Quick answer: Set the dehumidifier to about 50 % relative humidity. The US EPA advises keeping indoor humidity below 60 %, ideally between 30 and 50 % (US EPA). Go lower, to about 40–45 %, in rooms with cold outside walls, basements or stored goods, because mold grows on the coldest surfaces, where humidity is much higher than at the sensor.
Quick answer
- Normal rooms: 50 % RH setpoint.
- Cold walls, basements, warehouses with stock against walls: 40–45 % RH.
- Dry anything wet within 24–48 hours (US EPA). A dehumidifier cannot dry a leak that keeps leaking.
Why 50 % is the usual setting
Mold needs moisture at the surface it grows on. Above about 60 % room RH, surfaces that are a few degrees cooler than the air can reach the humidity mold needs. Keeping the room at 50 % gives a margin, and it is also low enough to discourage dust mites, without over-drying people or wood.
Why mold still grows at 50 %: cold surfaces
The humidity a surface sees depends on its temperature. When a wall is colder than the room air, the air next to it has a higher relative humidity than the room sensor shows.
| Room air | Wall surface temperature | Relative humidity at the wall surface |
|---|---|---|
| 20 °C, 50 % RH | 20 °C (warm inside wall) | 50 % |
| 20 °C, 50 % RH | 15 °C (outside wall, mild weather) | about 69 % |
| 20 °C, 50 % RH | 12 °C (cold corner, behind furniture) | about 84 % |
| 20 °C, 60 % RH | 12 °C | about 100 %: condensation |
Values are calculated from the saturation vapor pressure of water. The 2009 edition of ASHRAE Standard 160 used a 30-day running average surface RH of 80 % (at surface temperatures between 5 and 40 °C) as its design criterion for limiting mold growth; later editions use a mold-growth model. Both judge the surface, not the room average.
What to do: lower the setpoint in rooms with cold walls, keep furniture and stock a few centimetres away from outside walls so air can reach them, and insulate cold surfaces where you can.
Recommended setpoints by space
| Space | Suggested setpoint | Why |
|---|---|---|
| Living spaces, offices | 45–50 % RH | Comfort and mold prevention |
| Basements and crawl spaces | 45–50 % RH, lower in cold seasons | Cold walls and floors |
| Warehouses with cardboard, textiles, paper | 45–50 % RH | Packaging absorbs moisture and grows mold |
| Archives and libraries | Follow the collection's conservation standard, typically held steady near 40–50 % | Paper and bindings react to swings |
| Indoor pools | 50–60 % RH, with air 1–2 °C warmer than the water (ASHRAE guidance for natatoriums) | Lower RH increases evaporation from the pool |
| After a flood or leak | As low as the unit can reach until materials are dry | Dry materials within 24–48 hours |
Five checks if mold keeps coming back
- Find the water source. Leaks, rising damp, condensation on cold pipes and wet cleaning all add water faster than a dehumidifier removes it. Fix the source first.
- Check the unit reaches the setpoint. If it runs all day without reaching 50 %, it is undersized or the room is too cold for a refrigerant unit.
- Move air. A dehumidifier only dries the air that reaches it. Use the unit's discharge or a fan to move air across walls and corners.
- Measure surfaces. An infrared thermometer and a hygrometer let you calculate the dew point and see which surfaces are close to it.
- Clean and dry. Remove existing mold before relying on humidity control; the US EPA's mold guide explains cleanup.
Choosing a dehumidifier for mold control
- Warm spaces (above about 15–18 °C): a refrigerant dehumidifier is the efficient choice. Our industrial dehumidifiers run on continuous drainage from 60 to 960 L/day.
- Cold spaces (unheated stores, winter basements): a desiccant dehumidifier keeps working where refrigerant coils ice.
- Large or multi-room spaces: a ceiling-mounted or ducted unit distributes dry air without taking floor space.
- Size it right: see how much water a dehumidifier collects in a day and our sizing guide.
FAQ
Frequently asked questions
What humidity level prevents mold?
Below 60 % indoor relative humidity, ideally 30–50 % (US EPA). Keep cold surfaces in mind: they can be far more humid than the room.
Should a dehumidifier be set to 40 or 50?
50 % for most rooms. Use 40–45 % where walls are cold or mold has appeared before, as long as people are comfortable and wood furniture or instruments are not affected.
Can humidity be too low?
Yes. Below about 30 % people feel dry eyes and throats, static increases, and wood can crack. Most buildings do not need to go below 40 %.
Should I run the dehumidifier all the time?
Run it on its humidistat. It will switch on and off to hold the setpoint, so it only uses energy when needed.
Does a dehumidifier kill mold?
No. It stops new growth by removing the moisture mold needs. Existing mold must be cleaned off or the material replaced.
Sources
- US EPA, A Brief Guide to Mold, Moisture and Your Home.
- ANSI/ASHRAE Standard 160, Criteria for Moisture-Control Design Analysis in Buildings.
- ASHRAE Handbook—Fundamentals, Psychrometrics.
