Relative humidity | Dew point | Evaporation | Humidity control | Condensation | Interstitial condensation | Mould | Source of humidity | Reduce humidity

Every property suffers from condensation. Condensation accounts for over 85{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} of dampness in properties. Every damp survey considers the effect of condensation. Most remedial actions start with reducing humidity and awareness of humidity.

Relative humidity ({f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH)

  1. Air has capacity to absorb water vapour,
  2. Air’s capacity to absorb water vapour increases with temperature,
  3. Conversely air’s capacity to hold water vapour decreases with temperature,
  4. Relative humidity is the proportion of air capacity express as a {f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH,
  5. At 100{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH water is condensing, most houses feel comfortable between 45{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH and 55{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH. Over 65{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH feels humid.

Dew Point & dew point line

  1. {f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH increases as temperature decreases until 100{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH is reached, this is called the dew point.
  2. Once 100{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} capacity is reach, air borne vapour will condense,
  3. Above the dew point, water will evaporate,
  4. If there is a significant difference between the internal and external dew points of a wall, water will condense within the building material along a dew point line – this is known as interstitial condensation.

Condensation evaporation equilibrium

  1. Condensation releases energy, called entropy,
  2. Conversely evaporation absorbs energy,
  3. This change in energy causes a small change to the surface relative humidity, and therefore dew point,
  4. Condensation and evaporation is a constant equilibrium,
  5. The benefit is we can use thermal imaging cameras to determine if evaporation is taking place, as a damp wall will be significantly cooler than surroundings at 80{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH or below.

Humidity control

  1. The best and cheapest form of humidity control is ventilation especial bathroom, kitchen and bedroom windows (with doors closed).
  2. The next cheapest is humidity awareness using a humidity meter, rule of thumb is air for 50{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH, keep below 60{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH,
  3. Temperature should be kept constantly above the dew point, typically 10˚C to 12˚C. The easiest way to do this is by using a modern thermostat with multiple temperature settings, such as a Nest or Hive (there are other systems).
  4. Insulation around cold surfaces, such as
    1. constantly shaded or north facing walls,
    2. contact with cold metal downpipes,
    3. windows and metal window and
    4. door frames and the wall cut-out around windows and doors.
  5. A thermal imaging camera is useful for identifying cold spots.
  6. Air flow especially in and around cupboards, this because denser humid air flows in around cupboards almost like a liquid, but doesn’t flow out so easily.


  1. Condensation normally only causes surface dampness.
  2. Walls tend to evaporate sufficiently during the day to be dry by the time the wall falls back below the dew point.
  3. Condensation is more likely to take place when the heating is off
  4. or when the external temperature drops at night.

Interstitial condensation

  1. Interstitial condensation takes place when either;
  2. The wall temperature remains below the dew point more than above it, therefore daily condensation exceeds evaporation, the result looks remarkably like rising damp, but is in fact condensation.
  3. When there is a poor insulator within or against the wall, such as;
    1. metal beam (RSJ),
    2. old gas lamp fitting,
    3. electrical wiring,
    4. mains water pipe,
    5. wrought iron fitting e.g. down pipe,
  4. Interstitial condensation is often accompanied by the movement of salts, especially calcium sulphate (sulfate) that are drawn onto the surface by evaporation.


  1. Mould grows on surface debris between 85{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH and 100{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH.
  2. The mould point is the temperature where 85{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH is reached.
  3. Mould can be controlled with a final coat of fungicidal or anti-mould paint.
  4. Mould is inhibited by salts especially nitrates found in rising damp.
  5. Mould is “your friend” as by seeing mould you know there is an excess humidity problem.

Source of humidity

A typical flat contains 20 litres of atmospheric water, and house about 30 litres. Each day, each occupant and pet adds;

  1. 0.8 litre in respiration, (about 0.2 litres at night),
  2. 0.5 litre per shower,
  3. 0.5 litre for washing,
  4. 0.25 litre cooking,

Each day 2 litres of water per person and pet will condense on walls if it not extracted out

Action to reduce humidity

Action plan to reduce humidity. Damp is a particular issue on lower ground floors as occupiers are not always willing to keep rooms constantly ventilated.

  1. Ventilation, ventilation, ventilation. Make a strict open window policy. The driest properties have a permanently open window, often on each level.
  2. A humidity and temperature meter is useful to encourage greater humidity awareness. See our mould risk calculator which we have made freely available on A humidity meter, which cost £10 or more (some humidity can link to and activate heating systems), or by placing a mirror on a cold external wall.
  3. Keep the bathroom window open and door closed or ventilation unit operational for as long as there is excess humidity, below 60{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH. An effective way to do this is with a light switch controlled ventilation unit, with a time delay of at least 20 minutes. Many time delay can be lengthen by turning a small screw visible through the unit’s vents.
  4. Similarly ventilate a kitchen when cooking, with a window open and door closed – keep tops on pots and pans. Or better to use an externally ducted ventilation unit, regularly using the highest setting until steam has cleared.
  5. Don’t dry clothes in doors, especially on radiators. Dry clothes outdoors or use a drying machine, ideally an externally ducted drying machine, rather than condensing machine which allow a greater escape of humid air.
  6. Apply a final coat of fungicidal paint (anti-mould) on any areas that mould has grown on.
  7. Maintaining a minimum temperature, especially at night. The cost of steady heat is less than fluctuating temperature, given the reduced insulation caused by condensation.
    1. It is recommended to use a modern thermostat that allows many different minimum temperature settings.
    2. This method allows you to keep multiple settings including a minimum night time temperature, throughout the year, with a small amount of heat circulating through the property when the walls are cold.
  8. Insulate around cold spots, especially cold metal pipes.
  9. Consider using thermal insulating plaster board to insulate cold walls e.g. shaded, north facing or thin walls.

Relative humidity | Dew point | Evaporation | Humidity control | Condensation | Interstitial condensation | Mould | Source of humidity | Reduce humidity

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