Summer humidity and mould in a closed villa
Why a sealed house in July grows mould, and the one number that tells you whether yours will.
A villa with the doors shut and the AC off in July is not a cool, dark, dry place. It is a warm, dark, humid one, and that is the exact set of conditions mould needs.
The number that matters is 60
Mould needs moisture in the air, and the measure of that is relative humidity — how much water the air is holding compared to the most it could hold at that temperature. Below about 60 per cent, mould struggles. Above it, and especially above 70, it does not.
Outdoor humidity in Dubai in July and August regularly sits above 80 per cent. Every time a door is opened, that air comes in. Without air conditioning running, it stays.
Where it starts
Not usually on the walls you can see. It starts in the places where air does not move and surfaces stay cool: inside wardrobes against an outside wall, behind headboards, under sinks, in the corner of a guest room nobody opens, and on leather and fabric, which hold moisture better than paint does.
By the time there is a visible patch in a hallway, it has usually been growing somewhere else for weeks.
What actually prevents it
Leaving the AC running at 24 to 26 degrees, not switching it off. The cost of running it at that setting is far less than the cost of repainting, replacing a wardrobe or treating a wall.
Running the AC also means the drain lines have to work. A blocked condensate line is the most common way a well-intentioned owner comes home to water damage: the unit keeps pulling moisture out of the air, and the water backs up into the ceiling instead of going outside.
Opening windows for a short period when the outside air is drier than the inside air — early morning, not the middle of the afternoon.
And measuring. A reading in each main room tells you whether the house is actually at 54 per cent or actually at 72, and those two houses need different things.
What we do about it
We take a humidity reading in each main room on every visit and write it on the report, so you can see the trend rather than one number. We check the AC drain lines, the ceilings and under every sink for the leaks that cause the problem in the first place. If a room is climbing week on week, you hear about it while it is still a number rather than a stain.