"Keep it below 50 % and you won't get mould" is the most repeated piece of humidity advice in existence, and it is not reliable.
A room held at a comfortable 20 °C and 50 % RH will have roughly 83 % relative humidity at the surface of an exterior wall sitting at 12 °C. That surface is above the threshold at which mould growth becomes a concern, while the room's humidity reading says everything is fine.
Mould does not grow in air. It grows on surfaces, and the condition at a surface can be very different from the condition in the room.
The Criterion Is at the Surface
Mould needs moisture, a nutrient source, and time. On any building material that can support growth, the moisture available is described by water activity — which, at equilibrium, is numerically the relative humidity of the air immediately in contact with that surface, divided by 100. A surface at 80 % relative humidity has a water activity of 0.80.
Most moulds relevant to buildings require a water activity somewhere in the region of 0.7–0.8 or above to become established, with some species able to grow at lower levels and others requiring more. It is not a sharp cut-off, which is why standards express it as a threshold plus a duration.
ANSI/ASHRAE Standard 160, Criteria for Moisture-Control Design Analysis in Buildings, sets the widely used form of the criterion:
The 30-day running average relative humidity at the material surface should be less than 80 %, when the 30-day running average surface temperature is between 5 °C (41 °F) and 40 °C (104 °F).
Two features of that wording carry most of the meaning.
"At the material surface." Not in the room. Not at the thermostat. At the surface where growth would occur.
"30-day running average." Mould growth is a process with a time constant, not an instantaneous event. Brief excursions do not establish growth; sustained conditions do. This is also why a single spot humidity reading tells you very little.
A recent addendum to the standard replaces the simplified 80 % criterion with an empirical mould index model that accounts for the material's sensitivity, the surface temperature and the surface relative humidity over time — a more accurate treatment reflecting that different materials behave differently under the same conditions.
Room Humidity Is Not Surface Humidity
The gap between the two is created by temperature, and it is larger than most people expect.
Air in contact with a cool surface is cooled toward that surface's temperature. Its absolute moisture content does not change, but its relative humidity rises, because cooler air is closer to saturation at the same moisture content.
Worked example:
Room air: 20 °C, 50 % RH
Saturation pressure at 20 °C: 2.334 kPa
Actual vapour pressure: 2.334 × 0.50 = 1.167 kPa
Cold wall surface: 12 °C
Saturation pressure at 12 °C: 1.400 kPa
Surface relative humidity = 1.167 / 1.400 = 83 %The room reads 50 %. The wall surface is at 83 %, above the ASHRAE 160 threshold, and it will stay there for as long as those conditions persist.
This is why mould appears in predictable places: exterior wall corners, behind furniture pushed against outside walls, inside cupboards on exterior walls, at thermal bridges, in unheated rooms adjoining heated ones, and behind anything that restricts air movement over a cool surface. All of these are locations where the surface is colder than the room, the air movement is poor, or both.
The same relationship, taken further, gives the condensation case: when the surface reaches the air's dew point, surface RH reaches 100 % and liquid water forms. The general treatment is in condensation risk and dew point.
Find the Moisture Source First
A humidity setpoint manages the consequences of a moisture load. It does not remove the load, and where the load is a defect, no setpoint will resolve it.
| Source | Indication | Correct action |
|---|---|---|
| Leak — roof, plumbing, envelope | Localised, may correlate with rain or usage | Repair. Dehumidification is temporary mitigation only |
| Rising damp / ground moisture | Low level, persistent, does not decline | Building fabric remedy — see moisture barriers for concrete floors |
| Residual construction moisture | Declines over time under drying conditions | Time and environmental drying |
| Occupant activity — cooking, washing, drying laundry | Follows daily pattern | Extraction at source, then dehumidification |
| Inadequate ventilation | General elevation, worse in winter | Ventilation, then dehumidification for what remains |
| Cold surfaces — thermal bridges, poor insulation | Localised at specific surfaces, worst in winter | Insulation or surface temperature; dehumidification helps but does not fix |
| Condensation from equipment or process | Localised at that equipment | Address at source |
The diagnostic question that separates most of these: does the problem correlate with weather, with occupancy, or with neither? Weather correlation points to the envelope. Occupancy correlation points to internal generation. Neither points to a defect or an ongoing source.
Running a dehumidifier against an unrepaired leak is a recurring cost with no end point.
Why One Universal Setpoint Does Not Work
Given the above, a single number applied everywhere fails for several reasons at once.
Surfaces differ within one space. The setpoint that keeps an interior partition below 80 % surface RH may not keep an exterior wall corner below it.
Temperature changes the relationship. The same room RH produces different surface RH as room and surface temperatures move through the day and the season. Winter is generally the demanding case, because surface temperatures are lowest.
Materials differ in susceptibility. Paper-faced products, some organic-based materials and surfaces carrying dust or organic soiling support growth more readily than clean inorganic surfaces. The newer ASHRAE 160 approach accounts for exactly this.
The requirement may not be mould at all. Where the concern is condensation, the specification should be a dew point relative to the coldest surface. Where it is a stored product's condition, it is that product's equilibrium moisture content. These lead to different targets.
Over-drying has costs. Running a lower setpoint than needed increases energy and runtime, and in occupied buildings very low humidity brings its own complaints. Where wooden building elements or contents are present, excessive drying can cause shrinkage and damage.
A defensible approach instead:
- Identify the coldest surface where growth is a concern, and measure its temperature under the demanding condition.
- Determine the room humidity that keeps that surface below the threshold at that temperature.
- Set the target from that calculation, with a margin for sensor tolerance and spatial variation.
- Verify at the surface, not only at the thermostat.
Placement and Air Movement
Equipment position determines whether the setpoint means anything at the locations that matter.
Dead zones stay humid. Corners, spaces behind furniture, inside cupboards and unventilated voids exchange little air with the room. A dehumidifier can hold the open room at target while these locations sit well above it — and these are exactly the locations where mould appears.
Air movement over cool surfaces helps twice. It reduces the local humidity at the surface by mixing, and it slightly raises the surface temperature by improving convective heat transfer from the room. Both move the surface RH down.
Simple measures matter. Moving furniture a few centimetres off an exterior wall, ventilating cupboards on exterior walls, and leaving internal doors open to unheated rooms all address specific dead zones at no equipment cost.
Sensor placement. A sensor in the middle of a well-mixed room reports the best conditions in the space. Where a specific location is the concern, that is where the measurement belongs.
Equipment clearance. Inlet and outlet need space to work; a unit against a wall or behind furniture recirculates its own discharge.
When the Setpoint Cannot Be Reached
If equipment runs continuously without achieving target, the cause is on a short list — and adding capacity is rarely the first answer.
- The load exceeds capacity, either because it was underestimated or because it has grown.
- An unaddressed source is supplying moisture continuously.
- Air exchange with a wetter space — an adjoining unheated area, a crawl space, or outdoor air — is replacing treated air.
- Space temperature is too low for refrigerant equipment to work effectively; below roughly 15–18 °C entering air, coil frosting costs capacity to defrost cycles. Cold spaces point toward desiccant equipment.
- Capacity has degraded — filters, coil, airflow. Comparing condensate per running hour against a baseline distinguishes this, as described in how long should a dehumidifier run.
- The target is unachievable for the equipment at the current conditions.
Verify at the Surface
Since the criterion is a surface condition sustained over time, verification has to match.
Log room temperature and relative humidity continuously, not spot readings. The 30-day running average in the criterion cannot be assessed from occasional checks.
Measure surface temperature at the locations of concern. An inexpensive infrared thermometer identifies the cold spots; a logging contact sensor on the worst one converts the assessment from inference to measurement.
Calculate surface RH from room conditions and surface temperature, using the relationship shown above. This is the number that should be compared against the criterion.
Check the demanding period. Winter, and any period when the space is unoccupied and unheated. Summer data will not reveal a winter problem.
Inspect the locations, not just the readings. Behind furniture, inside cupboards, in corners. Early growth is visible before it is extensive.
Where growth is already present, its assessment and remediation is a separate matter from humidity control, and should be handled according to the applicable guidance and by appropriate specialists. Controlling humidity prevents recurrence; it does not remediate existing growth.
Information to Prepare
- The space: dimensions, construction, insulation, heating, and whether it is occupied.
- Where growth has appeared, and when — with photographs and dates.
- Whether the problem correlates with weather, with occupancy, or with neither.
- Known or suspected moisture sources, including any defects.
- Logged room temperature and relative humidity, covering the demanding season.
- Surface temperatures at the locations of concern.
- Existing ventilation, heating and any dehumidification, and what has been achieved.
- Materials present at the affected surfaces.
- Available power supply, drainage and equipment positions.
- Any specification the space must meet, and its basis.
Discussing a Project
Yakeclimate designs and manufactures industrial dehumidification equipment for complex climate applications, with a focus on agriculture and energy projects.
We co-develop application-specific dehumidification equipment around the operating conditions, interfaces, and integration requirements of the wider project or system.
Building fabric investigation, defect diagnosis, and the assessment and remediation of existing mould growth remain with the appropriate building and remediation specialists. Our scope is the dehumidification equipment used to control the environment.
Explore industrial dehumidifiers, review desiccant rotor dehumidifiers, or contact Yakeclimate to review the operating conditions for your space.
FAQ
Frequently Asked Questions
Is keeping the room below 50 % RH enough to prevent mould?
Not reliably, because mould grows on surfaces and surface conditions differ from room conditions. A room at 20 °C and 50 % RH produces roughly 83 % relative humidity at the surface of a wall sitting at 12 °C — above the threshold at which growth becomes a concern, while the room reading looks safe. The relevant criterion is at the surface: ANSI/ASHRAE Standard 160 uses a 30-day running average surface relative humidity below 80 % when the 30-day running average surface temperature is between 5 °C and 40 °C.
How do I calculate the humidity at a cold surface?
Calculate the actual vapour pressure of the room air, then divide by the saturation vapour pressure at the surface temperature. Using es(T) = 0.61094 × exp(17.625 × T / (243.04 + T)) in kPa: room air at 20 °C and 50 % RH has an actual vapour pressure of 2.334 × 0.50 = 1.167 kPa; saturation pressure at a 12 °C surface is 1.400 kPa; so surface relative humidity is 1.167 / 1.400 = 83 %.
Why does mould appear in corners and behind furniture?
Because those locations combine two conditions: the surface is colder than the room — exterior corners and thermal bridges lose more heat — and air movement is restricted, so the humid boundary layer at the surface is not mixed away. Both raise the surface relative humidity above the room value. Moving furniture a few centimetres off exterior walls and ventilating cupboards on exterior walls addresses specific dead zones at no equipment cost.
Will a dehumidifier fix mould caused by a leak?
No. A dehumidifier manages the consequences of a moisture load; it does not remove a source. Running one against an unrepaired leak is a recurring cost with no end point. The diagnostic that helps: if the problem correlates with rainfall or with a particular plumbing use, it points to a defect; if it correlates with occupancy, it points to internal generation; if with neither, an ongoing source such as ground moisture is likely.
What setpoint should I actually use?
Derive it rather than adopt a number. Identify the coldest surface where growth is a concern, measure its temperature under the demanding condition — usually winter — and calculate what room humidity keeps that surface below the threshold at that temperature. Add a margin for sensor tolerance and spatial variation. Then verify at the surface rather than only at the thermostat. A setpoint lower than needed costs energy and runtime, and in spaces with wooden elements or contents, over-drying can cause its own damage.
My dehumidifier runs constantly and never reaches the setting. Why?
Common causes, in rough order: the load exceeds the equipment's capacity; an unaddressed moisture source is supplying moisture continuously; the space exchanges air with a wetter area such as a crawl space or unheated adjoining room; the space is too cold for refrigerant equipment to work effectively, with coil frosting below roughly 15–18 °C entering air; or capacity has degraded through a fouled filter or coil. Adding capacity is rarely the right first step — identifying which of these applies is.
References
- ANSI/ASHRAE Standard 160, Criteria for Moisture-Control Design Analysis in Buildings — 2021 addendum a (PDF)
- ASHRAE Standard 160 — criteria for moisture-control design analysis (2009 base document, PDF)
- Analysis of Improved Criteria for Mold Growth in ASHRAE Standard 160 by Comparison with Field Observations
