Encapsulated Crawl Space, Dehumidifier or Supply Air

Under the 2018, 2021 and 2024 IRC, either one meets the code once the dirt has a sealed Class I vapor retarder. IRC Section R408.3 lets an unvented crawl space use 1 CFM of conditioned supply air per 50 sq ft of floor with a return air pathway and insulated perimeter walls, the same rate of continuous exhaust with an air pathway and insulated walls, or a dehumidifier, which the 2018 IRC sized at 70 pints a day per 1,000 sq ft and the 2021 and 2024 editions leave to the manufacturer. For the 1,700 sq ft crawl in the thread below, that's 34 CFM, or 119 pints a day under the 2018 text.

The code part is short, and the edition your inspector enforces changes the dehumidifier answer more than anything else. Once the method is picked, the free dehumidifier size calculator runs the moisture balance for the crawl itself, from its footprint and average clear height.

What does the IRC allow in an encapsulated crawl space?

An encapsulated crawl space is what the IRC calls an unvented crawl space, and R408.3 asks for two things. The dirt gets a sealed Class I vapor retarder, and the space gets one of four treatments, which are continuous exhaust, conditioned supply air, an existing-house plenum, or, from 2018 on, dehumidification. Supply air and dehumidification are both on that list in every edition from 2018 to 2024.

The ground cover rules read the same in all four editions checked here. Exposed earth gets "a continuous Class I vapor retarder," joints overlap 6 in. and are sealed or taped, and the edges run at least 6 in. up the stem wall, attached and sealed. Then comes the list.

Item 2.3 only applies when the crawl of an existing house is already used as a plenum, so a crawl that isn't a plenum picks from 2.1, 2.2 and, from 2018 on, 2.4. These are model code texts. A state or city can amend R408, so confirm the adopted edition before you quote the job.

How much supply or exhaust air does the code require?

One CFM for every 50 sq ft of crawl floor, so a 1,700 sq ft crawl needs 34 CFM. Supply air also needs a return air pathway back to the house, such as a duct or transfer grille, and exhaust needs an air pathway to the house. Both options also call for insulated perimeter walls, and the dehumidifier option has neither clause.

2021 IRC § R408.3, item 2.2

"Conditioned air supply sized to deliver at a rate equal to 1 cubic foot per minute ... for each 50 square feet ... of under-floor area, including a return air pathway to the common area (such as a duct or transfer grille), and perimeter walls insulated in accordance with Section N1102.2.10.1 of this code." - International Code Council, codes.iccsafe.org

In the source thread, a commenter in Virginia wrote that the county's code official told them a supply-air crawl needed a grille in the crawl door or something similar to get a return. That fits item 2.2 when the door opens into the house, since the return path has to reach the common area. A grille in an exterior crawl door leads outdoors and doesn't reach the common area at all.

The two air options are also worded differently. Item 2.1 says "continuously operated" exhaust. Item 2.2 says the supply is "sized to deliver" the rate and doesn't say continuously. By our reading, a supply branch that only moves air while the blower runs still matches the 2.2 wording, but the crawl gets no air between cycles. If that's a concern on the job, ask the inspector how they read it.

At an average clear height of 4 ft, 34 CFM works out to 0.30 air changes per hour of the crawl's 6,800 cu ft.

How big a dehumidifier does the code require?

It depends on the edition. The 2018 IRC says 70 pints of moisture removal a day for every 1,000 sq ft of crawl floor, which is 119 pints for 1,700 sq ft. The 2021 and 2024 IRC replaced that number with "sized in accordance with manufacturer's specifications," so the unit's own data sheet sets the size. In 2015 there was no dehumidifier option at all.

2018 IRC § R408.3, item 2.4

"Dehumidification sized to provide 70 pints ... of moisture removal per day for every 1,000 square feet ... of crawl space floor area." - International Code Council, codes.iccsafe.org

That 2018 sentence doesn't say what temperature or humidity the 70 pints is measured at, and that matters because rated pints change with the test. The Santa Fe Compact70 data sheet for model 4033600 prints 70 pints a day at 80°F and 60% RH and 47 pints at 70°F and 60% RH for the same unit.

Under 2021 and 2024 you go to the manufacturer's sizing. That same data sheet sizes the Compact70 for crawl spaces of 1,800 to 2,600 sq ft, or 9,000 to 13,000 cu ft, and notes that crawl space sizing is "based on 5 foot ceiling height." Held to the 2018 sentence, the Compact70's 70-pint rating at 80°F covers 1,000 sq ft. So by our reading, one unit can be sized for a 1,800 sq ft crawl by the 2021 rule and fall short of a 1,700 sq ft crawl by the 2018 rule.

The 1,700 sq ft crawl from the thread, worked three ways

For the thread's crawl, about 1,700 sq ft at an average 4 ft high, the code gives 34 CFM of supply or exhaust, or 119 pints a day under 2018. By our reading of its data sheet, one Santa Fe Compact70 covers it under 2021 and 2024, and it takes two under 2018. Our calculator puts the leakage moisture at about 15.6 pints a day.

The thread is on r/hvacadvice. The owner, also a general contractor, had just finished a house with a fully encapsulated crawl, no foundation vents and the gas furnace's air handler down there. Sensors in the crawl read 68 to 72% RH. The house is in Nashville, and the owner's HVAC contractor and home inspector both said it's either supply air or a dehumidifier.

1. Floor area and volume. About 1,700 sq ft at an average 4 ft clear height, the owner's own estimate, is 1,700 x 4 = 6,800 cu ft.
2. Supply or exhaust, items 2.1 and 2.2. 1,700 / 50 = 34 CFM, plus the return or air pathway and insulated perimeter walls.
3. Dehumidifier, 2018 item 2.4. 1,700 / 1,000 x 70 = 119 pints a day. One Compact70 at its 80°F rating gives 70, so by our reading it takes two of them, 140 pints, to clear 119.
4. Dehumidifier, 2021 and 2024 item 2.4. The Compact70 sheet lists 1,800 to 2,600 sq ft at a 5 ft height. This crawl is 100 sq ft under the low end by area and 2,200 cu ft under it by volume, 6,800 against 9,000 cu ft, because it's 4 ft high and the sheet assumes 5 ft. By our reading one unit covers it. The sheet doesn't say whether a crawl below its range should get a smaller unit.
5. Moisture balance on our page. Floor area 1,700, ceiling height 4, 0.18 ACH (the page's average preset, a BuildSolver assumption for whole houses, not a measured crawl rate), outdoor 85°F and 65% RH (the page's default humid day), space 70°F at a 60% RH target, no people. Result 15.6 pints a day from leakage.

Our balance only counts outdoor air leaking in and people. The 34 CFM of supply air in item 2.2 is conditioned house air, so it isn't one of these leakage rows. Our page doesn't model water coming up through the ground cover or the stem walls, so enter your own number for that under other moisture sources. It also doesn't turn a rated capacity into capacity at your conditions. Here the Compact70 sheet happens to print 47 pints at 70°F and 60% RH, the same condition as the target in step 5, so you can read the two side by side.

The thread's crawl also holds the gas furnace. Item 2.1 pulls air out of the crawl around the clock, and the 2021 IRC, in G2407.4, says that where exhaust fans "interfere with the operation of appliances, makeup air shall be provided." If that furnace burns crawl air, check its combustion air before you pick exhaust. A direct-vent furnace gets its combustion air by the manufacturer's instructions under G2407.1.

Can you use both supply air and a dehumidifier?

The code text asks for one method and doesn't stop you from adding a second. R408.3 says "one of the following shall be provided" in 2021 and 2024 ("is provided" in 2015 and 2018), and by our reading that's a minimum. The text doesn't say whether a supply branch added on top of a dehumidifier has to bring the 2.2 return pathway and wall insulation with it, so ask the inspector.

So the thread's "either or" is the code minimum. A dehumidifier-only crawl under item 2.4 carries no return path and no perimeter wall clause, and a crawl you treat under item 2.2 carries both.

Pick the method from what the job already has. Where the perimeter walls aren't insulated, item 2.4 is the only one of items 2.1, 2.2 and 2.4 that doesn't call for wall insulation. If the inspector enforces the 2018 IRC, the dehumidifier has to reach the 70 pints per 1,000 sq ft figure. If you're adding a supply branch, size it for the 34 CFM and plan the return pathway to the house along with the supply branch.

Checking the dehumidifier against the crawl's own moisture

Run the balance once the method is picked. Enter the crawl footprint, average clear height, a leakage rate and the conditions you want to hold on the dehumidifier size calculator, then add ground and wall moisture under other sources. Compare the result with the unit's rating at a condition close to the crawl's, then check the unit against the code minimum for your edition as a separate test.

1. Open the dehumidifier size calculator and enter the floor area and the average height from ground cover to subfloor.
2. Use a measured leakage rate if you have one. The air changes per hour calculator gives a rough natural ACH from a blower door number, ACH50 divided by 20, a rule of thumb from blower door research that the page itself calls a ballpark. A whole-house test doesn't measure the crawl alone.
3. Set the space temperature the crawl actually runs at and the RH you want to hold, then add your estimate for ground and wall moisture.
4. Check where ducts and pipes in the crawl will sweat at that target with the dew point calculator.

Take the pints a day from the dehumidifier size calculator, hold the unit's data sheet against that number at conditions close to the crawl's and against the R408.3 minimum for your edition, and pick a unit that clears both.

Sources

• International Code Council, 2015 International Residential Code, Section R408.3, codes.iccsafe.org
• International Code Council, 2018 International Residential Code, Section R408.3, codes.iccsafe.org
• International Code Council, 2021 International Residential Code, Sections R408.1 and R408.3, codes.iccsafe.org
• International Code Council, 2024 International Residential Code, Section R408.3, codes.iccsafe.org
• International Code Council, 2021 International Residential Code, Sections G2407.1 and G2407.4, codes.iccsafe.org
• Santa Fe (Therma-Stor), Compact70 data sheet, model 4033600, domyown.com
• Source thread on r/hvacadvice, an owner asking whether to condition or dehumidify an encapsulated crawl space, reddit.com/r/hvacadvice