Relative humidity (RH) and absolute humidity (AH): what they mean and how to compute them
Outdoor air at −5 °C and 90 % RH holds only 3.1 g/m³ of water (2.3 g/kg of dry air); warmed to 23 °C, the same air is at 14 % RH. That is why a “humid” winter day gives a dry house: relative humidity (RH) says how close the air is to saturation at its temperature, absolute humidity (AH) says how much water it actually holds. Below: a converter (temperature and % RH → g/m³, g/kg, dew point and the humidity after warming), the saturation curve chart, a table of typical cases, and what the numbers mean for a house and for ventilation (HRV or ERV).
Converter: relative humidity to absolute humidity
Chart: how much water air can hold
Below 30 % RH: dry air. You feel it in the throat and eyes; ventilate less before you humidify.
The 100 % RH curve shows how much water air can hold at each temperature; the 70, 50 and 30 % RH curves are fractions of it. The blue dot is the outdoor air; the arrow takes it indoors, to the indoor temperature, without changing its water (a horizontal line), and the yellow dot shows its relative humidity there. The open circle on the 100 % RH curve is the dew point.
Enter the outdoor temperature and relative humidity and the indoor temperature. The converter gives the water in the outdoor air (g/m³ and g/kg), its dew point, and the relative humidity the same air has once it comes indoors and warms up: its water stays the same, only the temperature changes. Air pressure is fixed at 101,325 Pa, sea level.
What relative humidity (RH) is
What is relative humidity
Relative humidity (RH) is the ratio of the water-vapour pressure in the air to the saturation pressure at the same temperature, in per cent: how much of the water the air could hold it actually holds. At 100 % RH the air is saturated and any extra water comes out as liquid (dew, fog, condensation). Here RH is always written with “% RH” after the number, so it is not confused with other percentages.
Relative humidity depends on temperature
Warm air can hold far more water than cold air: at 0 °C at most 4.8 g/m³; at 20 °C, 17.2 g/m³; at 30 °C, 30.3 g/m³. The same water therefore gives a high relative humidity in cold air and a low one in warm air. Warm the air without adding water and RH falls; cool it and RH rises, up to 100 % RH at the dew point. A hygrometer reading “90 % RH” outdoors in winter says nothing about how dry the house will be.
What absolute humidity (AH) is
What is absolute humidity
Absolute humidity (AH) is the mass of water in a cubic metre of air, in g/m³. Engineers and PHPP (the Passive House planning tool) use a related quantity, the mixing ratio x: grams of water per kilogram of dry air, in g/kg. For indoor air the two numbers are close (a cubic metre of air weighs about 1.2 kg): at 23 °C and 50 % RH, 10.3 g/m³ or 8.7 g/kg.
Relative vs absolute humidity: the difference
Absolute humidity does not change when air is only warmed or cooled: its water stays the same, until it is cooled below its dew point and water starts to condense. Relative humidity changes with every degree. Strictly, the mixing ratio x stays constant; the g/m³ figure drops a little on warming because warm air takes up more room (air at −5 °C has 3.1 g/m³, brought to 23 °C it has 2.8 g/m³, with the same water). That is why the chart below is drawn in g/kg: warming is a horizontal line.
Dew point and condensation on cold surfaces
How to calculate the dew point
The dew point is the temperature at which air, cooled without losing water, reaches 100 % RH. It comes from the Magnus formula solved the other way round (the formula is under Method). For indoor air at 23 °C and 50 % RH the dew point is 12.0 °C; at 23 °C and 40 % RH, 8.7 °C; for outdoor air at −5 °C and 90 % RH, −6.4 °C.
Condensation on windows and thermal bridges
Any surface in the house colder than the air’s dew point gets water on it: the window pane, the corner of an outside wall, an uninsulated concrete lintel (a thermal bridge). Mould comes earlier, with no visible water: when the relative humidity right at the surface stays above 80 % RH for long. That is the criterion used by the Passive House Institute and by the German standard DIN 4108-2 (Passipedia, thermal bridges). For air at 23 °C and 50 % RH, a surface reaches 80 % RH below 15.5 °C. DIN 4108-2 does the same sum with 20 °C and 50 % RH indoors and −5 °C outdoors: the surface must stay above 12.6 °C, hence the temperature factor fRsi ≥ 0.70.
Humidity in typical cases
Six everyday cases, with the water in the air and the dew point, and the relative humidity of the same air brought to 23 °C (the indoor temperature in winter). The Bucharest January mean (−0.4 °C and 88 % RH) comes from the site’s monthly climate data. The converter’s “Typical cases” buttons load the outdoor air from the table, plus Brașov in January.
| Case | The air | g/m³ | g/kg | Dew point, °C | At 23 °C |
|---|---|---|---|---|---|
| Outdoors in winter | −5.0 °C · 90 % RH | 3.1 | 2.3 | −6.4 | 14 % RH |
| Bucharest, January mean | −0.4 °C · 88 % RH | 4.1 | 3.2 | −2.1 | 19 % RH |
| Indoors in winter | 23.0 °C · 40 % RH | 8.2 | 7.0 | 8.7 | 40 % RH |
| Indoors, humid | 23.0 °C · 55 % RH | 11.3 | 9.6 | 13.5 | 55 % RH |
| Outdoors in summer | 30.0 °C · 60 % RH | 18.2 | 16.0 | 21.4 | 91 % RH |
| Bathroom after a shower | 25.0 °C · 90 % RH | 20.7 | 18.0 | 23.2 | 100 % RH, condenses |
Summer air (30 °C, 60 % RH) brought to 23 °C would have 91 % RH: that is why air conditioning dries it, condensing the water on its cold coil. Bathroom air after a shower has its dew point at 23.2 °C, above 23 °C: the mirror and the cooler tiles get wet.
What humidity means for a house
Ideal indoor humidity in winter
Between 30 and 60 % RH. Below 30 % RH air feels dry (Passipedia, air volumes); a reference medical review (Arundel et al., 1986) finds the fewest adverse effects (viruses, bacteria, dust mites, mould, allergies) between 40 and 60 % RH. Above 60 % RH dust mites and the mould risk on cold surfaces grow. At 23 °C, 40 % RH means 7.0 g/kg and 55 % RH means 9.6 g/kg.
Why indoor air is dry in winter
Because the fresh air brought in from outside holds little water. In January in Bucharest the air holds on average 3.6 g/kg (hourly mean of the PVGIS typical year); warmed to 23 °C with no other water it would be at 21 % RH. The water comes from the house: people, cooking, showers, plants. The Passive House Institute counts 330 g/h for 4 persons. With 120 m³/h of fresh air (the 150 m² house of the heat recovery ventilation calculator), the balance gives 34 % RH in January; with twice the flow, 240 m³/h, only 27 % RH.
Over-ventilation dries the air
More fresh air than needed means more water carried outside. The PHI remedy for air that is too dry, before a humidifier and before moisture recovery, is to lower the flow towards 20 m³/h per person.
Why windows get condensation
Because the window surface is colder than the dew point of the indoor air. With 23 °C and 50 % RH inside (dew point 12.0 °C) and −5 °C outside, the room side of single glazing (U ≈ 5.8 W/(m²·K)) sits at 1.9 °C: certain condensation. An old double-glazed unit (U ≈ 2.8) reaches 12.8 °C, borderline, and lower at the edges near the frame. Modern double glazing (U ≈ 1.1) sits at 19.0 °C, Passive House triple glazing (U ≈ 0.6) at 20.8 °C: far from the dew point. A simple centre-of-pane calculation with a surface resistance of 0.13 m²·K/W.
What it means for ventilation: HRV and ERV
What 78 % moisture recovery means on an ERV
An ERV (energy recovery ventilator, with a membrane core) returns part of the extract air’s water to the fresh air; an HRV (plain heat recovery ventilator) returns only heat. The moisture recovery figure (ηx) is defined on the absolute humidity difference: ηx = (xsupply − xoutdoor) / (xextract − xoutdoor). “78 %”, the PHI figure of the Zehnder ComfoAir Q350 ERV, means 78 % of the difference between indoor and outdoor air, not 78 % of the water indoors. With 3.6 g/kg outdoors in January and 5.9 g/kg indoors (34 % RH at 23 °C), the ERV returns 30 % of the extract air’s water.
HRV or ERV in Romania’s climate
For a normally occupied house, the water an ERV returns is too much in winter. The 150 m² house with 4 persons of the heat recovery ventilation calculator, in Bucharest, at 120 m³/h: with an HRV 34 % RH in January, already in the healthy range; with a 78 % ERV at the same flow, 80 % RH. That is why PHI requires units with more than 60 % moisture recovery to raise their flow on indoor humidity, holding the extract air at 55 % RH (PHI certificates), and says that with average occupancy the energy balance uses the raised flow. For the same house the ERV holds 55 % RH, but in 5 months its flow rises up to 270 m³/h: 182 kWh of extra heat loss and 112 kWh of extra fan electricity a year. The humidity gain is small and costs heat; an ERV makes sense where the air really comes out dry (few people in a large house, long cold spells). All the numbers, room by room and unit by unit, in the heat recovery ventilation calculator; see also the other calculators.
Relative and absolute humidity, in short
What is relative humidity?
The ratio of the water vapour in the air to what the air could hold at the same temperature, in per cent (% RH). It depends on temperature: air at −5 °C and 90 % RH, warmed to 23 °C, is at 14 % RH.
What is absolute humidity?
The mass of water in a cubic metre of air, in g/m³. Air at −5 °C and 90 % RH holds 3.1 g/m³; indoor air at 23 °C and 50 % RH holds 10.3 g/m³. Ventilation calculations use the mixing ratio x, in g/kg of dry air.
What is the difference between relative and absolute humidity?
Absolute humidity says how much water the air holds; relative humidity says how close it is to saturation at its temperature. Warmed or cooled, air keeps its water (absolute humidity), but its relative humidity changes with every degree.
How do you calculate the dew point?
With the Magnus formula solved the other way round: γ = ln(RH/100) + 17.625·t/(243.04 + t), then tdew = 243.04·γ/(17.625 − γ). For 23 °C and 50 % RH it gives 12.0 °C; for −5 °C and 90 % RH, −6.4 °C.
What is the ideal indoor humidity in winter?
Between 30 and 60 % RH, best 40–60 % RH. At 23 °C that is about 5.2–10.5 g/kg of water in the air.
Why is indoor air so dry in winter?
Because cold outdoor air holds little water: 3.1 g/m³ at −5 °C and 90 % RH. Warmed to 23 °C it reaches 14 % RH. If you ventilate much more than needed, the water produced indoors is not enough to moisten it.
Why do windows get condensation on the inside?
Because the glass is colder than the dew point of the indoor air (12.0 °C at 23 °C and 50 % RH). Single glazing is at 1.9 °C on the inside when it is −5 °C outside; triple glazing at 20.8 °C.
What does 70 % moisture recovery on an ERV mean?
That the fresh air receives 70 % of the absolute humidity difference between extract and outdoor air, not 70 % of the water indoors. For Bucharest winter air, a 78 % ERV returns about 30 % of the extract air’s water. PHI requires units above 60 % to raise their flow so the indoor air stays at or below 55 % RH.
How humid is a bathroom after a shower?
About 25 °C and 90 % RH: 20.7 g/m³, with its dew point at 23.2 °C. Any surface below that temperature fogs up; the bathroom extract removes the water within a few tens of minutes.
Method and sources
The Magnus formula over water, with the Alduchov and Eskridge (1996) coefficients, the same as in the ventilation calculator; air pressure fixed at 101,325 Pa.
Saturation pressure
ps(t) = 610.94 · exp(17.625 · t / (t + 243.04)) Pa, t in °C; over water also below 0 °C, as relative humidity is reported in meteorology.
Source: Alduchov and Eskridge, 1996
Water in the air
pv = RH/100 · ps(t); x = 622 · pv / (101,325 − pv) g/kg; AH = pv / (461.5 · (t + 273.15)) · 1,000 g/m³.
Source: Magnus
Dew point
γ = ln(RH/100) + 17.625 · t / (243.04 + t); tdew = 243.04 · γ / (17.625 − γ).
Source: Magnus
At another temperature
x stays the same; RH(t₂) = pv / ps(t₂) · 100, at most 100 % RH; beyond that the water condenses. Mould: the surface at which RH reaches 80 % RH.
Source: Passipedia · DIN 4108-2
Checks
| Check | Reference | This page |
|---|---|---|
| Saturation at 20 °C | 17.3 g/m³ · 14.7 g/kg | 17.2 g/m³ · 14.7 g/kg |
| 23 °C and 50 % RH | dew point 12.0 °C | 12.0 °C |
| Passipedia: −5 °C and 90 % RH warmed to 20 °C | ≈ 3 g/m³ · 17.6 % RH | 3.1 g/m³ · 16 % RH |
Passipedia reaches 17.6 % RH by dividing the grams per m³ (3.1 g/m³ outdoors by 17.2 g/m³ at saturation). With the vapour pressure, which allows for warmed air taking up more room, the same air is at 16.3 % RH at 20 °C and 13.5 % RH at 23 °C. This page uses the vapour pressure, as the ventilation calculator does.
Limitations
- Pressure is sea-level pressure; at 600 m altitude (Brașov) the air pressure is about 7 % lower and x is higher by about as much for the same RH.
- Below 0 °C saturation over water is used; over ice it is slightly lower, and the dew point becomes a “frost point”.
- Window temperatures are a simple centre-of-pane calculation; edges and frames are colder.
- The HRV / ERV numbers come from the ventilation calculator: steady monthly balance, the 150 m² house with 4 persons, Bucharest.
Sources
- Alduchov and Eskridge (1996), Improved Magnus form approximation of saturation vapor pressure, J. Appl. Meteor. 35, 601–609
- Passipedia, air volumes and humidity
- Passipedia, thermal bridges and the 80 % RH criterion
- Passive House Institute, criteria for ventilation units
- PHI component database, certified moisture recovery
- Arundel et al. (1986), Environ. Health Perspect. 65, 351–361
- PVGIS, typical climate year
Calculation and text: SEMPER SUPRA INITIATIVE S.R.L. · Climate data: PVGIS © European Union, 2001–2026.