What this category is about
Condensation is the one damp problem that is entirely about temperature. The water was already in your air; a cold surface simply took away the room it had to sit in. That is why the same house can run wet in November and dry in April with nobody changing a single habit — the air did not change, the walls did.
The distinction that matters on every page in this category is between condensation and the three kinds of damp that involve liquid water arriving from somewhere else: penetrating damp through the fabric of the wall, rising damp from the ground, and plumbing leaks. They look similar on a wall and they need completely different repairs. Getting that call right is worth more than any product you can buy.
The articles here work through the diagnosis in the order a surveyor would: where the water is, when it appears, what the surface temperature is, and only then what to do about it.
The four things it keeps coming back to
Reading the pattern, not the patch
Condensation appears on the coldest surfaces first and evenly across them — the whole pane, the whole north corner. Penetrating damp has a defined edge and gets worse after rain. Rising damp stops at a horizontal line about three feet up. A leak ignores all of those rules and follows a pipe run. The pattern tells you more than the moisture meter does.
Why morning is the honest time to look
Surfaces are coldest just before dawn, and that is when the room is closest to its dew point. A wall that is dry at 6 pm and beaded at 7 am is condensing, not leaking. Half the misdiagnoses on the internet come from inspecting a room in the afternoon.
The dew point as a working number
Once you know the dew point of your room air, you know exactly which surfaces are wet: every one colder than that figure. It turns a vague worry into a measurement you can take with a $20 infrared thermometer.
Fixing the surface, not just the air
Drying the air lowers the dew point. Warming the surface raises the surface temperature. Both close the gap, but only one of them removes the cause — which is why insulation appears in this category as often as ventilation does.
Articles
When opening a window makes your humidity worse
Outdoor air at 80% can be drier than your indoor air at 60%. It can also be much wetter. The percentage alone cannot tell you which.
Why your windows sweat and your neighbor’s do not
Same street, same weather, same thermostat — and one house wipes the glass every morning. Four variables explain it.
Common questions
Is condensation always a sign that something is wrong with the house?
No. Every occupied house produces water vapor, and on a cold enough night some of it will land on the coldest glass. Occasional light misting at the bottom of a window on a freezing morning is normal. Water running down the pane most mornings, or any condensation on walls rather than glass, is not — that is a room whose humidity is too high for its surface temperatures.
Can condensation damage a house, or is it only cosmetic?
It damages. Persistent surface water swells and rots timber window frames, lifts paint and wallpaper, corrodes fixings, and gives mold the sustained wetness it needs to colonize plaster. The damage is slow rather than dramatic, which is exactly why it usually gets ignored until the frame is soft.
Why does my house feel damp even though the hygrometer reads under 50%?
Two common reasons. The first is that the reading is taken in the warmest room and the failing room is several degrees colder, where the same water reads much higher. The second is that damp smell comes from microbial growth in materials, which persists after the air has dried — the air is fixed but the carpet, plaster or stored cardboard has not dried out yet.