Mould and Condensation Risk Checker

The room

Mould grows where a cold surface meets moist air. Start with the room and how cold its surfaces get. The answer updates as you go.

Room you are worried aboutThe one with the black spots, or the wettest room in the house
Which way does the outside wall face?South walls get no winter sun in Melbourne and stay coldest
External walls in the roomCorners of two external walls run coldest
Wall shaded by a fence, shrubs or a neighbourShade keeps masonry a degree or two colder all winter
Window typeThe window frame is almost always the coldest surface in the room
Ceiling insulation
Home floor areaIn square metres. Used to work out the air volume the moisture spreads through.

Extraction

Where the fan discharges matters more than whether you have one. A fan blowing into the roof cavity moves the problem, it does not remove it.

Exhaust fan in this room
Fan run-on after showeringMost of the moisture leaves in the ten minutes after you turn the water off

Moisture the household makes

Every litre put into the air has to leave through ventilation, or it lands on the coldest surface.

People living here1.5 to 2 litres a day each, breathing and washing
Showers per day, whole household
Washing loads dried indoors, per weekOn an airer, or a vented dryer blowing inside
Main heatingUnflued gas puts about a litre of water into the room for every hour it burns
Hours the gas heater runs per dayOnly counts for an unflued gas heater

What you can see

Visible evidence overrides the model. If it is already growing, the conditions are already right for it.

Mould risk for this room
Working it out

The moisture balance

Your top three drivers

Ranked by how far each one pushes the surface humidity up. Fix the top one first.

Fixes, cheapest first

Work down this list. Painting is the last step, never the first.

Book a mould and repaint assessment Call 1300 044 206
What this assessment assumes
  • A Melbourne winter night outside: 8C at 80% relative humidity. Indoors heated to 20C.
  • Moisture generation from the research: 1.5 to 2 litres a day per occupant, plus 1.5 litres per hour of unflued gas heating, plus 2 litres per washing load dried indoors, plus 0.3 litres per shower taken without working extraction.
  • Air volume is your floor area at a 2.4m ceiling. Background air change rates assumed: 0.7 per hour with single glazed timber windows, 0.5 with single glazed aluminium, 0.35 with double glazing, plus 0.15 where the ceiling has no insulation and leaks through downlights.
  • Extraction is credited only when the fan discharges outside the building. AS 1668.2 and the NCC set the minimums: bathroom 25 L/s, kitchen 40 L/s, laundry 20 L/s. A fan discharging into the roof space is credited at zero, because the moisture returns through the ceiling.
  • Surface temperatures come from standard building physics: surface temperature equals indoor temperature minus U value times internal surface resistance times the temperature difference. Frame U values used are 10.0 for single glazed aluminium, 7.5 for single glazed timber and 3.0 for double glazing. That puts a single glazed aluminium frame at about 4C on a July night, which matches what people measure.
  • Dew point uses the Magnus formula. Mould germinates where the relative humidity at the surface stays above 80%, which happens well before you see liquid water.
  • Costs are indicative Melbourne ranges for 2026, not a quote. Ducting, glazing and heater work is done by the relevant trade, not by a painter.
  • This is a screening tool. It does not replace a data logger, a thermal camera or a building inspection.
Mould risk for this room
Working it out
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