Brits are being warned that a common cooling mistake during this week's returning 33C heatwave could leave the coolest room in the house damp, musty and at greater risk of mould.
Why Opening Windows at the Wrong Time Backfires
As temperatures climb across parts of the UK, many homeowners will instinctively open every window, door and garage entrance to “air the house out”. But Ryan Gorenflo, basement and home improvement expert at The Basement Doctor, says opening cooler spaces at the wrong time of day can backfire.
Cellars, basements, garages, utility rooms, pantries and lower-ground rooms are often much cooler than the rest of the house. When very warm air is allowed to rush into these spaces, moisture in that air can settle on colder walls, floors and surfaces.
Ryan warns this can leave rooms smelling musty, feeling clammy or showing condensation, even during a dry heatwave. Ryan said: “The coolest room in the house is not always the driest. If hot, humid air hits cold walls or floors, it can leave moisture behind. That can create musty smells, condensation and mould risk, especially in cellars, garages, basements and lower-ground spaces. The mistake people make during a heatwave is opening everything at the hottest part of the day because they think any fresh air is good air.”
The Heatwave Window Mistake to Avoid
Ryan says homeowners should avoid throwing open windows and doors in naturally cool rooms during the hottest part of the day. That includes cellar windows, garage doors, basement doors, utility room windows and lower-ground entrances. He explained: “If the air outside is hotter and more humid than the air inside, opening the room up may not dry it out. It can do the opposite. The warm air enters, hits cooler masonry, concrete floors, tiles or stored items, and moisture can settle. That is why a cellar or garage can suddenly smell damp after a very hot day, even if there has been no rain.”
Ryan says this does not mean rooms should be sealed up completely, but timing matters. He added: “Ventilation is important, but it needs to be controlled. In hot weather, the best time to air cooler spaces is usually early morning or later in the evening, when the outside air is cooler. Opening those rooms at peak heat can bring moisture in rather than remove it.”
Rooms Most at Risk
The rooms Ryan says homeowners should be most careful with are the ones that naturally stay cool, shaded or below ground level. This includes basements, cellars, garages, utility rooms, storage cupboards, lower-ground bedrooms, wine storage areas and rooms with concrete, stone or tiled floors. Ryan said: “These rooms often feel like a relief during a heatwave because they are cooler than the rest of the house. But that temperature difference is exactly why moisture can become an issue. Cold surfaces attract condensation when warm, moisture-heavy air reaches them. You may not see water running down the wall, but you might notice a musty smell, damp cardboard, clammy floors or a stale feeling in the room.”
Garages can be especially tricky because they are often poorly insulated and badly ventilated, while also being used to store cardboard boxes, tools, paint, soft furnishings, gym equipment and old furniture. Ryan added: “A garage can swing between very hot and surprisingly cool depending on the time of day, shade and ventilation. If people open the garage wide during peak heat, then close it again, any moisture that came in can sit with stored items. Cardboard, fabrics and timber are the first things to absorb it.”
Why This Matters During 33C Heat
Ryan says the warning is especially important when temperatures jump suddenly, as homes do not all warm up at the same speed. Upstairs bedrooms, loft rooms and conservatories may become unbearable quickly, while cellars and garages remain cooler for much longer. That makes people more likely to use them as cooling spaces or storage areas during extreme heat. Ryan said: “During a 33C day, people are looking for the coldest part of the home. They may open a cellar window, leave the garage door up, or move items into a cooler utility room. But if that cool room is also poorly ventilated, it can become a moisture trap. Heatwave advice often focuses on keeping living rooms and bedrooms cool, but homeowners should also think about what is happening in the spaces they do not sit in every day.” He says damp problems often begin quietly.
Ryan added: “By the time you see black mould, the moisture problem has usually been there for a while. A musty smell after hot weather is one of the early warnings homeowners should not ignore.”
Signs Your Cool Room Has Become Damp
Ryan says homeowners should check cool rooms after the hottest part of the day and again the following morning. Warning signs include a musty smell, condensation on pipes or windows, damp cardboard, soft boxes, clammy floors, peeling paint, swollen timber, rust on metal items, water marks on walls, mould spots or white powdery marks on masonry. He said: “If the room smells different after a hot day, pay attention. A musty smell means moisture is present somewhere. It might be in cardboard boxes, fabric, wood, walls or stored belongings. Do not just mask it with air freshener. Find where the moisture is coming from and how air is moving through the room.” Ryan says homeowners should also check behind stored items, especially where furniture or boxes are pushed against external walls. He explained: “Air needs space to move. If boxes, furniture or storage tubs are pressed tightly against a cold wall, that area can stay damp for longer. Pull items slightly away from walls and avoid storing cardboard directly on concrete floors.”
What to Do Instead
Ryan recommends keeping cooler rooms shaded during the hottest hours, then ventilating them when the outdoor air is cooler. He said: “During peak heat, keep the hot air out of the coolest rooms where possible. Use blinds, curtains or internal doors to reduce heat gain, but do not block air bricks, vents or any proper ventilation points. Then air the room early in the morning or later in the evening. The goal is not to trap stale air forever. The goal is to avoid dragging hot, moisture-heavy air into a colder space at the worst time of day.”
He also recommends removing damp-prone storage from garages, cellars and basements during heatwaves. Ryan said: “Cardboard boxes, soft furnishings, clothes, books, paperwork and wooden furniture are not ideal in damp-prone rooms. If the space already smells musty, those items can absorb moisture quickly.” For homeowners with a dehumidifier, Ryan says it can help manage moisture, but it should not be used as a substitute for fixing leaks, blocked vents or poor drainage. He added: “A dehumidifier can reduce moisture in the air, but it will not solve water coming through a wall, condensation caused by poor airflow, or a drainage problem outside.”
