Relative Humidity Versus the Moisture Actually in the Air

Why this matters

A homeowner reads 41 percent on the hallway thermostat and 65 percent on the basement dehumidifier and concludes the basement has a moisture problem the upstairs does not. A tech opens the basement windows on a breezy summer afternoon "to dry it out" because the forecast says 50 percent. Both are reading relative humidity as if it were the amount of water in the air, and both are wrong in a way that leads to the wrong job. Relative humidity is a ratio that moves with temperature. The quantity that tells you how much water is really there is the dew point, or its cousin, grains of moisture per pound of dry air.

Three measures, three different questions

Relative humidity (RH) is how much water vapor the air holds compared with the most it could hold at its current temperature, as a percentage. Warm air can hold far more vapor than cool air, so the same water reads as a low RH in a warm room and a high RH in a cool one. RH is the right number for one question: how close is this air, at this temperature, to depositing water? That is why mold, dust mites, wood movement and corrosion respond to it.

Dew point is the temperature at which the air's actual vapor would start to condense. It does not change when the air warms or cools, as long as no water is added or removed. Two rooms with the same dew point hold the same water, whatever their RH readings say. It is the right number for the question "where will this air condense?": any surface colder than the dew point gets wet.

Grains per pound (humidity ratio) is the weight of water vapor mixed with a pound of dry air, with 7,000 grains to a pound. Like dew point, it stays put through heating and cooling. It is the number dehumidifier sizing and psychrometric calculations work in, and the one that lets you add and subtract moisture.

The way temperature changes what an instrument reports is the subject of How Humidity Changes What You Measure. This card owns the distinction itself: which number answers which question.

One house, one moisture level, three readings

 SAME AIR, dew point 50 F, about 53 gr/lb
 -----------------------------------------
 Location            Temp     RH reads
 Upstairs hallway    75 F     41 %
 Basement, mid-room  62 F     65 %
 Basement closet,
  air at the cold
  exterior wall      52 F     93 %
 -----------------------------------------
 Dew point at all three: 50 F
 Water in the air:   same at all three

The air is the same air, moving freely through an open stairwell. The only thing that changed between readings is temperature. Upstairs it is far from condensing. In the closet, 2 degrees above its dew point, it is on the edge.

The rule of thumb behind it. Near room temperatures, RH roughly doubles for each 20 degree F drop in temperature at a fixed moisture level. Check it against the table: 75 F to 55 F would take 41 percent to about 83 percent, roughly double. From 75 down to 52 is a 23 degree drop, a little more than 20, and the reading goes from 41 to 93, a ratio of about 2.25, which is what slightly more than one doubling looks like. The rule loses accuracy as you move far from room temperature, so for anything that decides a job, use a psychrometric chart or a meter that computes dew point.

Worked example: two calls that turn on the distinction

Call one: "the basement is twice as humid as upstairs." A homeowner sees 41 percent upstairs and 65 percent in the basement and wants a second dehumidifier for the basement.

• Read the temperatures with the RH: 75 F upstairs, 62 F in the basement.
• Convert both to dew point with a meter or chart: both come out near 50 F.
• Conclusion: the basement does not have more water in its air. It has cooler air holding the same water. A second dehumidifier would be treating a temperature difference.

That is not the end of the call. A basement at 65 percent RH is high enough that sustained readings at that level and above are a reason to look at its coldest surfaces, and the closet at 93 percent is a real problem whatever caused it. But the fix is likely about that closet's surface temperature and air movement, not the basement's water load. See related: The Cold Surfaces Where a House Condenses First.

Call two: "open the windows, it is only 50 percent outside." Midsummer, 90 F and 50 percent RH outdoors. The basement walls and slab run near 62 F.

• Outdoor dew point at 90 F and 50 percent: about 69 F, which is roughly 106 grains per pound.
• Basement air at a 50 F dew point holds about 53 grains per pound, about half as much water.
• Any surface in the basement colder than 69 F, which is every wall and the slab, is below the incoming air's dew point.

Opening the windows roughly doubles the water in the basement air and puts the walls below its dew point, so they sweat. The 50 percent outdoors sounded drier than the 65 percent inside, and it carried about twice the water. In summer, basements are dried by closing them up and removing water mechanically, not by ventilating them with outdoor air.

The winter mirror image. In January, outdoor air at 30 F and 80 percent RH sounds damp. Its dew point is in the mid 20s F and it holds roughly 19 grains per pound. Warmed to 70 F indoors with nothing added, that same air reads about 18 percent RH. That is why houses with high winter air leakage run dry, and why tightening a house raises winter indoor RH: less dry outdoor air is diluting the moisture the occupants make. See related: Why Air Sealing Changes What a House Needs for Ventilation.

A pocket conversion at 70 F

Most meters now display dew point directly, and that is the number to write down. When all you have is an RH reading in a room near 70 F, this short table, computed at sea-level pressure from the ASHRAE Handbook, Fundamentals psychrometric relationships, gets you close enough to compare rooms and decide where to look next.

Two things it teaches at a glance. Going from 30 to 60 percent at the same temperature doubles the water in the air, from about 33 to about 65 grains. And a house held at 50 percent and 70 F in winter puts any surface colder than about 50 F at risk of condensation, which on a cold night includes many window frames and some exterior corners. The table holds only at 70 F; at any other room temperature the same RH means a different dew point, which is the whole point of this card.

Why RH readings mislead even when the meter is good

The meter reads the air at its sensor, not the surface you care about. An RH reading taken in the middle of a room tells you about mid-room air. The wall behind the dresser may be 10 degrees colder and sitting at a far higher RH right at its face. To judge a surface, measure the surface temperature and compare it with the room's dew point.

Small temperature errors become large RH errors. Near room temperature, a 1 F change shifts the saturation point by about 3.5 percent of itself, so at 50 percent RH a sensor reading 1 F off the true air temperature reports RH off by close to 2 points. A sensor near a warm electronic case, in sun or in a hand reads high in temperature and low in RH. A typical consumer hygrometer's own accuracy is a few points of RH under normal conditions and worse near the extremes; the instrument's data sheet states its figure.

RH comparisons across rooms and days are apples to oranges. "It was 55 percent yesterday and 48 today" can mean the house dried out or that the thermostat ran three degrees warmer. A dew point trend answers the question; an RH trend needs the temperature beside every reading to mean anything.

What changes the answer

At altitude, the grains-per-pound figure for a given dew point is higher than at sea level, because the air itself is thinner. Dew point, which is the number that decides condensation, does not need a correction. If you work in grains for dehumidifier sizing at elevation, use a chart for your elevation.

Once condensation starts, the dew point of the air near that surface can fall, because water is leaving the air onto the surface. A surface already wet will read a lower local dew point than the room, which is a sign it is pulling water out of the air, not that the room is drier.

Hygroscopic materials such as wood and drywall store and release water, so the dew point of a room can stay stubbornly high after the source is gone. See related: Hygric Buffering and Why a House Swings Slowly.

The failure mode in the field

The expensive version of this mistake is a solution sized to an RH reading. A whole-house dehumidifier sold because the basement reads 65 percent when the upstairs at the same dew point reads 41. A humidifier turned up in a tight house in winter because a cold bedroom reads low, when the hallway at 72 F is already at a dew point that will frost the windows. A crawlspace "ventilated" in summer because the vents seemed like the answer to a high reading. In each case the RH number was correct and the conclusion drawn from it was about the wrong quantity.

The cheap habit that prevents it: never write down an RH without the temperature it was taken at, and convert to dew point before comparing any two places or any two days. The procedure for taking those readings belongs to How to Measure Indoor Humidity and Dew Point Properly.

References

• ASHRAE Handbook, Fundamentals, psychrometrics chapter, for saturation properties and humidity ratio
• See related: How Humidity Changes What You Measure; Dew Point and Condensation, the Reasoning Behind the Fault
• See related: How to Measure Indoor Humidity and Dew Point Properly; The Cold Surfaces Where a House Condenses First
• Instrument manufacturer data sheets for stated RH and temperature accuracy