Houses contain about 4 litres of water as vapour. A normal house typically contains 10 grams per cubic meter at 65%RH at 20°C. A 2.5M high room and foot print of a house is 150M2 = 375 x 10 = 3,750 g or 3.7 litres. (1,000 grams in a litre)
Tooltip Categories: FACTS
Facts use to help professional and property owners to determine root of causes and improve outcomes.
Warm air holds more water vapour, a similar amount is absorbed into walls
White surfaces salts
Mould only grows where relative humidity
RSJs are prone to condensation as the metal is normally in contact with the cold external temperatures, either supporting an external wall, adjoing wall or in contact with the sub-floor void or cold ground. Metal loses heat rapidly when there is a relatively large heat difference between the inside and out.
90% of vapour remains in a bathroom after a shower has been switched off and the occupier left the room. That is why it is so important to keep the extractor fan running for 30 minutes. Strangely most extractor fan manufacturers set fans up with a 1 or 5 minute default and builder’s or electricaians, rarely know to change the setting. This shortcoming probably accounts for more isdiagnosed rising damp than any other cause of dampness, probably accounting for £100M of wasted economic activity in the UK every year.
Cupboards by external walls are prone to mould as vapour moves through diffusion, but heat moves mainly through air currents. Humid air become trapped for hours. Mould needs long term high relative humidity to grow, 6 or more hours at 85%RH.
Vapour causes 85% of internal damp issues
Mould is inhibited by nitrates. Nitrates are used in foods to prolong the shelf lives of meats and prepared foods. The presence of mould in a wall exlcudes, or elminates risng damp as the root cause, as groundwater, the source of wateer in rising damp, is rich in nitrates as it passes through the soil.
Solid floors are prone to condensation because of heat loss close to the ground. The impermeable surface acts as an amplifier, resulting in water under the skirting board. It is also common for plasters to allow the plaster to touch the solid floor. plaster is absorbent. The result is often misdiagnosed as rusing damp. The simpest solution is 1) improve ventilation, 2) seal up at the base of the skriting board, 3) cut the plaster back at the base of the wall by 10 – 20mm back to brick (chisel, grinder or better still Oscillating Multi Tool, such as Bosch), 3) Run a bead ~10mm of a absorption reducing Thixotropic cream such as DryZone by Safeguard (Amazon etc), along the exposed brick. Replace the skirting board.