Ducts & air
Humidity, dew point and comfort at home
Relative humidity changes with the temperature; dew point does not. Once you read the air by its dew point, comfort, condensation and mould all make more sense.
Two ways to describe moisture
Air holds water vapour, and warm air can hold much more than cold air. Relative humidity is the amount of vapour in the air as a percentage of the most it could hold at its current temperature. Dew point is the temperature at which the air would be saturated, the temperature to which you would have to cool it for water to start condensing.
The difference matters because relative humidity changes whenever the temperature does, even when the amount of water in the air stays the same. Air at 70 °F and 50% relative humidity, cooled to 60 °F, is at about 70% with no water added or removed. Dew point stays at about 50 °F throughout. For comfort, and for whether a surface will get wet, dew point is the better number.
How dew point feels
At a room temperature of 75 °F, the dew point and humidity calculator gives:
| Relative humidity at 75 °F | Dew point | How it feels |
|---|---|---|
| 30% | 41.5 °F | Comfortable, a little dry |
| 40% | 49 °F | Comfortable |
| 50% | 55.1 °F | Noticeably humid |
| 60% | 60.2 °F | Sticky |
| 70% | 64.6 °F | Sticky, close to uncomfortable |
The bands are the common meteorological descriptions: below 55 °F comfortable, 55 to 60 noticeably humid, 60 to 65 sticky, 65 to 70 uncomfortable, and above 70 oppressive. Indoors in summer, a dew point at or below about 55 °F is a good target, which at 75 °F is about 50% relative humidity.
Summer: what makes a house clammy
The air conditioner removes moisture as well as heat, but only while it runs and only once its coil is cold. A system that runs long cycles keeps the indoor dew point down; one that short-cycles, often because it is oversized, cools the air without drying it. The result is a house at 72 °F that feels damp, the problem described in the guide to oversized systems. Other causes are a fan set to run continuously, which blows the water on the coil back into the house between cycles, and ventilation air brought in on humid days.
Fixes, in order of cost: set the fan to auto, lower the fan speed for cooling if the air handler allows, run bathroom and kitchen fans when they are needed, and add a dehumidifier if the indoor dew point stays above 60 °F. The guide to dehumidifier sizing covers that last step.
Winter: dry air and wet windows
Cold outdoor air holds very little water. When it leaks into a house and is warmed, its relative humidity falls sharply, often to 20% or less in a cold snap. Dry air feels cooler, dries skin and eyes, and causes static. A humidifier puts some water back.
The limit is condensation. The inside of a window is much colder than the room, and when it falls below the room’s dew point, water condenses on it. In a cold climate the window sets the ceiling on indoor humidity:
A window on a very cold night
A room at 70 °F and 40% relative humidity, with the inside of the glass at 35 °F.
- Dew point: 44.6 °F
- The 35 °F glass is well below it, so it condenses, and may frost
- The glass stays dry only below about 28% relative humidity
That is why the usual winter guidance is to lower the target humidity as it gets colder outside: around 35 to 40% in mild weather, 30% around 0 °F, and 25% or less in deep cold, with better windows allowing more. Condensation that runs down onto sills and walls feeds mould, so a little dry air is the lesser problem.
How much water a humidifier adds
The humidifier size calculator works out how much water the house loses to air leakage and must replace to hold a target:
For an average 2,000 sq ft house at 0.5 air changes per hour, 20 °F outdoors and a 35% target, it comes to 2.29 lb of water an hour, or 6.6 gallons per day, a console or small whole-house unit. A tight 1,500 sq ft house at 0 °F and a 30% target needs only 3.1 gallons per day. Air sealing reduces the humidifier’s job as well as the heating bill.
Heat index outdoors
The heat index combines air temperature and humidity into the temperature it feels like to the body, because humid air slows the evaporation of sweat. The US National Weather Service uses the Rothfusz regression, which the heat index calculator applies. At 92 °F and 60% relative humidity, it feels like 105 °F, in the Danger category; at 100 °F and 50%, like 118 °F. It is an outdoor, shaded measure, but it explains why a humid room at 80 °F is so much less comfortable than a dry one, and why air conditioning is a health matter for older and vulnerable people in a heat wave.
Ventilation and humidity together
Ventilation dilutes indoor moisture in winter and brings outdoor moisture in during humid summers. A balanced energy recovery ventilator moderates both. The guide to whole-house ventilation covers how much outdoor air a house needs, and the dehumidifier size calculator sizes the equipment when ventilation and the air conditioner are not enough.