually a symptom of a bigger moisture problem — and understanding that is the key to solving it.

Why Windows Get Mouldy

Windows are the coldest surfaces in most rooms. When warm, moist indoor air meets cold glass and metal or timber frames, the moisture condenses — exactly like water forming on a cold glass on a hot day. This condensation problems provides a constant moisture supply for mould growth. Single-glazed windows (common in older Hobart homes) are particularly problematic because they offer minimal thermal insulation, creating extreme temperature differentials.

The condensation and mould risk explains in detail how temperature, humidity, and surface temperature interact to create condensation — and why windows are always the first place it appears.

Types of Window Mould

Window mould manifests differently depending on the frame material:

• Timber frames: Mould penetrates into the wood grain, causing dark staining and eventual rot. The most difficult to clean and most expensive to replace
• Aluminium frames: Mould grows in the condensation channel and on rubber seals. Easier to clean but indicates severe condensation
• uPVC frames: Mould grows on the surface and in seal grooves. Relatively easy to clean but still indicates a moisture problem
• Window sills: Paint blistering and mould growth on sills indicates persistent pooling of condensation water

Effective Cleaning Methods

For non-porous surfaces (aluminium, uPVC, glass), spray with undiluted white vinegar, let sit for 15 minutes, and scrub with an old toothbrush for detailed areas. Rinse and dry completely. For timber frames, sand any mouldy areas back to clean wood, treat with a timber-safe antifungal, and repaint with exterior-grade paint. For rubber seals, clean with vinegar and replace if the mould has permeated the rubber (this is usually a sign to replace them anyway).

If the frames are severely damaged or the mould extends into the wall around the window, professional mould removal can assess the full extent of the problem. free mould risk assessment to understand your risk level.

Long-Term Solutions

Cleaning window mould without addressing condensation is like mopping up water without turning off the tap. Effective long-term solutions include:

• Improve ventilation: Open windows briefly each morning (even in winter) to exchange humid indoor air for drier outdoor air
• Reduce indoor humidity: Use exhaust fans, dehumidifiers, and avoid drying clothes indoors
• Upgrade glazing: Double or triple glazing dramatically reduces condensation by keeping the inner glass surface warmer
• Window insulation film: A budget alternative to double glazing that creates an insulating air pocket
• Trickle vents: Small vents in window frames allow continuous background ventilation without security or weather concerns

For more on window-specific condensation problems and their connection to broader home moisture issues, see our guide on garage mould removal for full-room solutions. Understanding the overall causes helps you implement a whole-home approach.