January and February are the most damaging months of the year for garden offices, studios and other garden rooms, yet many homeowners won’t realise there is a problem until spring.
Cold temperatures, persistent rainfall and high humidity create the perfect conditions for moisture to become trapped inside garden buildings. With shorter days and limited opportunities for natural drying, condensation can build up unnoticed, often behind furniture, along window frames or at the base of walls, allowing mould and damp to take hold long before any visible signs appear.
According to Christopher Murphy MBE, founder of Dunster House, garden building specialists, winter weather rarely causes issues on its own, but it can quickly expose weaknesses in how a garden building was installed or ventilated: “Winter doesn’t usually create problems from scratch, but it does make existing ones much worse,” he says. “Once moisture is present, cold conditions and poor airflow allow damp to linger, turning what might have been a minor issue into visible mould or even rot by the time spring arrives.”
Early warning signs many homeowners miss
Murphy says the first indicators of a moisture problem are often subtle and easy to overlook: “A musty smell, light condensation on windows, darkening timber near the base or patches that feel damp to the touch are often early warning signs. If these are ignored through winter, damage can become much more difficult and expensive to fix later.”
A simple test foil to tell if damp is structural or condensation
For homeowners unsure where moisture is coming from, Murphy suggests a quick at-home check.
Tape a small square of foil or cling film to an internal wall overnight:
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If moisture forms behind the foil, the moisture is likely coming through the structure.
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If moisture appears on the front, condensation is the main issue.
“This simple test can help homeowners understand whether they need better ventilation or professional help,” Murphy says.
Murphy adds that winter is also the best time to assess whether a garden building is suitable for year-round use: “If damp or mould is appearing now, it’s a sign something needs attention. Addressing it during winter can prevent long-term damage and help ensure the space is comfortable, dry and usable when the warmer months return.”
7 simple winter checks to protect your garden room
1. Check drainage around the base
Waterlogged ground allows moisture to sit against the structure for long periods. Clear leaves and debris, remove pooled water and ensure the ground slopes away from the building to help keep timber dry.
2. Clear gutters and run-offs
Blocked gutters can cause rainwater to overflow down walls and collect at ground level. Keeping them clear helps surfaces dry more quickly after wet weather.
3. Inspect exposed timber
Look for dark staining, cracking or soft patches, particularly near ground level. Untreated timber should be protected with a suitable outdoor treatment to help prevent moisture penetration.
4. Check the roof covering
Small gaps, loose panels or worn felt can allow moisture in without being obvious. Regular winter checks help prevent rising humidity inside the building.
5. Improve airflow where possible
Poor ventilation is one of the main causes of winter mould. On dry days, open doors or windows briefly, keep vents clear and avoid sealing the space completely. In garden offices that are used year-round having an extractor fan installed can help manage moisture.
6. Store items to allow air to circulate
Avoid storing contents of garden buildings tightly against the interior walls. Items should be fully dry before storing and raised slightly off the floor to allow airflow and prevent damp spots forming behind them.
7. Two-finger furniture rule
Ensure there is a gap of approximately two finger widths between the furniture and the external walls.
Large items, such as cupboards, desks, sofas, or storage units, placed directly against external walls can block airflow and trap moisture, creating ideal conditions for condensation and mould to develop unnoticed. This is especially common in winter, when walls are colder, and humidity levels are higher.
By leaving even a small gap, air can circulate more freely around the furniture, helping moisture evaporate instead of settling on cold surfaces. This simple adjustment can significantly reduce the risk of damp patches forming behind furniture and prevent mould from taking hold in areas that are rarely seen or cleaned.
This is one of the most overlooked causes of winter mould in garden rooms, particularly in home offices and studios, where furniture layouts are rarely changed once set up.