12 studies, every one cited on the page where it is used. Mould is not a cleaning problem. It is a temperature problem — and the temperature that matters is not the room’s, it is the wall’s. A hygrometer on the shelf reads the air in the middle of the room. Mould does not grow in the middle of the room. It grows on the coldest surface in it, and the air touching that surface is not at the room’s humidity — it is at whatever that same water amounts to once it has been chilled to that surface’s temperature. One room has as many humidities as it has surfaces, and only the worst one matters. The finding this book is built on There are two thresholds, not one. Water appears on a surface at 100% humidity. But the building standard treats 80% surface humidity as the level at which mould becomes possible — and 80% is reached at a warmer temperature. In a room at 20 °C and 55% humidity, water appears at 10.7 °C and mould becomes possible at 14.1 °C. That is a 3.4-degree band in which your wall is growing mould and is completely dry to the touch. The gap holds between 3.3 and 3.6 K across every ordinary combination of room temperature and humidity, which is what makes “about three and a half degrees” safe to remember rather than a fact about one particular evening. Every “wipe it and open a window” answer is aimed at the bottom of that band. The mould is already living at the top of it. What is in it The arithmetic, printed in full. The saturation vapour pressure equation and the 80% threshold, with a worked derivation, so you can check the claim rather than trust it. A table of your two temperatures for every normal room condition — 16–22 °C, 40–70% humidity. Find your row, get your mould line and your dew line. Why it is always the corner. Geometry, not a defect: an external corner loses heat through more outside wall than it has inside surface to recover it from. Same for window reveals, ceiling edges, and the wall behind the wardrobe. What one winter night actually looks like — the room cooling, the corner cooling faster, the humidity climbing, and the two curves crossing at about two in the morning while everyone is asleep. What double glazing actually did to this problem. The old single-glazed window was acting as a dehumidifier in plain sight. Warm the glass and the water does not go away — it moves to the reveal, which is porous and nobody wipes. Why opening a window in winter is not nonsense, computed rather than asserted. What a dehumidifier can and cannot do, with the test to run before buying one. The order of work — measure, make less water, take the water out, warm the surface, and only then clean. A chapter for renters. A dated record of room temperature, room humidity and surface temperature settles the tenant-versus-landlord argument in a way a photograph of mould never will. A fault finder arranged by what you can see, a printable survey sheet, and the whole book on one page. What it is not It names no products at all — no paint, no dehumidifier, no ventilation system, no treatment. It does not tell you how many days you have, because the published criteria were written for monthly means and inventing precision from them would be dishonest. It covers condensation, and it tells you plainly when what you are looking at is rising damp or a leak instead — and then tells you to stop, because nothing in it will fix those. Three works consulted during research are listed at the back as not cited, because they were not read at their original publication. Nothing in the book rests on them. Formats: PDF (US Letter, laid out page by page) and reflowable EPUB for Kindle, Kobo, Apple Books and phones. Both included. Instant download, nothing ships. Plain English throughout. Metric first. The two prevalence surveys quoted are British and labelled as such; the physics is not.