How to Measure Indoor Humidity and Dew Point Properly

Why this matters

"The basement reads 70 percent humidity" is the start of a lot of expensive jobs: a dehumidifier, a vapor barrier, an encapsulation, a waterproofing contract. Sometimes it is the right start. Often the number came from a cheap hygrometer set on a shelf for ten minutes after someone took a shower upstairs, and it says almost nothing about whether the basement has a moisture problem or is simply cooler than the rest of the house. Relative humidity depends on temperature as much as on moisture, so the same air reads 48 percent in a 72 F kitchen and 65 percent in a 63 F basement without a drop of water added. Getting a reading that supports a decision takes three things: loggers placed where they read the space and not a local event, a record long enough to catch the weather and the household, and a comparison done in dew point rather than RH. Why dew point is the right comparison belongs to Relative Humidity Versus the Moisture Actually in the Air.

What you need

• Three temperature and RH data loggers with stated accuracy (commonly plus or minus 2 to 3 percent RH and under 1 F), set to the same logging interval.
• A notes sheet for the household: showers, cooking, laundry, guests, humidifier or dehumidifier use, windows opened, and the HVAC mode and setpoint.
• A source for the outdoor weather, including rain, if your outdoor logger is not rain-proof.

The steps

1. Put the three loggers side by side for an hour before you place them. Same shelf, same room, out of drafts. Afterward, compare their readings at the end of the hour. If they agree within about 1 percent RH and a few tenths of a degree, use them as they are. If one reads consistently higher or lower, write down its offset and correct its readings before you compare them. Skip it and two loggers that each meet a plus or minus 3 percent spec can disagree by 6 percent RH with nothing wrong in the house, and at basement conditions 3 percent RH is worth about 1.2 F of dew point. A real difference smaller than that is invisible.

2. Place the main-floor logger in a central room, away from local sources. About 3 to 5 ft off the floor, on an interior wall, out of direct sun, not within several feet of a supply register, and not in or next to a bathroom, kitchen or laundry. Skip it and you log the shower, the stove or the cold dry air coming off a supply, and the record describes a register instead of a house.

3. Place the basement logger in the open basement, not in a corner or on the slab. Same height range, away from the furnace, the dryer, the sump and any running dehumidifier's discharge. If the complaint is a specific room or corner, a fourth logger there is worth adding, but keep the open-basement logger as the reference. Skip it and a logger set on the floor reads the cold slab's air layer and looks wetter than the space really is.

4. Place the outdoor logger in shade, sheltered from rain, away from the house's own outlets. The north side under an eave or deck works, a few feet off the wall, away from a dryer vent, a bath fan hood, a condensing unit discharge or a lawn sprinkler. Skip it and the sun or a dryer vent writes its own weather into the comparison you are going to make.

5. Log for at least three days, at an interval of 5 to 15 minutes. Three days is the minimum that catches daily household patterns and usually one weather change. A week is better, and a rain event inside the window is worth waiting for if the question is foundation moisture. Skip it and you are reading one afternoon, and an afternoon can be a laundry day, a rainy day or a dry day with the windows open.

6. Keep the household notes running the whole time. Ask the customer to jot down showers, cooking, laundry, guests and anything opened or turned on. Record the HVAC mode and setpoint. Skip it and a spike in the record has no explanation, and you will guess.

7. Convert every reading to dew point and compare dew points. Most loggers' software computes it; if yours does not, a psychrometric chart or calculator does. Dew point is set by how much water vapor is in the air, regardless of the air's temperature, so two spaces with the same dew point hold the same moisture. Skip it and you will compare RH across spaces at different temperatures, which tells you mostly that the spaces are at different temperatures.

Worked example: a three-day record

A two-story house with a finished-looking but unconditioned basement. The customer wants a quote for basement waterproofing because "the humidity is always high down there." The house is in cooling mode at a 72 F setpoint. Co-location before placement: the three loggers agreed within 1 percent RH and 0.3 F, so no correction was applied.

3-DAY LOGGER RECORD     daily averages
dew point from the averaged temperature and RH
---------------------------------------------------
          MAIN FLOOR     BASEMENT       OUTDOORS
Day       F   RH   DP    F   RH   DP    F   RH   DP
Tue      72   48  51.2   63   65  51.0  81   62  66.7
Wed      72   47  50.7   63   66  51.5  79   64  65.8
Thu      72   49  51.8   64   74  55.5  77   70  66.5

Notes: showers 6-7 am daily; laundry Wed 7 pm;
heavy rain Wed 10 pm to Thu 4 am, gutters
overflowing at the NE corner per owner.
No dehumidifier running. AC cooling, 72 F.

Tuesday and Wednesday say the basement has no moisture of its own. The basement read 65 and 66 percent RH, which sounds high, but its dew point (51.0 and 51.5 F) sits within 0.8 F of the main floor's (51.2 and 50.7 F) on both days. That difference is inside what the co-located loggers can resolve. The basement holds the same air the main floor does; it reads higher RH because it is 9 F cooler. Nothing is adding water down there on a dry day.

Thursday says something changed after the rain. The basement dew point rose to 55.5 F while the main floor stayed at 51.8 F, so the basement now holds noticeably more moisture than the house above it: 55.5 minus 51.8 is 3.7 F of dew point. The outdoor dew point barely moved across the three days (65.8 to 66.7 F), so this is not outdoor air leaking in more on Thursday. The source is in the basement, and it appeared after six hours of heavy rain with gutters overflowing at one corner. That points to water reaching the foundation, not to the air.

The gate, and how each day lands on it. A shop default worth setting: a basement whose dew point sits more than 2 F above the main floor's, sustained across a day, has a moisture source of its own worth finding. Tuesday's gap is 0.2 F the other way (51.0 against 51.2). Wednesday's is 0.8 F (51.5 against 50.7). Thursday's is 3.7 F. Two days pass under the gate and one crosses it, and that split is the finding.

What the customer should hear. On dry days the basement's high RH is a temperature effect. Lowering it means drying or warming the basement air, and a dehumidifier sized for the space does that. After heavy rain the basement takes on water of its own, and the place to start is the gutter at the northeast corner and the grading there, before anyone prices waterproofing. Keeping indoor RH below 60 percent, which is EPA's guidance for limiting mold growth, is a reasonable target for the dehumidifier; the basement averaged 65 to 66 percent even on the dry days, so it misses that target either way.

What would have happened with a spot reading. A tech who read 74 percent on Thursday morning would have quoted waterproofing. A tech who read 65 percent on Tuesday and compared it with the main floor's 48 percent RH would have quoted the same thing for the wrong reason, and a tech who compared dew points on Tuesday alone would have said there was no source at all. Each of those is a single reading of a three-day story.

What changes the reading

• Heating season. Outdoor dew point in winter is usually far below indoor, so the comparison that matters is indoor against outdoor. An indoor dew point well above outdoor in winter is the household's own moisture (people, cooking, showers, plants, an unvented appliance), and the question becomes ventilation and cold surfaces rather than foundation water. See related: The Cold Surfaces Where a House Condenses First.
• A running dehumidifier or humidifier. Either one sets the dew point in its space. Note when it runs, or turn it off for the logging window with the customer's agreement so the record shows the space and not the appliance. If you turned it off, turn it back on when the logging ends and confirm it is running: the fan starts, the compressor or element cycles, and a dehumidifier's bucket or drain line is collecting water.
• A conditioned basement. If the basement has supplies and a return on the same system, its air mixes with the main floor's and a source down there is diluted across the house. A basement dew point that matches the main floor proves less, and the main floor's dew point rising after rain becomes the clue.
• A sealed or encapsulated crawlspace. Treat it like the basement in the example, with its own logger, and compare it with both the house and the outdoors.

How to verify the record is trustworthy

• The co-location check is written down, with the readings at the end of the hour.
• Each logger's placement is described in enough detail that someone could put it back in the same spot for a follow-up.
• The record covers at least three full days and every gap or spike has a note beside it.
• Every comparison in your report is in dew point, and any difference you call real is larger than what the co-location showed the loggers can resolve.
• Rerun the same placement after the fix (the gutter repaired, the dehumidifier running). The basement dew point after a comparable rain should stay within about 2 F of the main floor.

References

• US EPA, A Brief Guide to Mold, Moisture and Your Home (indoor RH below 60 percent)
• Logger manufacturer's documentation for stated accuracy and dew point calculation
• See related: Relative Humidity Versus the Moisture Actually in the Air; Hygric Buffering and Why a House Swings Slowly
• See related: How Moisture Moves Through a Building; Moisture and Vapor Management Reference