A conditioned unvented crawl space must meet four connected requirements under the 2024 International Residential Code: a sealed ground vapor retarder, insulated perimeter walls sized to your climate zone, one of four approved methods of conditioning the air inside, and supporting details for fire protection, drainage, and radon where applicable. Section R408.3 is the controlling provision, and every element has to work with the others. Miss one and the assembly can trap moisture, fail inspection, or waste the energy you were trying to save.
Ground Vapor Retarder
The soil is the largest moisture source under your house, so the assembly starts there. IRC R408.3 requires a continuous Class I vapor retarder over all exposed earth. Class I means a permeance rating below 0.1 perms. Six-mil polyethylene sheeting meets that threshold, and heavier products tested to ASTM E1745 hold up better where anyone will walk on the liner during service calls.1UpCodes. R408.3 Unvented Crawl Space
Joints in the sheeting must overlap at least 6 inches and be sealed with compatible tape or mastic. The edges extend at least 6 inches up the interior face of the stem wall and are fastened and sealed to the wall or to any insulation covering it. That turns the liner into a continuous tray so soil vapor cannot slip in at the wall-to-floor junction.1UpCodes. R408.3 Unvented Crawl Space
If your jurisdiction has adopted IRC Appendix F for radon, the same sheeting doubles as a soil-gas retarder, but joints must overlap 12 inches and a vent pipe fitting is installed beneath the membrane.2International Code Council. Appendix F Radon Control Methods
Foundation Wall Insulation by Climate Zone
Sealing the crawl space shifts insulation from the floor above to the perimeter walls, which brings ducts, pipes, and equipment inside the thermal envelope. Required R-values come from IRC Table N1102.1.2:3UpCodes. N1102.1.2 Insulation and Fenestration Criteria
- Zones 1 and 2: no crawl space wall insulation required.
- Zone 3: R-5 continuous or R-13 cavity.
- Zone 4 except Marine: R-10 continuous or R-13 cavity.
- Zones 5 through 8 and Marine 4: R-15 continuous or R-19 cavity, or a combined R-13 cavity plus R-5 continuous.
Continuous insulation is rigid foam or spray foam applied directly to the wall with no thermal break at framing. Cavity insulation is batt or blown material between framing members. Rigid foam board or closed-cell spray foam applied to the concrete or block is the most common choice because it insulates and air-seals in one step. The insulation runs from the top of the wall down to the crawl space floor.
Rim Joist Air Sealing
The rim joist is the leakiest spot in most crawl spaces. Left unsealed, it lets outdoor air pour into the conditioned zone and creates a cold surface where indoor moisture condenses in winter, which invites rot. Spray polyurethane foam is the standard fix because it conforms to the irregular geometry of joists, sill plate, and subfloor in one pass. Closed-cell (about 2 pcf) also adds vapor resistance; open-cell (about 0.5 pcf) does not, but both air-seal.4ENERGY STAR. BSC Information Sheet 408 – Critical Seal (Spray Foam at Rim Joist)
For the foam to function as an air barrier, it must bridge continuously from the wall face, up over the rim joist, and onto the underside of the floor sheathing with no gaps. Rigid board cut to fit each joist bay and caulked around all edges works too, but it takes far more labor to reach the same airtightness.
Four Approved Conditioning Methods
Because the space is cut off from outdoor air, the code requires a mechanical strategy to keep it dry. IRC R408.3 lists four options, and every option still requires the perimeter wall insulation above.1UpCodes. R408.3 Unvented Crawl Space
Continuous Mechanical Exhaust
A small fan runs continuously, pulling air out of the crawl space and discharging it outdoors. Size it at 1 CFM per 50 square feet of crawl space floor area, so a 1,500-square-foot crawl space needs at least a 30 CFM fan. A duct or transfer grille from the living area supplies the makeup air, meaning your conditioned indoor air is what keeps the crawl space dry. The fan is not on a timer or humidistat.5Georgia Department of Agriculture. 2012 International Residential Code Section R408 Under-Floor Space
Conditioned Air Supply
Instead of exhausting, this method pushes conditioned air from the HVAC system into the crawl space at the same 1 CFM per 50 square feet. A return pathway, either a duct or a transfer grille, connects back to the common living area so air circulates rather than pressurizing the space. Homes with ductwork already running through the crawl space often take this route because adding a supply register is straightforward.5Georgia Department of Agriculture. 2012 International Residential Code Section R408 Under-Floor Space
Dehumidification
The 2024 IRC added a standalone dehumidifier as a recognized method. The unit must remove at least 70 pints of moisture per day per 1,000 square feet of crawl space floor area. That is a meaningful threshold; a typical 30- or 40-pint residential dehumidifier will not comply for most crawl spaces. The code sets compliance by rated removal capacity relative to floor area, not by a target humidity reading.1UpCodes. R408.3 Unvented Crawl Space
Under-Floor Plenum, Existing Buildings Only
The crawl space itself can serve as a supply plenum for the HVAC system, but the 2024 IRC prohibits this in new construction. In a retrofit, the plenum must comply with IRC M1601.5: tightly enclosed, cleaned of combustible debris, covered with a moisture barrier, and any lining insulation must have a flame-spread index no greater than 200.1UpCodes. R408.3 Unvented Crawl Space
Fire Protection Over Foam
Foam plastic insulation is combustible, so the code requires a protective layer between the foam and the crawl space interior. Where the crawl space is entered only for utility service and not used for storage, an ignition barrier is allowed in place of a full 15-minute thermal barrier like drywall. Materials that qualify as an ignition barrier without further testing:
- 1½-inch mineral fiber insulation
- ¼-inch wood structural panels
- ⅜-inch particleboard
- ¼-inch hardboard
- ⅜-inch gypsum board
- Corrosion-resistant steel at least 0.016 inches thick
Some spray foam products can skip the ignition barrier entirely if the manufacturer has obtained approval through a fire test such as NFPA 286, but that approval is product-specific. Check the evaluation report before relying on it. Spray foam left exposed at the rim joist may also qualify for an exemption when it is less than 3¼ inches thick, depending on local interpretation.4ENERGY STAR. BSC Information Sheet 408 – Critical Seal (Spray Foam at Rim Joist)
Drainage and Grading
A sealed crawl space is far less forgiving of bulk water than a vented one because there is no outdoor airflow to help it dry. IRC R401.3 requires the ground surface to slope away from foundation walls at least 6 inches within the first 10 feet, with surface water directed to a storm sewer or other collection point that does not create a hazard.6International Code Council. International Residential Code Chapter 4 Foundations
Where the crawl space floor sits below exterior grade, IRC R405.1 requires a perimeter foundation drain. Drain tile or perforated pipe is installed at or below the top of the footing, surrounded by washed gravel, and covered with a filter membrane so soil cannot clog it. Where gravity drainage is not possible, a sump pump provides the mechanical alternative. In a conditioned crawl space, cover and seal any sump pit so ground moisture cannot evaporate into the conditioned air and defeat your humidity control.
Group I soils under the Unified Soil Classification System (clean gravels and sands) can qualify for an exception to the perimeter drain requirement if a geotechnical report confirms the classification. Most inspectors still expect the grading requirement to be met regardless.
Radon Where Appendix F Applies
Sealing a crawl space concentrates soil gases in ways that a vented design does not. The EPA’s Map of Radon Zones identifies high-potential areas but does not mandate mitigation; that authority rests with state and local jurisdictions, many of which adopt IRC Appendix F for new construction.7U.S. Environmental Protection Agency. EPA Map of Radon Zones and Supplemental Information
Where Appendix F applies, the crawl space needs a soil-gas retarder (the same polyethylene, but with 12-inch joint overlaps) and a passive vent pipe. The pipe connects to a fitting beneath the sheeting through a plumbing tee, runs vertically through the building, and terminates at least 12 inches above the roof. The termination must be at least 10 feet from any operable window or other opening into conditioned space that is less than 2 feet below the exhaust point.2International Code Council. Appendix F Radon Control Methods
A passive system uses the stack effect to draw soil gas up the pipe. If post-construction testing shows radon at or above the action level of 4 picocuries per liter, an inline fan converts the passive pipe into an active depressurization system. Roughing the pipe in during construction is cheaper than retrofitting, so even outside Appendix F jurisdictions it is a reasonable precaution in moderate- to high-risk zones.
Access Opening
The crawl space needs an access opening at least 18 inches by 24 inches. Where mechanical equipment such as a furnace or air handler sits inside, local mechanical codes may require a larger opening and a clear working space around the unit. Locate the opening so it does not compromise the vapor retarder or air seal, and fit it with a gasketed, insulated cover that keeps the conditioned envelope closed when not in use.