Is It Condensation Or Damp?

What's happening?

Condensation and damp are different faults, and the cure for one does nothing for the other. The builders page on a damp patch on an internal wall sets out the signs of penetrating damp, condensation and rising damp. This page is about testing which you have. A hygrometer, a cheap humidity meter left in the room for a few days, shows whether the air is wet enough to condense on a cold wall. A damp meter shows moisture in a wall but not its cause, and it is fooled by hygroscopic salts, which keep holding water after the original damp is cured, and by foil-backed wallpaper and dense materials, so a wall that reads damp may be dry. A note of when the damp appears, in cold weather or after rain, and whether it changes after some warm, ventilated days, tells you more than one reading.

What usually causes it?

• A damp meter reading taken once, on a wall that holds salts or has foil-backed paper
• Humid air from showers, cooking and drying washing with too little ventilation or heating
• Cold surfaces such as external walls, lintels, corners and window reveals
• Rain entering through cracked render, failed pointing or a leaking gutter
• Groundwater drawn up from below a failed or bridged damp proof course
• A hidden leak from a pipe, waste or radiator that behaves like damp
• Hygroscopic salts left in plaster that keep the wall damp after the cause is cured

Is it dangerous?

Damp and mould are a health issue as well as a building one: the Government's guidance for rented housing says damp and mould can affect breathing and are a hazard under the Housing Health and Safety Rating System. The first risk is a wrong diagnosis: a chemical damp course on a wall that only has condensation does nothing, and a dehumidifier does nothing for a wall that has a failed gutter above it.

Can I fix it myself?

Leave a hygrometer in the room for several days and note the readings. Write down the date and weather whenever damp appears, where it is and whether there is mould or a white salt bloom. Check gutters, downpipes and outside ground levels. Ventilate wet rooms, keep furniture a hand's width off cold walls and heat evenly. Do not buy a damp treatment on the strength of one meter reading.

When should you call a damp proofer?

Call a damp surveyor for an independent diagnosis, ideally one who does not sell the treatment. Call them sooner if the wall is wet to the touch, the plaster is crumbling, or damp has appeared with no change in habits. If the wall is plainly condensation, the fix is ventilation and heat, not a damp proof course.

What will the damp proofer actually do?

• Ask about the property, the heating and ventilation and when the damp appears
• Measure moisture at several heights and in several places, and compare with a dry reference wall
• Check gutters, pointing, render, ground levels and the damp proof course from outside
• Check for plumbing leaks and note hygroscopic salts and mould type
• Say in writing which kind of damp it is and which of the causes is behind it
• Treat what is found: ventilation and mould treatment for condensation, building repair for penetrating damp, damp proofing for rising damp

Who fixes this

• Damp survey
• Condensation and mould treatment
• PIV installation

Questions people ask

Sources & References

1. Property Care Association: condensation
2. Property Care Association: rising damp
3. Property Care Association: penetrating damp
4. GOV.UK: damp and mould, understanding the health risks for rented housing providers