How to understand and avoid it.
We've all experienced welcome condensation when we grab a cold drink on a hot day. The water vapour in the air meets the cold glass or bottle and condenses into liquid droplets. The same happens in our homes when the moist, humid air meets the cold glass surface of windows- an unwelcome guest on cold winter mornings and a breeding ground for mould growth.
In order to prevent it, we need to understand some basic facts.
warm air can hold more moisture as vapour than cold air.
air inside our homes is generally more moist than outdoor air because of cooking, breathing, bathroom and laundry use and even indoor plants.
water vapour won't condense until it meets a cold surface
So the keys to avoiding condensation are to reduce the amount of moist air inside your home and to reduce any cold surfaces. We can reduce moist air by using exhaust fans that vent to the outside while showering, cooking or drying clothes inside. Also opening up windows during the day in winter when weather permits. Showerdomes cover the shower recess and mean the steam can condense in that enclosed space, drip down into the plughole, never reaching the rest of the bathroom.
We can reduce cold surfaces by ensuring floors, walls and ceilings are 100% covered with insulation. Windows are always a weak spot for insulation, even with double glazing. The worst condensation will happen on single pane windows that get very cold. Consider upgrading to double glazed units or you can install secondary glazing- another layer of perspex or even clear film to create the same still pocket of air as double glazed windows. The inside layer then remains warmer than the outer pane and condensation is less likely to happen. There are companies that can do this for you or you can do it yourself especially with a product like Clear Comfort.
Another thing to consider is condensation happening in spaces you can't see. Warm inside air can filter into wall spaces and condense on the first bit of cold material it meets. Perhaps that is timber framing which can gradually rot and deteriorate without you seeing it. That can happen even faster if there are gaps and cracks that allow the passage of water vapour much faster than diffusing through a plaster wallboard for example. The use of building wraps outside the timber frame that do not allow water vapour to pass through them can trap that moisture around that timber. If you're building, ensure you use a vapour permeable wrap to avoid these issues down the track.
The motto for energy efficient building is "build tight, ventilate right" and it goes without saying that 100% coverage with insulation is needed. Building tight means ensuring uncontrolled air leakage like gaps and cracks, pipe and electrical penetrations, door and windows are sealed. Ventilation should be provided in a way that you can control like opening windows or turning on exhaust fans or for very efficient homes, using a heat recovery ventilation system.
For a more in depth look at these issues you can also read Jenny Edwards' article in Renew here.