Mould in buildings: what the science actually says — and why we still want it gone
Mould is one of those topics where everyone has an opinion, a horror story, or a bottle of bleach they want to recommend. The conversation tends to swing between two unhelpful extremes: it's catastrophically toxic and will kill you, or it's basically harmless and people are overreacting.
The truth, as usual, is more nuanced — and more useful — than either of those positions.
What the science actually says
Two podcasts worth your time have recently tackled the evidence on mould and health: the Skeptics' Guide to the Universe and the Unbiased Science Podcast. Both are worth a listen if you want to go deep. The short version of what they found: the relationship between mould exposure and specific health outcomes like asthma is more complicated than the headlines suggest.
A Finnish birth cohort study — which followed children from infancy through early childhood and analysed the fungi present in household dust — found no significant correlation between mould in home dust and the later development of asthma. That surprised a lot of people. Mould is often cited as a classic asthma trigger, and here was a well-designed study suggesting the link isn't as direct as assumed.
The Unbiased Science Podcast reached a similar conclusion: yes, mould can cause allergic reactions and respiratory problems in sensitive individuals, but the scientific community is divided on the extent of its impact, particularly for more serious health conditions. The "toxic mould" narrative — the one that drives panicked remediations and eye-watering quotes from specialists — turns out to be significantly overblown in many cases.
So why do we still care about mould?
Here's the building science perspective, and it's the one that shapes everything we do at BEO: mould is a symptom, not the disease.
When mould appears in a building, it's telling you something is wrong with the building itself — persistent moisture it can't shed, inadequate ventilation, insufficient heating, a building envelope that's letting condensation form somewhere it shouldn't. The mould is just the visible evidence of underlying conditions that are already degrading the structure and compromising the indoor environment.
Even if the direct health link to specific conditions like asthma is contested, those underlying conditions are not good for you. Poor insulation means cold surfaces, which means condensation, which means moisture in the fabric of your home. Poor ventilation means stale, humid air — elevated CO₂, accumulated pollutants, and the damp conditions that mould needs to thrive. These are independently problematic, separate from any mould that results from them.
And it's worth noting that the people most likely to be living in mould-affected homes are often also dealing with other compounding disadvantages — inadequate heating, overcrowding, limited access to healthcare. Mould may be one thread in a larger web of conditions. That doesn't make it irrelevant; it makes it a useful indicator of a broader set of problems that need addressing.
Where mould actually comes from
Mould needs three things: a surface, organic material to feed on, and moisture. The first two are everywhere in buildings. Moisture is the variable you can control.
| Condensation Most common | Cold surfaces — windows, uninsulated walls, thermal bridges — attract moisture from warm indoor air. Classic in poorly insulated homes. |
| Inadequate ventilation Very common | Cooking, showering, breathing — everyday life generates moisture. Without ventilation, it accumulates until it finds a cold surface to settle on. |
| Leaks and ingress Common | Roof leaks, plumbing failures, or water getting through the building envelope. Usually easier to identify than condensation-driven mould. |
| Rising damp Less common | Ground moisture wicking up through foundations or subfloor spaces. More of an issue in older homes without adequate subfloor ventilation. |
The fix isn't bleach
Treating mould with bleach or anti-mould paint and calling it done is the building equivalent of putting a sticking plaster over a crack in your foundation. It addresses the visual problem while leaving the underlying cause completely intact. Within weeks or months, you're back where you started.
Effective mould management means fixing what caused the moisture in the first place. That might mean improving insulation to eliminate cold surfaces where condensation forms. It might mean installing or upgrading ventilation — a heat recovery ventilation system in a tight house, or at minimum proper extraction in kitchens and bathrooms. It might mean fixing a leak, improving subfloor ventilation, or addressing a thermal bridge in the building envelope.
These aren't small interventions, and they're not cheap. But they're the only ones that actually work — and they come with benefits well beyond mould prevention. A better-insulated, better-ventilated home is warmer, healthier, cheaper to run, and less likely to develop the condensation and moisture problems that cause structural decay over time.
Our position, plainly stated
We think mould should be eliminated from buildings — not primarily because of the (contested and complex) health evidence, but because its presence is a clear signal that a building is failing to do its job. A home that can't manage moisture isn't protecting its occupants or its own structure. That's a problem regardless of which specific health outcomes we can or can't pin to mould exposure.
And for what it's worth: I have yet to read a negative review from someone living in a Passive House. Buildings that are properly insulated, airtight, and well-ventilated don't have mould problems — because they don't have the underlying conditions that allow mould to exist. That's the standard we should be working towards, even if most of us are starting from a long way behind it.
If you've got mould in your home and you're not sure where it's coming from or what to do about it, get in touch — we'd rather help you understand the cause than watch you repaint the ceiling for the third time.
