It is the question we get most: how do you get mould in a desert? The answer is that Arizona mould has almost nothing to do with ambient humidity. It comes from specific water failures inside the building — and because the climate is dry, those failures tend to run unnoticed far longer than they would somewhere wet.
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Two reasons, and they compound.
First, nobody is looking. In a humid climate people expect mould and check for it. In Phoenix the assumption is that it cannot happen here, so a slow leak runs for months before anyone investigates the musty smell.
Second, the failures are hidden by design. Condensate lines are in attics and closets. Slab leaks are under concrete. Neither announces itself until there is staining or an odour — by which point the affected area is considerably larger than what you can see.
Because the water arrives from concealed places, visual inspection alone is not much use. Moisture meters and thermal imaging find the temperature differentials and wet cavities behind finished surfaces, which is how the real extent gets mapped before anyone starts demolition.
It also means fixing the mould without fixing the cause is pointless. A remediated wall over a live slab leak will be a remediated wall with mould in it again by next summer.
Two peaks. Mid-summer, when air conditioning runs hardest and condensate systems fail under load. And monsoon season, when roof and exterior envelope failures let water in fast.
If you are going to check anything proactively, check the AC drain line before summer and the roof before monsoon.
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Ambient-humidity mould is, yes. Mould from plumbing and AC failures is not — those happen everywhere, and the desert climate mainly changes how long they go unnoticed.
No. Mould needs a moisture source, and if that source is a leak inside the wall it does not care what the outdoor humidity is. Dry air can mask the problem by drying the surface while the cavity stays wet.
Before summer, and again mid-season if the unit runs hard. It is a small maintenance item that prevents a disproportionate share of the mould claims we see.
It can. Evaporative coolers add moisture by design, and where drainage or ducting is poor that moisture ends up in the structure. They warrant more attention than refrigerated air.
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