Blog
Condensation & moisture
There is moisture in the air all around us, and we add to it ourselves just by breathing. Add cooking, showering and washing, and a family of four puts roughly 10 litres of water into the air every day.
If the house is not ventilated well enough, most of that stays inside. Over time it shows up as mould, fungus, pests, swelling or peeling timber, timber rot and streaming windows — a smelly, unhealthy home.
Warm air holds more moisture than cold air. When it drifts past a cold surface such as a window it cools, and the water condenses on the glass. Even double glazing does this if the aluminium is not thermally broken: it starts on the frames and sills, and keeps going until the humidity indoors drops to around 55 percent. A thermal bridge like that is a construction flaw, not bad luck.
Even new houses have no moisture barrier on the inside of the exterior walls, so the excess moisture pushes into the structure and quietly does its damage. In unheated rooms below about 15°C the air holds less moisture again, which is why bedrooms and bathrooms are usually the worst.
A new house built of concrete and timber takes 12 to 18 months to dry out. Open the doors and flush the warm, damp air out for 15 minutes as often as you can.
Solutions
- Insulate walls, roof and ceiling to the highest R value you can.
- Ventilate every day, for at least 15 minutes.
- Consider a mechanical ventilation system that draws air through a ground pipe, so the incoming air is preheated.
- Use double glazing with the lowest Ug value you can get.
- Use thermally broken windows.
- Know where the thermal bridges in your house are.
- Keep bedrooms and bathrooms above 15°C.
- Put a moisture barrier on the inside of exterior walls too — 9mm ply sheathing does this best.