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Facts use to help professional and property owners to determine root of causes and improve outcomes.
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.
Drain leaks are often misdiagnosed. Drains and wastewater under buildings can behave like rising damp (which comes from groundwater). While chemical damp proofing treatment and slurry can slow down the effects of a broken drain, the water could go on to cause greater damage, such as rot, subsidance, heave, and is liable to need retreatment until the cracked drain is dealt with. The cost of fixing broken drains is normally considerable less expensive than chemical damp proofing.
Dampness in bricks reduces insultion by a factor of about 50%, so even if the rain doesn’t actually penetrating through the wall, the increased susepticibility of one a section of wall to become damp, reduces internal temperature increasing the risk of condensation and mould. A good example is where the uppeer section of wall is painted or render, and the lower section exposed, resulting increased dampness on the external skin causing bricks to lose heat..