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.
Table of Contents
Relative humidity ({f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH)
- Air has capacity to absorb water vapour,
- Air’s capacity to absorb water vapour increases with temperature,
- Conversely air’s capacity to hold water vapour decreases with temperature,
- Relative humidity is the proportion of air capacity express as a {f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH,
- 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
- {f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH increases as temperature decreases until 100{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH is reached, this is called the dew point.
- Once 100{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d} capacity is reach, air borne vapour will condense,
- Above the dew point, water will evaporate,
- 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
- Condensation releases energy, called entropy,
- Conversely evaporation absorbs energy,
- This change in energy causes a small change to the surface relative humidity, and therefore dew point,
- Condensation and evaporation is a constant equilibrium,
- 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
- The best and cheapest form of humidity control is ventilation especial bathroom, kitchen and bedroom windows (with doors closed).
- The next cheapest is humidity awareness using a humidity meter, rule of thumb is air for 50{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH, keep below 60{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH,
- 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).
- Insulation around cold surfaces, such as
- constantly shaded or north facing walls,
- contact with cold metal downpipes,
- windows and metal window and
- door frames and the wall cut-out around windows and doors.
- A thermal imaging camera is useful for identifying cold spots.
- 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.
Condensation
- Condensation normally only causes surface dampness.
- Walls tend to evaporate sufficiently during the day to be dry by the time the wall falls back below the dew point.
- Condensation is more likely to take place when the heating is off
- or when the external temperature drops at night.
Interstitial condensation
- Interstitial condensation takes place when either;
- 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.
- When there is a poor insulator within or against the wall, such as;
- metal beam (RSJ),
- old gas lamp fitting,
- electrical wiring,
- mains water pipe,
- wrought iron fitting e.g. down pipe,
- Interstitial condensation is often accompanied by the movement of salts, especially calcium sulphate (sulfate) that are drawn onto the surface by evaporation.
Mould
- Mould grows on surface debris between 85{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH and 100{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH.
- The mould point is the temperature where 85{f898b7ccba74aedaf6f9f165b1651d39d829939f1b62e7036dcda8fdc0245a9d}RH is reached.
- Mould can be controlled with a final coat of fungicidal or anti-mould paint.
- Mould is inhibited by salts especially nitrates found in rising damp.
- 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;
- 0.8 litre in respiration, (about 0.2 litres at night),
- 0.5 litre per shower,
- 0.5 litre for washing,
- 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.
- Ventilation, ventilation, ventilation. Make a strict open window policy. The driest properties have a permanently open window, often on each level.
- A humidity and temperature meter is useful to encourage greater humidity awareness. See our mould risk calculator which we have made freely available on mouldpoint.co.uk. 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.
- 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.
- 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.
- 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.
- Apply a final coat of fungicidal paint (anti-mould) on any areas that mould has grown on.
- Maintaining a minimum temperature, especially at night. The cost of steady heat is less than fluctuating temperature, given the reduced insulation caused by condensation.
- It is recommended to use a modern thermostat that allows many different minimum temperature settings.
- 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.
- Insulate around cold spots, especially cold metal pipes.
- 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