Mould and Damp Doesn’t Take a Summer Holiday

Image from bobfrewin.co.uk

Why homes still get damp even when the sun comes out

If you ask most people when mould and damp are a problem, they'll say winter.

Cold walls, condensation on the windows, that unmistakable musty smell as soon as the heating goes on. It's a fair association — cold surfaces and warm, wet indoor air are a classic combination for condensation. But it's also given people a dangerous blind spot: the idea that once summer arrives, the risk goes away. It doesn't.

We recently monitored a real property through a week in late July — outdoor temperatures peaking at 26.6°C, and found conditions that would concern any surveyor at any time of year. No cold snap. No condensation on a freezing window. Just a home that stayed persistently, quietly humid, every single day, for the entire monitoring period.

Here's what that actually looked like, and why it happened.

Warm outside, humid inside, and moisture going nowhere

This was a mid-terrace flat, built around 1900, monitored for a week from 21–27 July. Outdoor conditions were genuinely warm: daytime highs ranged from 22°C to 26°C across the week, with the outdoor peak hit mid-afternoon on 23 July.

Indoors, though, humidity told a completely different story from what most people picture in “warm season”:

• Every single one of the 5 monitored rooms sat between 76% and 85% relative humidity — Kitchen 82%, Lounge 78%, Hallway 81%, Bedroom 76%, Bathroom 85%; sustained humidity where mould-risk conditions start to build

• The bathroom wall itself measured 86% RH on average, and was at or above 75% RH continuously for all 7 days of monitoring — not spiking after a shower and recovering, but sitting there, permanently.

• Mould-risk conditions were recorded as present for 99% of monitored hours in the bathroom, with a longest continuous run of over five days.

None of this was a cold-weather story. It was a ventilation and moisture-generation story, happening in the warmest part of the year.

A converted flat with no way to dry out

We got one of our partners to visit the property to get a floorplan and visually check how widespread the mould was, how severe and check whether our data aligned with reality. 

It was a mid-terraced house converted into a ground floor and 1^st floor flat with an small extension on the back for the bedroom.

It didn’t take much to see why our data was right. There's no working extractor fan in the bathroom, and no window in the kitchen. There were only 2 windows and two entry points into the home.

Our data showed there was no meaningful extraction happening at all in the bathroom and kitchen. 

The air outside couldn't help dry the home.

When we checked where the excess humidity was originating, the numbers pointed squarely at an internal source rather than humid weather blowing in. But here's the twist: the data was saying that even if there was perfect ventilation in the home, the humidity would still be above 55%.  The analysis was saying the building was in a ‘moisture laden’ area.  The visit to the property revealed there was a stream at the front of the house.

The high humidity wasn’t masked or explained away by heat. The home was on average 18°C - 20°C and no room showed a sustained overheating or underheating pattern. This home wasn't struggling because it was too cold. It was struggling because moist air had nowhere to go, in a spot where even the air outside doesn't offer much relief.

The myth, and why it's dangerous

The “Mould is a winter thing” myth persists because winter makes the mechanism visible. You can see your breath fog a cold window and understand immediately why. Summer humidity doesn't announce itself the same way. There's no dramatic condensation pooling on the glass, so it's easy to assume everything's fine.

But mould doesn't care what season it is. It cares about sustained relative humidity, ventilation, surface temperature, and time. A bathroom sitting at 85% RH for a week in July is arguably a bigger mould risk than the same room briefly touching high humidity after a shower in December, because in this case there was no daily recovery — no window of dry air to reset the clock.

Persistence is what turns “a bit steamy after a shower” into an actual mould problem, and persistence doesn't need cold weather. It needs blocked ventilation and nowhere for moisture to go, and both of those are entirely possible — arguably easier — in summer, when windows might stay shut for security, or (as here) the extraction that should be doing the job simply isn't there.

What this means practically

If you're only thinking about damp and mould when the heating comes on, you're only watching half the year. The questions worth asking apply just as much in July as in January:

• Do your kitchen and bathroom actually have a working way to get moist air out — an extractor fan that runs, or a window that opens?

• Does your home sit somewhere naturally humid — near water, in a valley, somewhere the outdoor air itself rarely gets genuinely dry — where “just open a window” won't be enough on its own?

• Is humidity in your home settling somewhere and staying there, rather than spiking briefly and clearing?

None of these questions are seasonal. Warm air can hold more moisture than cold air, which means a poorly ventilated home in summer can end up as humid, not less, than the same home in winter — just without the visible warning sign on the window pane to tell you.

Mould doesn't take a summer holiday. It can settle in quietly while everyone else is looking the other way.