What are the three types of damp?
By The Damp GuyLast updated
Condensation, penetrating damp and rising damp. Condensation forms when warm, moist air cools against a cold surface. Penetrating damp is water crossing the structure from outside through a defect. Rising damp is moisture moving up through permeable building materials by capillary action. What separates them is when they appear, not what they look like. Condensation tracks the cold and the way you use the room. Penetrating damp tracks the rain. Rising damp tracks neither.
Hold on to that last point. All three put the same dark patch on a wall at skirting height. That is why they get mixed up, and why a photograph settles nothing.
Condensation
Everything you do in the house feeds this one: cooking, washing, drying clothes indoors, breathing. Warm air holds more moisture than cold air, and when that air meets a cold enough surface the moisture comes out of it and lands there.
It shows up on the coldest surfaces first. That is why your windows and the outside corners of external walls go before anything else. It is worst in cold still weather with the windows shut. Your building does not need a single defect for it to happen.
Penetrating damp
This is water getting through the structure from outside, and something is always letting it in. The usual suspects: failed pointing, cracked render, slipped roof coverings, leaking gutters, defective seals around windows, porous masonry, or a plumbing leak inside the structure.
In a cavity wall the water needs help to cross the gap. It gets that help from bridged cavities, blocked weep holes or damaged cavity trays. The weather gives it away. Penetrating damp appears or worsens during and after rain, then eases off in a dry spell.
Rising damp
Moisture climbing up the wall from the ground by capillary action. The same way liquid climbs into a sugar cube.
It is the one people diagnose most confidently and test least rigorously. It also carries the biggest bill.
It does not track the rain and it does not track the cold, which is what sets it apart from the other two. What it does do is carry salt. The moisture dissolves soluble salts out of the building materials and takes them up with it.
The fourth possibility nobody lists
Your wall can read damp with no water arriving at all. Hygroscopic salts left in the plaster pull moisture straight out of the air once relative humidity passes about 75%. The wall really is damp. Nothing is leaking, rising or condensing on it today.
Salt deposits and tide marks also stay put long after the water that caused them has gone. A tide mark proves a wall was once wet. It does not prove anything is wet now. Treating it as proof is how the wrong work gets sold.