Grains, and why relative humidity misleads you here
Relative humidity is a percentage of what the air could hold at its own temperature, so two air masses at the same percentage can carry completely different amounts of water. That makes it useless for comparing outdoor air with crawl air. The number that compares directly is the humidity ratio, usually written in grains of water per pound of dry air, with 7,000 grains to the pound.
Working the default conditions: outdoor air at 82°F and 75 percent comes to 123.7 grains per pound with a dew point of 73.3°F. Crawl air at 62°F and 70 percent comes to 57.8 grains with a dew point of 52.1°F. The difference is 65.8 grains per pound, and it is a real, transportable quantity of water rather than a percentage.
Turn that into a flow. A 40 by 30 crawl with 3 ft of clear height is 3,600 cubic feet. At 1.5 air changes an hour that is 90 CFM. Air at standard density delivers 4.5 pounds of dry air per hour for each CFM, so 405 pounds of air an hour. Multiply by 65.8 grains and divide by 7,000 and you have 3.81 pounds of water an hour. Over ten hours a day that is 38.1 pounds — 4.56 gallons, or 36.5 pints.
| Step | Value | Arithmetic |
|---|---|---|
| Crawl volume | 3,600 cu ft | 40 × 30 × 3 |
| Airflow at 1.5 ACH | 90 CFM | 3,600 × 1.5 ÷ 60 |
| Outdoor air, 82°F / 75% | 123.7 grains/lb | dew point 73.3°F |
| Crawl air, 62°F / 70% | 57.8 grains/lb | dew point 52.1°F |
| Difference | 65.8 grains/lb | |
| Water carried in | 3.81 lb/hr | 4.5 × 90 × 65.8 ÷ 7,000 |
| Latent load | 4,029 BTU/hr | 0.68 × 90 × 65.8 |
| Over 10 hours | 4.56 gal | 38.1 lb ÷ 8.345 |
The 0.68 and the 4.5 are the same constant seen twice. Standard air weighs about 0.075 pounds per cubic foot, and 60 minutes times 0.075 is 4.5 pounds of dry air per hour per CFM. Multiply that by roughly 1,061 BTU to evaporate a pound of water and divide by 7,000 grains and you get 0.68 BTU per hour per CFM per grain. The 1.08 that turns up in every sensible heat calculation is the same 4.5 multiplied by the 0.24 BTU per pound per degree specific heat of air instead.
The dew point is where the damage shows
Air at 73.3°F dew point does not have to be cooled much to give up water — anything it touches below 73.3°F starts collecting it. On the defaults the coolest surface named is 60°F, which is 13.3 degrees below the dew point. That is why the classic finding in a vented crawl in August is water beaded on the cold water lines and running down the outside of the supply ducts, on a day with no rain in the forecast.
This is also the trap in opening the vents to dry the place out. It works in a dry climate and in winter, when the outdoor air really is drier in absolute terms. In a humid summer the same action pumps water in. The dew point calculator will check a single reading quickly if you just want to know whether today is one of those days.
Adding it to the other sources
On the numbers here, ventilation air brings in 36.5 pints a day and bare soil on the same footprint contributes about 57.5 — see the ground moisture calculator. Neither dominates completely, which is the useful point: covering the ground and leaving the vents wide open on a humid day still leaves a substantial load, and closing the vents over bare dirt leaves the larger one. Whether a crawl may be sealed at all is a code and construction question that this site will not answer for you.
Once you have a total, the run time calculator turns pints per day into hours and dollars. If you are working on the air leakage itself rather than the moisture, the air sealing payback calculator handles blower door numbers and the heating side, and the rim joist page covers the band that most of that leakage goes through.
Questions people ask
Should I close my crawl space vents in summer?
This page will not answer that, and anyone who answers it without seeing the house is guessing. What it will do is show you the size of the effect: at 82 degrees and 75 percent outside against a 62 degree crawl at 70 percent, ventilation air is carrying in about 4.6 gallons of water a day at 1.5 air changes an hour. Whether vents may be closed depends on the code your jurisdiction has adopted, on what else is under the house, and on whether closing them changes how a combustion appliance vents. Ask the building official and a contractor who has been down there.
What are grains of moisture and why not just use relative humidity?
Grains per pound of dry air is an absolute measure — the actual weight of water each pound of air is carrying, at 7,000 grains to the pound. Relative humidity is a percentage of the maximum at that air temperature, so 70 percent at 82 degrees and 70 percent at 62 degrees are very different amounts of water. Comparing two air streams needs the absolute number. Outdoor air at 82 and 75 percent holds 123.7 grains; crawl air at 62 and 70 percent holds 57.8.
Where does the 0.68 in the latent load formula come from?
Standard air weighs about 0.075 pounds per cubic foot, and 60 minutes an hour times that gives 4.5 pounds of dry air per hour for every CFM. Evaporating a pound of water takes roughly 1,061 BTU, and there are 7,000 grains in a pound of water. So 4.5 times 1,061 divided by 7,000 is 0.682, rounded to 0.68 BTU per hour per CFM per grain. The 1.08 in sensible heat calculations is the same 4.5 multiplied by the 0.24 specific heat of air.
Why is there water on my ducts and pipes in the crawl space in August?
Because their surface temperature is below the dew point of the air around them. On the default conditions the incoming air has a dew point of 73.3 degrees and a supply duct might sit at 60, which is 13.3 degrees of subcooling — the air touching it is pushed past saturation and the water comes out as liquid. It is condensation, the same thing that happens on a cold glass, and it has nothing to do with rain.
How do I find the air changes per hour for my crawl space?
Properly, with a blower door, which is what an energy auditor brings. Without one you are estimating, and the estimate is the weakest link in the whole calculation — an open-vented crawl on a windy day can be several air changes an hour while the same crawl on a still day is a fraction of one. Run the page at 0.5, 1.5 and 3 and read the answer as a range rather than a figure. The spread you get is an honest picture of how much you actually know.