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Module 1 · Moisture, Air and Materials

1.2 Relative Humidity Is a Ratio, Not an Amount

8 min read

Relative humidity is the most-quoted and least-understood number in damp surveying. It appears on every logger, in every report, and in almost every argument about whether a property is 'damp'. It is a ratio between the vapour pressure present and the vapour pressure that temperature could sustain — which means it changes when nothing about the water has changed at all. Reading RH as if it were a quantity of moisture is how a surveyor concludes that a cold room is wetter than a warm one when the opposite is true.

Learning objectives

By the end of this lesson you will be able to:

  • Distinguish relative humidity, absolute humidity, moisture content of air and dew point.
  • Explain how RH can rise while the amount of water in the air is unchanged or falling.
  • Interpret a logged RH trace correctly by reading it alongside temperature.
  • Choose the right humidity measure for the question being asked.

Key terms

Relative humidity (RH)
Actual vapour pressure as a percentage of the saturation vapour pressure at the same temperature. A ratio, and temperature-dependent by definition.
Absolute humidity
The mass of water vapour in a given volume of air, in g/m³. Independent of temperature.
Moisture content of air (mixing ratio)
Mass of water vapour per kilogram of dry air, in g/kg. The measure that stays constant as air is heated or cooled without gain or loss.
Surface relative humidity
The RH in the thin layer of air immediately against a surface, governed by that surface's temperature rather than the room's. What mould actually responds to.

Take air at 10°C and 90% RH — a cold, apparently very damp hallway. Its mixing ratio is about 6.9 g/kg. Now heat the same air to 20°C without adding or removing a single gram of water. The RH falls to about 47%.

Nothing was dried. No moisture left. The air is exactly as wet as it was, and by the measure that matters for how much water is present, it is unchanged. What changed is the denominator: 20°C air can sustain roughly twice the vapour pressure of 10°C air, so the same vapour is now a much smaller fraction of what is possible.

This is why heating alone appears to 'cure' condensation, and also why it does not remove the water — it moves it. Warm the room and the RH drops; the moisture is still there, and it will find the next surface cold enough to take it.

The four numbers, and which question each answers

  • Relative humidity — how close is this air to condensing at its own temperature? Use it for mould risk at a surface, and for comfort.
  • Mixing ratio (g/kg) — how much water is in this air? Use it to compare two spaces at different temperatures, and to decide whether ventilation would help.
  • Absolute humidity (g/m³) — how much water is in this volume? Use it for drying calculations and dehumidifier sizing.
  • Dew point — what surface temperature will condense this air? Use it directly against measured surface temperatures.

The one that answers 'is this room damper than that one' is the mixing ratio, and it is the one almost nobody logs. Most loggers record temperature and RH, from which it can be derived — but a report comparing raw RH between a heated living room and an unheated bedroom is comparing two different denominators and concluding something about the numerator.

Reading a logged trace properly

A month of half-hourly temperature and RH is the single most useful piece of evidence you can gather in a disputed case, and it is routinely read wrong. What to look at, in order:

  • Plot RH and temperature together. An RH trace that mirrors temperature inversely, with no independent movement, is a temperature story, not a moisture story.
  • Derive and plot the mixing ratio. Now the moisture-generation events show up as what they are — spikes when someone showers, cooks or dries washing.
  • Compare the internal mixing ratio with the external. Consistently higher inside means moisture is being generated indoors; roughly equal means the ventilation rate is high and the problem is elsewhere.
  • Look at what happens overnight. A mixing ratio that does not fall back overnight indicates the space is not clearing, which points at ventilation rather than generation.

That internal-minus-external mixing ratio difference — the moisture excess — is the number that actually distinguishes an occupancy problem from a building problem, and it is the number to put in the report. It is developed further in Module 4.

Why surface RH, not room RH, governs mould

Mould does not respond to the humidity of the room. It responds to the water activity at the surface it is sitting on, which is governed by the RH in the boundary layer against that surface — and that is set by the surface temperature.

A room at 20°C and 55% RH is comfortable and unremarkable. A single-glazed window reveal in that room at 9°C has a surface RH of essentially 100%. A thermally bridged corner at 13°C has a surface RH around 80% — enough for the more tolerant moulds to establish over weeks.

This is why mould appears in patterns rather than everywhere: it maps the cold spots. Read as a map of surface temperature, a mould pattern is one of the most informative things in a property, and Module 5 treats it that way.

It is also why 'keep the RH below 60%' is incomplete advice. Below 60% where? A room average of 55% with a 9°C reveal has a mould problem, and the occupant will be told they are causing it.

Common mistakes to avoid

  • Comparing raw RH between rooms at different temperatures and concluding one is wetter. Convert to mixing ratio first — the denominators differ.
  • Reading an RH trace without its temperature trace. Most of the movement in an RH plot is usually temperature, not moisture.
  • Quoting a room RH as evidence about mould risk when mould responds to surface RH, which the room figure does not describe.
  • Treating a fall in RH after heating as evidence the moisture has gone. Heating changes the ratio; it removes nothing.
  • Deploying loggers without an external reference. Without it the moisture excess cannot be calculated, and the most decisive number in the dataset is unavailable.

Lesson summary

Relative humidity is a ratio between the vapour present and the vapour that temperature could hold, so it moves when temperature moves and nothing else has. To ask how wet a space actually is, use the mixing ratio; to ask what will condense, use dew point against measured surface temperatures; to ask about mould, use surface RH, which is governed by surface temperature and not by the room. The moisture excess — internal mixing ratio minus external — is the number that separates an occupancy problem from a building problem.

Next: capillarity and pore structure — why two materials at the same moisture content behave completely differently.