Crawl space condensation calculator

Water shows up on crawl space framing, ductwork and pipes for one reason more than any other around Huntsville: the air reaching them holds more moisture than a surface that cool can tolerate. Whether a given joist will sweat comes down to two numbers, the dew point of the air and the temperature of the surface. Give this calculator the outdoor temperature and humidity (or pick a Huntsville preset built from NOAA's climate normals) plus a surface reading from under the house, and it works out the dew point and says whether that surface will get wet.

With JavaScript off, use the table further down: it gives the dew point for each Huntsville preset, and any surface at or below that temperature will collect water.

How to get the surface reading

The calculator is only as good as the surface temperature you feed it. An inexpensive infrared thermometer pointed at the wood or metal works, or a contact thermometer taped to a duct for a few minutes. Take readings in the spots most likely to be coldest: the supply ducts and the refrigerant line while the air conditioner is running, the subfloor near the middle of the house, and joists on the shaded north side. Take one in the early morning and one in the afternoon if you can; the dew point outside changes through the day, and the cold spots underneath change with how hard the AC is working.

If you have a cheap digital hygrometer, set it in the crawl space for a day and add its temperature and humidity to the optional boxes. Comparing the two dew points tells you which way the vents are moving water.

Dew point in plain terms

Relative humidity changes whenever the temperature does, so on its own it says little about condensation. The dew point is steadier. It is the temperature where the air's water vapor starts turning into liquid, and EPA's moisture control guidance for buildings puts it simply: when the dew point of the surrounding air is above the temperature of a surface, water condenses on that surface; when it is below, it doesn't. EPA's own example is air at 85 °F and 60 percent relative humidity, which has a dew point of about 70 °F and will wet anything at 70 °F or colder. The same guide notes that damp crawl spaces can add as much water vapor to a low-rise building in a day as all its other internal sources together.

The calculator uses the Magnus form of the saturation vapor pressure equation with the coefficients Alduchov and Eskridge recommended in 1996 (17.625 and 243.04 °C), as presented by Mark Lawrence in the Bulletin of the American Meteorological Society in 2005. Lawrence reports those coefficients keep the error under 0.4 percent from −40 °C to 50 °C, far tighter than any household thermometer.

Huntsville presets, from NOAA normals

The presets come from NOAA's 1991–2020 hourly climate normals for Huntsville International Airport. For each month, we averaged the normal temperature and normal dew point at hour 6 and hour 15. NOAA lists those hours in local standard time, so in summer they are 7 a.m. and 4 p.m. on the clock. The relative humidity shown is computed from the two. The muggy-day preset swaps in the 90th-percentile dew point, the level only one day in ten exceeds at that hour.

Read down the last column and the summer pattern is plain. A duct carrying chilled air, or a joist sitting in the high 60s, is below the normal outdoor dew point for much of June, July and August, morning and afternoon alike. That is why vented crawl spaces here so often show their worst condensation in the hottest months rather than the rainiest ones.

What to do with the answer

• Surfaces sweat on normal summer days. Open vents are feeding the problem. The U.S. Department of Energy's Building America research found that in humid climates outside air is often wetter than crawl space air, and that closed, conditioned crawl spaces lowered humidity by more than 20 percent in the homes studied. See crawl space mold for what sealing and drying involve.
• Only the ducts or refrigerant line sweat. The fix may be insulation and air sealing on that equipment rather than the whole space; an HVAC contractor can check it.
• Nothing sweats, but the space smells musty. Look for liquid water instead: gutters, grading, a plumbing leak, or bare soil with no ground cover.
• Growth is already on the wood. Drying the air won't remove it. It has to be cleaned off, and the mold inspection page covers when testing is worth paying for.

The month-by-month background is in the Huntsville humidity guide. Sources: EPA, Moisture Control Guidance for Building Design, Construction and Maintenance (2013); Lawrence, The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air (BAMS, 2005); NOAA NCEI 1991–2020 hourly normals for Huntsville International Airport; U.S. DOE Building America, Unvented, Conditioned Crawlspaces; EPA, A Brief Guide to Mold, Moisture and Your Home.

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