Relative Humidity
The percentage of water vapour in air relative to the maximum the air can hold at that temperature. It determines comfort, condensation, and mould growth.
What is relative humidity and how do we measure it?
Relative humidity (RH) is the ratio of water vapour present in air to the maximum the air can hold at that temperature, expressed as a percentage. At any given temperature, air has a saturation point: the maximum water vapour it can contain before condensation begins. Warm air holds more moisture than cold air. At 25 °C, air holding 10 g/m³ of water vapour registers about 40% RH; at 15 °C, the same air parcel registers about 80% RH. This is why it is called relative: it depends on both moisture content and temperature. In buildings, relative humidity is measured with a digital hygrometer or hair-tension psychrometer. An RH reading of 50% means the air holds half the water vapour it can possibly hold before saturating.
Why does the comfort band sit at 40-60% relative humidity?
The comfort band for winter is 40-60% RH, based on occupant surveys and research on residential indoor climate. Below 30%, air feels dry: mucous membranes in nose and throat dry out, susceptibility to colds increases, and respiratory irritation occurs. Static electricity builds up. Above 65%, dust mites proliferate, mould grows, and air feels stuffy. Occupants prefer the 40-60% band because it balances physiological comfort, material durability, and perceived air quality. In summer, up to 65% RH is accepted because higher temperature partially offsets the discomfort. This commonly recommended range is supported by occupant satisfaction research across North American, European, and Scandinavian populations.
How does winter heating and airtight construction affect indoor humidity?
In a traditional, drafty house, outdoor air infiltrates around doors, windows, and leaks. Outdoor winter air in Slovakia is very dry (often 10-20% RH when temperature drops to minus 10 °C). This cold, dry air enters the house, is heated to 20-22 °C, raising its RH to perhaps 20-35%, and mixes with indoor air. In an airtight passive house or modern retrofit, infiltration is eliminated. Instead, all fresh air comes through controlled ventilation, often via MVHR (heat recovery ventilator). The MVHR exchanges heat between exhaust and intake air but cannot add moisture. Incoming winter air enters the house unhumidified. Indoor RH in well-sealed homes can drop to 20-35% by mid-winter, well below comfort range. Occupants experience dry throats, static electricity, and increased infection risk. Wooden doors and furniture shrink; plaster may micro-crack. The paradox is sharp: the more energy-efficient the building, the drier the winter air becomes, unless humidification is added or occupants accept the discomfort.
What is the relationship between relative humidity and condensation risk?
Condensation forms on a surface when that surface temperature falls below the dew point. The dew point is a function of both air temperature and relative humidity. At 20 °C and 60% RH, the dew point is around 11.7 °C. If a window pane or wall cools to 11 °C or lower, water condenses on that surface. The risk escalates with higher RH. At the same temperature drop, 80% RH dew point is around 16 °C, so condensation begins earlier as RH rises. This is why dew point and RH are inextricably linked in building physics: you must track both to predict whether a given surface will wet. A bedroom at 70% RH, 20 °C has a dew point of about 14 °C. If the window frame gets down to 14 °C on a winter night (common in poorly insulated frames), condensation forms reliably.
How do surface condensation and mould growth relate to relative humidity?
Mould does not require standing water or visible condensation. It grows when RH at a surface exceeds approximately 80% for sustained periods (hours to days). Surface condensation appears when air temperature drops below dew point and water droplets form; that is the visible sign. But mould colonization often begins before condensation is visible, in the form of microscopic spore germination in the film of moisture between 70 and 85% RH. By the time a household notices water droplets on a wall, the mould problem has usually been developing for weeks, especially in corners, behind wardrobes, and on north-facing exterior walls where air circulation is poor. This is why building standards and hygiene guides flag RH above 65% as a risk zone and above 80% as a danger zone. Mould growth risk is thus both a direct function of RH and an indirect function of surface temperature, since colder surfaces develop condensation and elevated RH sooner.
| Indoor RH Range (%) | Winter Comfort | Health & Material Risk | Mould / Dust Mite Risk |
|---|---|---|---|
| <30% | Too dry, irritation | Respiratory irritation, static shock | None |
| 30–40% | Acceptable but dry | Slight irritation, wood shrinkage | Very low |
| 40–60% | Optimal comfort | Minimal risk to occupant or materials | Low |
| 60–70% | Acceptable; slightly humid | Dust mite proliferation begins | Moderate (condensation on cold surfaces) |
| >70% | Muggy, uncomfortable | Mould, dust mites, material swelling | High (visible condensation likely) |
How do ventilation strategies control relative humidity in residential design?
Several approaches exist. Natural cross-ventilation (opening windows) dilutes indoor moisture but loses heat and may introduce pollution. MVHR supplies fresh air while recovering heat, but unless equipped with humidity recovery (desiccant or counter-flow core), the outgoing moisture is lost and RH drops. Humidity-recovery cores can transfer 50-80% of moisture from exhaust to intake. In renovations, adding mechanical ventilation improves control over fresh air entry compared to random infiltration. Design teams typically target 35-40% RH in winter (accepting slight dryness for mould prevention) and maintain below 60% in summer. Extract fans during showers and cooking quickly remove moisture peaks. Hygroscopic materials (wood, plasterboard, natural insulation) can dampen RH swings but cannot control absolute humidity alone.
| Ventilation Method | Winter RH Control | Heat Loss | Moisture Recovery |
|---|---|---|---|
| Natural infiltration (old houses) | Very dry (15–30% RH) | High energy loss | None; moisture exits via leaks |
| Window opening (manual) | Rapid drying, user-dependent | Significant heat loss per open window | Depends on duration and frequency |
| MVHR without humidity recovery | Moderate drying (25–40% RH) | Minimal; heat recovered | Water vapour exits with exhaust |
| MVHR with humidity recovery core | Less dry (35–50% RH possible) | Minimal; heat and some moisture recovered | Partial; 50–80% of moisture transferred |
| MVHR plus humidifier | Controlled (can hold 45–60% RH) | Minimal; heat recovered | Humidifier adds back moisture to target |
What common misconceptions cloud understanding of relative humidity in homes?
Many occupants confuse absolute humidity (water mass per cubic metre) with RH (percentage). Air with 8 g/m³ of water feels dry at 22 °C (about 50% RH) but damp at 15 °C (about 75% RH). The water content is identical; perceived dampness reflects temperature alone. Another error: opening windows in winter adds moisture. Cold outdoor air holds little water; when heated indoors, its RH drops further, drying the house. A third misconception: insulating a wall eliminates condensation risk. Insulation raises surface temperature but does not lower indoor RH. A poorly ventilated, humid room with a cold corner still condenses. The key is the gap between air RH and saturation at that surface temperature. Finally, many assume thermal comfort depends only on air temperature. In fact, humidity is a significant independent factor. A room at 22 °C and 25% RH feels different from 22 °C and 60% RH, even with identical air temperature.
Frequently asked questions
- What is the comfortable range of relative humidity indoors?
- Commonly recommended guidance for residential comfort is 40-60% RH in winter and 30-65% in summer. Below 30%, air feels dry and irritates respiratory passages. Above 65%, mould and dust mites thrive. This band is supported by occupant surveys and research across European and North American homes.
- Why does MVHR reduce humidity in winter?
- Mechanical ventilation with heat recovery (MVHR) exchanges indoor and outdoor air. In winter, outdoor air is cold and holds very little water. MVHR recovers the heat from exhaust air but cannot add moisture to the incoming fresh air. The result is dry indoor air, often 20-35% RH, which requires either humidification or acceptance of discomfort and health effects.
- Does high humidity alone cause mould, or do you need condensation?
- Mould does not require visible condensation. It grows when relative humidity at a surface exceeds 80% for sustained periods. Condensation appears when air temperature drops below the dew point, and that is a sign the problem has already begun. Most mould growth occurs before any visible water forms.
- Can you reduce relative humidity by heating a room?
- Heating alone does not remove water from the air; it reduces RH by diluting the same amount of moisture across a larger air volume. A room heated from 18 to 22 °C with the same moisture content drops from 65% to about 50% RH. To actually lower humidity, you must either ventilate to exhaust moist air or add moisture-absorbing materials.
- How does insulation and airtightness affect indoor humidity?
- Airtight construction prevents outdoor air infiltration, so the water content indoors depends entirely on occupant activity (showers, laundry, cooking) and controlled ventilation. In winter, outdoor air is dry. If MVHR does not humidify incoming air, the indoor RH stays low. In summer, airtight buildings can trap moisture if ventilation is limited.
- What is the link between relative humidity and the dew point?
- Dew point is the temperature at which air becomes saturated and water condenses. Relative humidity and temperature together determine where the dew point sits. At 50% RH and 20 °C, the dew point is about 9.3 °C. If a wall surface is colder than that, condensation forms regardless of how much air volume surrounds it.