What is dew point, and why does mould grow before any water appears?

Every condensation problem in every house comes down to two numbers: how cold a surface is, and what the dew point of the air touching it is. Everything else — ventilation, heating, insulation, furniture placement, drying washing — is a way of moving one of those two numbers. Once that is clear, the advice stops being a list of tips and becomes a choice between two levers.

Step by step

1. Understand what the air is holding.Warm air can hold more water vapour than cold air. Cool a parcel of air without removing any water and its relative humidity rises, until at some temperature it reaches 100 per cent and has to condense. That temperature is its dew point.
2. Work out your dew point roughly.A room at 20 degrees and 60 per cent relative humidity has a dew point somewhere around 12 degrees. Any surface in that room below about 12 degrees will get wet. Most hygrometers and any online calculator will give you the exact figure.
3. Measure your cold surfaces.An infrared thermometer on the corner, the reveal, the wall behind the wardrobe and the ceiling at the eaves. Compare each against the dew point you just calculated.
4. Understand the mould margin.This is the part people miss. Mould germinates at around 80 per cent relative humidity at the surface, not 100. A surface several degrees warmer than dew point still has high enough humidity right against it to grow mould, which is why the wall is furry but never wet.
5. Follow the dew point through the wall.The temperature falls across the construction from room to outside. Where it crosses the dew point is where water forms. In a well-insulated wall that is out in the cold outer masonry, where it does no harm. In a cold wall it is at the room face.
6. See why hidden condensation is the dangerous one.Insulate a wall or a roof without controlling vapour and the crossing point moves inside the construction. Water then forms out of sight, in the insulation and against the timber, where nothing dries it and nobody sees it.
7. Pull lever one — raise the surface temperature.Insulation, cavity fill, insulated reveals, background heat, moving furniture off the wall, opening curtains. All of these lift the temperature line above the dew point line.
8. Pull lever two — lower the dew point.Extract at source, air the house, dry washing elsewhere, run a dehumidifier. All of these remove water from the air, which lowers the dew point line.
9. Notice what heating alone does not do.Warming the air raises the temperature line but does not move the dew point line at all, because the amount of water in the air has not changed. Heat without ventilation moves you along, not out of, the problem.

Tips

• This is why a house feels better after a short sharp airing on a cold day: cold outside air holds very little water, so bringing it in and warming it drops the indoor dew point substantially.
• It is also why a vapour control layer goes on the warm side of insulation — to stop room air carrying vapour into the construction to a point where it will cross its dew point.
• An infrared thermometer plus a hygrometer is under twenty pounds of kit and between them they diagnose every condensation problem in a house.

Common mistakes

• Trying to fix condensation by heating harder without ventilating — It raises the air temperature and the surface temperatures somewhat, but the dew point is unchanged, so the coldest surfaces still condense and the bill goes up.
• Insulating a wall or roof internally without thinking about vapour — The masonry behind the insulation becomes colder, and if room air can reach it the dew point crossing lands inside the construction. That is interstitial condensation, and it rots timber quietly for years.

Questions people ask