Glossary · Insulation & windows

Relative humidity (RH)

Also called: indoor humidity · humidity level · humidity levels

How much moisture the air holds compared with the most it could hold at that temperature, shown as a percentage.

What it means

Warm air can hold more moisture than cold air, so the same amount of water vapor gives a lower RH once air is heated. An inexpensive hygrometer measures it. Too high encourages mold, dust mites and condensation; too low can feel uncomfortably dry.

Why it matters in an audit

High indoor humidity is a warning sign of poor ventilation or a moisture problem, and sealing a house tighter can raise it, so auditors check RH and may recommend an HRV, ERV or dehumidifier.

Typical numbers

Reference values with their sources. Your auditor’s report gives the figure for your own house.

What	Value	Source
US EPA	Keep indoor RH below 60%, ideally 30%–50%	epa.gov ↗

Sources

Learn more here

Related terms

Dew point The temperature at which air becomes saturated and water starts to condense on surfaces, like water beading on a cold glass.
Condensation Water that forms when humid air touches a surface colder than its dew point, on windows, cold pipes or hidden inside walls and attics.
Mold Fungi that grow on damp surfaces indoors; they can damage materials and affect health, and moisture is always the root cause.
Dehumidifier An appliance that removes moisture from indoor air, most often used in basements and humid summers to keep humidity in a healthy range.
HRV (heat recovery ventilator) A ventilation system that swaps stale indoor air for fresh outdoor air while transferring heat from the outgoing air to the incoming air.
ERV (energy recovery ventilator) Like an HRV, but its core also transfers some moisture between the outgoing and incoming air, which helps manage humidity.
Hygrometer An inexpensive gauge, often combined with a thermometer, that shows a room's relative humidity so you can keep it in a healthy range.

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