A Swedish laboratory sprayed six common indoor molds onto 21 building products and held them in humid air at four fixed humidity levels for twelve weeks. The lowest humidity that produced growth turned out to be a property of the individual product, not of the material it is made from.
Most mold advice starts with an event. A supply line lets go, a roof fails, a washing machine hose splits, and the clock starts: dry it inside 24 to 48 hours or expect growth. That advice is sound, and it is only half of the picture. The other half has no event in it at all, which is exactly why it goes unnoticed for a season at a time.
What they did
Pernilla Johansson, Lukas Lång, Gunilla Bok and Carl-Magnus Capener at RISE Research Institutes of Sweden tested 21 building products used in internal insulation systems across Europe, as part of the EU Horizon 2020 RIBuild project. The set included calcium silicate boards, gypsum boards, polystyrene, polyurethane foam, mineral wool, cellulose fiber insulation and seven mortar mixes.
Every test piece was clean, unused and dry before testing. Each was sprayed with a water suspension carrying spores of six common indoor molds at a million spores per milliliter of each species: Eurotium herbariorum, Aspergillus versicolor, Penicillium rubens, Aureobasidium pullulans, Cladosporium sphaerospermum and Stachybotrys chartarum.
The specimens then went into four climate chambers held at 80%, 85%, 90% and 95% relative humidity, all at 22 degrees Celsius, for twelve weeks. Seven specimens per board product, three per mortar. At the end, each piece was read under a microscope at 10 to 40x magnification, because, as the paper notes, extensive growth on a material “is often not seen by the naked eye.”
Once the spore spray dried, the only moisture anywhere in the experiment was water vapor in the air.
What they found
Mold grew. On most products it took above 90% relative humidity: eight of the eighteen board and insulation results, and three of the seven mortars, landed in the 90% to 95% band. Seven board results and four mortars held out above 95%.
The range ran lower than that. Two board results failed between 85% and 90%, a gypsum board and a foil facer on polyurethane. And one cellulose fiber insulation board grew mold at 80%, the lowest humidity that was tested.
*75% to 80% relative humidity* is where that board’s threshold sits: it grew mold at the 80% chamber, the lowest level tested, and the paper treats 75% as the practical floor for mold growth on building materials. No liquid water was involved beyond the spore spray at inoculation.
The scatter is the finding
The spread inside a single material group was as wide as the spread across the whole study. Four calcium silicate boards: three needed 90% to 95%, one held above 95%. Four gypsum board results: three at 90% to 95%, one at 85% to 90%. The cellulose fiber products split harder, one at 75% to 80% and another above 95%. Two different faces of the same mineral wool product landed in different classes.
The authors state the conclusion without hedging: “it is not possible to predict RHcrit for one specific product from the result from another product belonging to the same group of materials.” Each product has to be tested on its own.
One more line from the discussion is worth carrying around. Conventional mold-resistance tests are run at high RH, generally above 95%, and “Many of the products in this study would have failed such a test.” A pass or a fail on a datasheet describes one extreme condition. It says very little about the wall you are standing in front of.
Why this matters in the Portland and Vancouver winter
A 90% threshold sounds comfortably far away when the hygrometer in the hallway reads mid-range. Surface humidity is a different number. Relative humidity climbs as air cools, so the film of air touching a cold exterior wall, an uninsulated corner, or the underside of a subfloor over a vented crawlspace can sit far higher than the room average. That is standard building physics rather than a finding of this paper, but it is the reason the threshold gets crossed in ordinary houses with nothing visibly wrong.
And the timescale here is twelve weeks. Autumn to spring in this region is a great deal longer than twelve weeks.
What to do with this
Measure at the cold surface, not in the middle of the room: exterior corners, the wall behind a bed or a sofa, the closet floor on an outside wall, the underside of the subfloor over a crawlspace. EPA’s target is a reasonable one to hold: “keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity.” And do not read a threshold off a material class. A gypsum board is not a known quantity, and neither is a cellulose board.
The limits, stated plainly. This is a laboratory study published in peer-reviewed conference proceedings, not a field survey of real homes and not a health study. No health outcome was measured and nothing here speaks to symptoms. The specimens received a heavy, deliberate spore dose, which is not how a wall gets seeded in practice. Most samples appear to have come from a single board per product, and the authors say the results are valid for the specific samples tested. Twelve weeks is the window that was measured, so a shorter damp spell may not produce growth at the same humidity. What survives all of that: given enough time, humid air by itself is sufficient. Mold does not require a plumbing failure.
If your walls are damp, the useful question is how long, not just what leaked.
illumenair measures moisture and airborne mold room by room in Portland and Vancouver homes. We measure air. We do not diagnose or remediate.
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Sources: Johansson P, Lång L, Bok G, Capener C-M. “Threshold values for mould growth: Critical moisture level of 21 different building materials.” E3S Web of Conferences 2020;172:20002 (12th Nordic Symposium on Building Physics), open access. US EPA, “A Brief Guide to Mold, Moisture and Your Home.”