What humidity causes condensation on windows?
There is no single percentage. Condensation forms when the glass is colder than the dew point of the room air, so the humidity that causes it depends on how cold the window is. At 20°C indoors, 50% relative humidity gives a dew point near 9°C — so any pane colder than that will stream, while the same 50% causes nothing on a warmer, better-insulated window.
That is why two rooms at the same reading behave differently, and why there are two ways to fix it. Lowering the moisture lowers the dew point; warming the cold surface, or insulating it so its face sits warmer, removes the cold target. In an unheated room against an outside wall the second is usually the real fix — a dehumidifier there fights the weather every night.
Humidity Control Guide
Find your ideal indoor humidity and get equipment recommendations
Humidity Calculator
Results
Comfort Zone Chart
Recommended indoor relative humidity by outdoor temperature
Seasonal Humidity Guide
| Season | Outdoor Temp | Ideal Indoor RH | Key Concern |
|---|---|---|---|
| Winter | Below 20°F | 25-30% | Condensation on windows |
| Early Spring / Late Fall | 20-40°F | 30-40% | Static electricity, dry skin |
| Spring / Fall | 40-60°F | 35-45% | Mold prevention |
| Summer | Above 60°F | 40-50% | Mold, dust mites, comfort |
Health Effects of Humidity
Too Low (<30%)
- Dry skin, cracked lips
- Irritated sinuses and throat
- Increased static electricity
- Worsened asthma symptoms
- Damage to wood furniture
Too High (>60%)
- Mold and mildew growth
- Dust mite proliferation
- Musty odors
- Wood rot and structural damage
- Respiratory issues
Natural Humidity Control Tips
- Use exhaust fans in kitchens and bathrooms during and 30 min after use
- Fix all plumbing leaks promptly
- Ensure proper grading around foundation to direct water away
- Open windows when outdoor humidity is lower than indoor
- Use houseplants moderately — they release moisture
- Avoid drying clothes indoors without ventilation
- Keep gutters clean and downspouts directing water away from home
- Insulate cold water pipes to prevent condensation
Further reading
Interactive humidity control guide and calculator. Find ideal indoor humidity levels, sizing for humidifiers and dehumidifiers, and condensation risk assessment.
Related tools
Sources
Keeping indoor humidity down is a mould-control measure, not just a comfort one. The EPA explains what damp surfaces do and when growth starts.
Links checked 2026-09-09.
Frequently asked questions
What indoor humidity should I aim for?
It moves with the outdoor temperature rather than sitting at one number. The seasonal table on this page runs from 25–30 percent when it is below 20°F outside up to 40–50 percent in summer, and the reason winter is lower is condensation: air that feels comfortable indoors will wet cold window glass and start the damp that mould needs.
What goes wrong if the air is too damp?
Above about 60 percent you get mould and mildew, dust mites, musty smells, wood rot and respiratory irritation. Humidity control is a mould-control measure before it is a comfort one, which is why the EPA covers it under mould rather than under comfort.
Is drier always safer?
No. Below about 30 percent you get dry skin and cracked lips, irritated sinuses and throat, worse asthma symptoms, static, and damage to wooden furniture. Both ends of the range cause problems, so the target is a band rather than a direction.
What can I do without buying equipment?
Run the extractor fans in kitchens and bathrooms during use and for about thirty minutes afterwards, fix plumbing leaks, grade the ground away from the foundation so water drains off, open windows only when it is drier outside than in, and stop drying laundry indoors. Those are the measures on this page that cost nothing.
Reading relative humidity correctly
Relative humidity is a percentage of what the air could hold at its current temperature, not an amount of water. That is why the same air reads differently in two rooms: warm air can hold far more moisture, so cooling it raises the reading without adding a drop. It is also why a cold outside wall is where condensation appears first while the middle of the room feels fine.
Indoor guidance generally lands between 30 and 50 per cent through the heating season. Below that range, static and dry airways become common complaints; above it, condensation on glass and cold surfaces starts, and sustained high readings are what mould needs.
Dew point is the number that predicts condensation
Dew point is the temperature at which the air in a room would become saturated. Condensation forms wherever a surface is colder than that. This is the practical test: if the dew point indoors is 12°C and a window pane is at 8°C, that pane will stream regardless of how comfortable the room feels.
It follows that there are two ways to stop condensation, and only one of them involves a dehumidifier. Lowering the moisture lowers the dew point; warming the cold surface, or insulating it so its face sits warmer, removes the cold target. In an unheated room against an outside wall the second is usually the real fix.
Measuring it without being misled
Put the meter where people are, not on an outside wall and not above a radiator: both read the local microclimate rather than the room. Give any meter half an hour to settle after moving it, and compare two readings in different rooms before deciding the whole house has a problem — bathrooms and kitchens spike and recover, which is normal and not the same as a damp house.
A single reading means little. A reading taken at the same time each day for a week shows whether the room is actually wet or whether you measured it ten minutes after a shower.