Enter the air temperature, humidity and the temperature of the coldest surface in the room. The calculator will tell you the dew point, whether condensation is likely to form on that surface, and whether conditions are suitable for mould growth.
Enter your readings
Measure the coldest surface in the room — typically a window reveal, external corner, or north-facing wall.
A planning and triage tool. Does not replace calibrated instruments used on site. Results update automatically as you adjust the sliders.
What do these results mean?
Dew point is the temperature at which water vapour in the air begins to condense into liquid water. If the surface temperature of a wall, window or floor is at or below the dew point, condensation will form there.
Vapour pressure tells you the actual moisture load in the air. High vapour pressure relative to the outside means the building is retaining moisture — a sign of inadequate ventilation or high moisture production (cooking, washing, breathing).
Surface water activity (aw) is the key number for mould risk. Mould species begin to germinate at surface water activity above 0.70–0.75 aw. At 0.80 aw or above, active mould growth is likely regardless of the type of mould.
Frequently asked questions
Why is the coldest surface temperature the important reading?
Condensation forms on surfaces, not in the air. The coldest point in a room — often a window reveal, an external corner, or an uninsulated wall behind furniture — will reach the dew point first. That’s where you’ll see water droplets, mould growth, or tide marks first.
What’s a safe relative humidity level indoors?
BS 5250 and most UK guidance considers 40–60% RH at room temperature to be a healthy indoor environment. Above 65–70% sustained, condensation on cold surfaces becomes increasingly likely. Above 80% RH at a surface, mould growth is essentially certain over time.
My dew point looks safe but I still have mould — why?
A single reading is a snapshot. Dew point risk varies throughout the day and across the year — it’s typically worst in autumn and winter mornings, and after high-moisture activities like cooking, showering and drying laundry indoors. It’s also worth checking whether the mould is in a hidden cold spot (behind furniture, in a cupboard on an external wall) where surface temperatures are lower than your reading.
Does this replace a professional damp survey?
No. This tool gives you an indicative picture based on the readings you enter. A professional survey involves calibrated thermo-hygrometers, moisture meters, and an experienced surveyor who can identify the cause of damp — whether it’s condensation, penetrating damp, or rising damp — and recommend the appropriate remedy. It also produces a written report you can show to landlords, solicitors or mortgage lenders.
What is the BS 5250 standard referenced here?
BS 5250 is the British Standard code of practice for the control of condensation in buildings. It defines the psychrometric approach used to assess condensation risk — the same methodology this calculator uses (the Magnus-Tetens approximation for saturation vapour pressure).
Not sure what your readings mean?
Damp Detectives provides independent professional damp surveys for homeowners, landlords and home buyers. We take calibrated readings throughout your property, identify the cause of any damp, and give you a written report with clear recommendations — with no product to sell you.