Why the ground beats the machine
Put a bucket of water in a room and the room gets humid. A dirt crawl space is a 1,200 square foot bucket with no lid, and the surface is being kept topped up from below. That is why a dehumidifier alone in a dirt crawl runs continuously, costs money every hour, and never gets the humidity where the owner wanted it.
The arithmetic on the default settings: 40 by 30 is 1,200 square feet, all of it bare, at 0.05 pounds per square foot per day. That is 60.0 pounds of water a day. Water weighs 8.345 pounds a gallon, so 7.19 gallons a day, and since a pint is an eighth of a gallon, 57.5 pints a day. Over a 180 day damp season it is 1,294 gallons — five and a half tons of water lifted off the ground and pushed up through the floor of the house.
| Bare soil | Barrier at 95% coverage | |
|---|---|---|
| Soil giving off water | 1,200 sq ft | 60 sq ft open, 1,140 sq ft covered |
| Water per day | 60.0 lb | 8.70 lb |
| Gallons per day | 7.19 | 1.04 |
| Pints per day | 57.5 | 8.3 |
| Over 180 days | 1,294 gal | 188 gal |
| Against a 50 pint machine | 1.15× its rating | 0.17× its rating |
The barrier removes 85.5 percent of the load in this example, and it does it without drawing a watt. The 10 percent leakage figure on the covered area is doing a lot of work in that number and it is a guess, which is exactly why it is a field rather than a constant. Set it to 25 percent for a loose untaped sheet and the after figure goes from 8.70 to 17.25 pounds a day, still a 71 percent reduction.
The rate is the whole calculation, so treat it honestly
Published evaporation rates for crawl space soil are not measurements of your crawl space. They come from a handful of studies on particular soils at particular water tables, and the range in general circulation — roughly 0.03 to 0.10 pounds per square foot per day for damp soil — is a three-fold spread that swamps every other input on the page. Running the default with 0.03 gives 4.31 gallons a day; running it with 0.10 gives 14.37.
You can measure it instead, and the measurement is not difficult. Tape a square of plastic tight to the soil at the edges, leave it for a full day, then lift it and weigh the water that has collected on the underside. Water on a 4 ft by 4 ft square, divided by 16 square feet and by the days it sat there, is your rate in pounds per square foot per day. A kitchen scale reading in ounces divided by 16 gives pounds. Three squares in three parts of the crawl will disagree with each other, and the disagreement is itself the useful result.
What is left for the machine
Ground evaporation is one of three moisture sources in a crawl space. The second is the air coming in through vents and gaps, which on a humid summer day carries a startling amount of water of its own — the vent air calculator puts a number on that, and on the default conditions it is around 36 pints a day, comparable to the bare ground. The third is plumbing: a weeping supply line, a condensate drain that ends under the house, a duct that sweats.
Add the sources you have and hand the total to the run time calculator, which turns pints per day into hours per day, kilowatt hours and a bill. If you want a capacity recommendation from a description rather than from physics, the dehumidifier sizing calculator works from room size and how damp the space feels.
Questions people ask
How many gallons of water evaporate out of a dirt crawl space?
It depends on the exposed area and how wet the soil is, and the honest range is wide. At 0.05 pounds per square foot per day — a middling figure for damp crawl soil — 1,200 square feet gives 60 pounds a day, which is 7.2 gallons or 57.5 pints. At the low end of the range in general circulation, 0.03, the same crawl gives 4.3 gallons. At 0.10 it gives 14.4. The number is tens of gallons a day rather than a trickle, and that is the finding that matters more than the precise value.
How do I measure the evaporation rate in my own crawl space?
Tape a square of plastic tight to the soil at all four edges, note the date, and come back a day or two later. Lift it and weigh the water clinging to the underside — a kitchen scale in ounces is fine, divide by 16 for pounds. Divide that by the square footage of the sheet and by the number of days and you have pounds per square foot per day. Do it in three places, because one crawl space is not one soil condition, and the spread between the three tells you how much to trust any single figure.
Will a vapor barrier alone fix a damp crawl space?
It removes the largest single moisture source in most dirt crawls, which on the default numbers here is 85.5 percent of the ground load. Whether that is enough depends on what else is going on: humid outdoor air coming in through vents, a plumbing leak, a condensate line discharging under the house, or actual groundwater entering. This page only calculates the ground path and offers no opinion on whether any crawl space is adequate.
Why does a dehumidifier struggle in a dirt crawl space?
Because the load is larger than the rating. A 50 pint machine is rated at a warm humid test condition, removes materially less than that in a 60 degree crawl, and on the default numbers here is being asked to keep up with 57.5 pints a day off the ground alone before any air infiltration is counted. Running flat out and losing is the normal outcome. Cover the soil first and the same machine has a fraction of the work to do.
Does covering 95 percent instead of 100 percent really matter?
Less than people fear. The 5 percent left open — the strip at the hatch, the ground behind a duct trunk — contributes 3.0 pounds a day on the default settings against 60.0 from the whole floor. What matters far more is the leakage figure through the covered part, because that applies to the other 1,140 square feet. Going from a taped liner at 10 percent to a loose sheet at 25 percent adds 8.55 pounds a day, nearly three times what the uncovered strip costs.