Damp is a subject where the vocabulary does real work. The difference between relative and absolute humidity is the difference between opening a window helping and hurting; the difference between condensation and penetrating damp is the difference between free and five figures.
Definitions below are the practical versions rather than the textbook ones.
Measuring the air
| Term | What it means | Why it matters |
|---|---|---|
| Relative humidity (RH) | How full the air is, as a percentage of what it could hold at its current temperature | The number on your hygrometer. Changes with temperature even when no water moves |
| Absolute humidity | The actual mass of water in the air, usually grams per cubic meter | Does not change when air warms or cools — so it is what you compare between indoors and out |
| Grains per pound (gr/lb) | The US HVAC unit for moisture per pound of dry air | 7,000 grains to a pound. The unit the window calculator compares |
| Dew point | The temperature at which air starts releasing water | Any surface below it is condensing. The single most useful number in a damp house |
| Vapor pressure | The share of air pressure contributed by water vapor | What drives moisture from wet places toward dry ones, including through walls |
| Psychrometrics | The physics of moist air | The field all of the above belongs to |
What goes wrong in a building
| Term | What it means | How to recognize it |
|---|---|---|
| Condensation | Water leaving the air onto a surface below the dew point | Diffuse, in corners and on glass, worst on winter mornings |
| Penetrating damp | Water coming through the wall from outside | Defined edge, worse during and after rain |
| Rising damp | Groundwater drawn up through masonry | Level band under about 3 ft, tide mark, salt deposits |
| Thermal bridge | A path where heat escapes faster than through the surrounding wall | A cold spot: corners, lintels, window reveals. Where mold appears first |
| Efflorescence | Salt left on a surface as water evaporates out of masonry | White crystalline residue. Proof water is moving through the wall |
| Interstitial condensation | Condensation forming inside a wall rather than on it | Invisible until damage appears. Why vapor control layers matter |
| Stack effect | Warm air rising and drawing replacement air in low down | Why crawl space air ends up in your living room |
Equipment and building terms
| Term | What it means | Note |
|---|---|---|
| Hygrometer | An instrument that measures relative humidity | Cheap ones drift — check with the salt test |
| Humidistat | A control that switches equipment on and off to hold a humidity target | Run dehumidifiers and humidifiers on one rather than continuously |
| Desiccant | A material that adsorbs water — silica gel, zeolite | Also the dehumidifier type that works in the cold |
| Deliquescent salt | A salt that absorbs so much water it dissolves in it | Calcium chloride. What is in a DampRid tub |
| HRV | Heat recovery ventilator — exchanges air while keeping most of the heat | Better value in a cold, dry climate |
| ERV | Energy recovery ventilator — also transfers some moisture between airstreams | The one to specify in a humid climate |
| PIV | Positive input ventilation — gently pressurizes the house with filtered air | Common in the UK, less so in the US |
| DPC | Damp proof course — a barrier in a wall that stops water rising | A "bridged" DPC is far more common than a failed one |
| Vapor barrier / retarder | A layer that slows moisture moving through an assembly | Placement matters enormously and is climate-dependent |
| Encapsulation | Sealing a crawl space with a lapped, sealed ground barrier | Current practice, replacing the old advice to vent |
| Latent vs sensible load | Energy spent removing moisture vs energy spent lowering temperature | An oversized AC handles sensible well and latent badly |
The four ways to describe the same water
Almost every confusing argument about indoor air comes from two people using different measures without saying so. These four describe the same thing and behave completely differently.
Relative humidity is the percentage of the water the air could hold at its current temperature that it is actually holding. It is the number on every hygrometer and it is genuinely useful for comfort, mold and health thresholds — but it moves whenever the temperature moves, even when nothing has been added or removed. Heat a room from 60 to 72 °F and the relative humidity falls sharply while the water content is unchanged. That single property causes most of the confusion in this subject.
Dew point is the temperature at which the air would have to be cooled for that water to start condensing. It does not move when you change the temperature, which makes it the right measure for two jobs: deciding which surfaces in a building are wet, and comparing two air masses — indoors against outdoors, one room against another. If you take one idea away from the glossary, take this one.
Absolute humidity is the actual mass of water in a given volume of air, usually stated in grams per cubic meter. It is intuitive — it answers "how much water is in this room" — but it shifts slightly with temperature because air expands and contracts, so it is not quite as clean for comparisons as the next one.
Humidity ratio, expressed in the United States as grains of water per pound of dry air, is the measure heating and cooling engineers use. It is a pure mass ratio and does not change with temperature at all, which is why load calculations and psychrometric charts are built on it. Comfortable indoor air sits roughly between 40 and 60 grains per pound; a humid Gulf Coast summer day is over 100.
The practical translation: use relative humidity to decide whether a room is comfortable and whether it is in the safe band, and use dew point or grains whenever you are comparing two different air temperatures. A basement at 68% and an outdoor afternoon at 55% is not a comparison until you convert both, and once you do the answer frequently reverses.
Common questions
What is the difference between relative and absolute humidity?
Relative humidity is a percentage of what the air could hold at its current temperature, so it changes when air warms or cools. Absolute humidity is the actual amount of water present and does not change with temperature — which is why it is the right thing to compare between indoors and outdoors.
What is a thermal bridge?
A path through the building envelope where heat escapes faster than through the surrounding wall — a corner, a lintel, a window reveal. These are the coldest interior surfaces and the first place mold appears.
Which terms actually change what I should do?
Three: the difference between relative and absolute humidity decides whether ventilating helps, the dew point decides whether a surface condenses, and thermal bridge explains why one corner grows mold while the room reads fine.
What is the difference between damp, condensation and humidity?
Humidity is a property of air — how much water it carries. Condensation is an event: that water turning liquid on a surface cold enough to force it. Damp is a condition of a material: something in the building holding water, whether it arrived as condensation, as rain through a wall, from the ground, or from a leak. Confusing the three is what leads to a dehumidifier being bought for a gutter problem.
What does "psychrometric" actually mean?
It is simply the study of the properties of moist air — temperature, humidity, dew point, enthalpy and the relationships between them. A psychrometric chart plots all of them together so an engineer can read off what happens when air is heated, cooled or mixed. Every calculator on this site is doing psychrometry on your behalf; the word sounds forbidding and the underlying idea is just that warm air holds more water than cold air.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions