Condensation and mould in winter

Water running down the inside of the windows on a winter morning means the house is holding more moisture than it can get rid of. Most of that water comes from daily life, and most of it can be moved out with a few habits. When the habits don't fix it, the cause is in the building, and that is a job for a trade.

Why winter

Insulation has only been required in new New Zealand houses since 1978. Many older homes still have little of it, with single glazing and no planned ventilation. They were built to breathe through open windows. In winter the windows stay shut, the heating goes on, and the moisture has nowhere to go.

Warm air holds more water than cold air. When warm, moist air meets a cold surface, the water comes out of the air and settles on it. That is condensation. The cold surfaces in a house are the windows, the outside walls, the corners of rooms and the backs of wardrobes against an outside wall. Those are the places mould starts.

Where the water comes from

Consumer NZ puts numbers on the main sources:

• Showers and baths: up to 1.5 litres per person a day.
• Cooking: up to 3 litres a day.
• A load of washing dried inside: 5 litres.
• An unflued gas heater: up to 1 litre for every hour it runs.
• The ground under a suspended timber floor: as much as 40 litres a day for every 100 square metres.

Breathing adds more on top, and nobody can stop doing that. The aim is not a dry house. It is moving the water out faster than it builds up.

The early signs

• Water on the inside of the windows most mornings.
• Pooled water on the sills, or black spots in the window seals.
• A musty smell in a bedroom or a wardrobe that does not go away.
• Grey or black speckling in the corner of a ceiling, behind furniture or above the shower.
• Bedding or clothes that feel cold and slightly damp.

The daily habits

• Air the house each morning. Open the windows for ten to fifteen minutes, before the heating goes on. The walls and furniture keep their warmth, and the wet air leaves.
• Heat the rooms you use. EECA's guidance is between 18°C and 21°C. A cold room is a damp room.
• Run the fans. Turn the bathroom fan on before the shower and leave it running until the mirror clears, with the door shut. Use the rangehood when you cook, and keep lids on pots.
• Dry washing outside, or in a dryer vented to the outside.
• Leave the unflued gas heater off. It puts water into the room as it burns. A heat pump or an electric heater adds none.
• Give the air some room. Pull beds and wardrobes a hand's width off outside walls, and leave wardrobe doors slightly open.
• Wipe the windows each morning rather than letting the water sit on the sill and the frame.

Cleaning mould

Clean mould while it is still damp, because dry mould gives off its spores more easily. Wear gloves, eye protection and a mask. White vinegar works well. Use it straight on most hard surfaces, and half and half with water on painted walls so it does not mark the paint. Rinse the cloth often so you are not spreading the spores around. If the mould covers a large area, or is in the carpet or the ceiling lining, get advice before you start.

One note from the painting side: never paint over mould. If it is not killed first, the spores grow back under the new paint and lift it. In bathrooms and laundries, ask for a paint made for wet rooms, with a mould inhibitor in it.

When the habits are not enough

Some damp is built into the house, and no amount of airing will shift it.

• Fans that vent into the ceiling space, or no fan at all. The moist air has to go outside, through ducting of the right size. An electrician can put this right.
• Bare ground under the house. A polythene ground moisture barrier laid over the soil, with the subfloor vents clear of plants and soil, cuts the moisture rising into the rooms above.
• Missing insulation in the ceiling or under the floor.
• A dehumidifier helps with condensation, but it treats the symptom. It will not fix the source.

The Healthy Homes standards for rentals make a good benchmark for any home: an extractor fan vented outside in each kitchen and bathroom, a ground moisture barrier under an enclosed subfloor where one can be fitted, and spouting, downpipes and drains that work.

When to call a professional

Call someone if mould comes back within weeks of cleaning it, if a wall or ceiling is wet to the touch rather than just cold, or if the damp shows up in one spot only. Swollen skirting boards, a stain that grows, or a musty smell from a wall rather than a room all point the same way. These are signs of water getting in, from the roof, a pipe, the cladding or the ground around the foundations, and opening windows will not fix them. A roofer, a plumber or a builder can find the source.

Photograph the patch and note the date each time it appears. On your Archive record, those photos show the trade whether it is growing.