Crawl Space Spray Foam: Vented vs Sealed Crawls

Vented vs sealed crawl spaces

A vented crawl space is treated as outdoors. Openings in the foundation let outside air through, and the insulation goes in the floor above. Under the IRC, the net vent area has to be at least 1 square foot for every 150 square feet of crawl floor, with an opening within 3 feet of each corner. That drops to 1 square foot per 1,500 when the ground is covered with a Class I vapor retarder.

A sealed crawl space (also called unvented or conditioned) is treated as part of the house, like a short basement. The vents are closed, the ground is covered and sealed, the walls are insulated, and the air is exhausted, supplied with conditioned air or dried.

Vented crawls have a seasonal problem. In summer, warm outdoor air comes in through the vents and cools as it meets the ground and foundation, and its relative humidity rises as it cools. That is why vented crawls often smell damp in humid weather and why sweating ducts and pipes are common there. In winter, those same ducts and pipes sit in near-outdoor air.

Why crawl space spray foam goes on the walls, not the floor

When you seal a crawl, the line between inside and outside moves from the floor down to the foundation walls and the ground. The insulation has to follow it.

Spray the underside of the floor in a sealed crawl and you get the worst of both: no vents to flush moisture, bare walls, and a crawl cut off from the warmth of the house above. The crawl ends up colder and damper than before.

In a sealed crawl, closed-cell foam goes on:

• the inside face of the foundation walls, from the vapor retarder up to the sill,
• the sill plate and rim joist, usually the leakiest line in the crawl (see rim joist spray foam),
• around pipe, wire and duct penetrations through the walls.

Closed-cell is the foam for this job. It tolerates ground moisture, sheds bulk water and becomes a Class II vapor retarder by about 2 inches; the exact thickness depends on the product. The DOE's insulation guide says open-cell foam should not be used below ground level, where it could absorb water.

How thick

The 2021 IECC lists crawl space walls in Climate Zone 5 at R-15 continuous insulation, R-19 cavity insulation, or R-13 cavity plus R-5 continuous. At about R-7 per inch, a typical aged value for HFO closed-cell, 2 inches is about R-14, just short. A 2.5-inch spec (about R-17.5) clears R-15 with margin for the small variation in any sprayed surface. The product's data sheet gives the exact R-value at each thickness.

Pennsylvania's energy code is based on the 2021 IECC with state amendments, and those amendments include an alternative path that allows R-10 continuous or R-13 cavity insulation on basement and crawl space walls in Zone 5. Your code official will tell you which path your permit uses.

Can the foam stay exposed?

If the crawl is entered only to service pipes and wiring, the IRC (R316.5.4) lets foam go without drywall as long as it is covered by an ignition barrier such as 1½ inches of mineral fiber, ¼-inch wood structural panel or ⅜-inch gypsum board. Products that pass specific fire testing (ICC-ES AC377 Appendix X) can skip the barrier, and some closed-cell foams have. Their evaluation reports attach conditions, typically: the crawl is entered only to service utilities, nothing is stored there, and crawl air is not circulated to other parts of the building. Not every product qualifies, so check the report for the foam on your quote. If your crawl will hold storage or get supply air from the furnace, plan on a covering and confirm with your code official. Thermal barrier vs ignition barrier explains the difference.

The ground vapor retarder

The ground is the biggest moisture source in most crawls. R408.3 requires the exposed earth to be covered with a continuous Class I vapor retarder (0.1 perm or less):

1. Overlap seams at least 6 inches and seal or tape them.
2. Run the edges at least 6 inches up the foundation wall, and attach and seal them to the wall.
3. Wrap and seal piers and posts the same way.

Six-mil polyethylene meets the minimum. If someone will be crawling over it for years to reach plumbing, a heavier reinforced liner holds up better.

Managing the air: the R408.3 options

A sealed crawl still needs its air managed. R408.3 requires one of these:

1. Continuously operated mechanical exhaust at 1 cubic foot per minute (cfm) for each 50 square feet of crawl floor, with an air pathway to the house.
2. Conditioned supply air at the same rate, with a return air pathway.
3. In existing homes, use of the crawl as an HVAC plenum under the code's plenum rules.
4. A dehumidifier, an option in newer editions of the IRC.

Take a 25 × 40 crawl, 1,000 square feet. Options 1 or 2 work out to 20 cfm. That is a small, steady flow, not a big fan.

One safety point: if a furnace or water heater in the crawl takes its combustion air from the crawl, closing the vents changes its air supply. Have your HVAC contractor sort out combustion air before the vents are sealed.

Drainage first

A sealed crawl over standing water is a sealed pond. Before any liner or foam goes in:

• Make sure the ground outside slopes away from the foundation and the downspouts discharge well away from the house.
• Look for water lines on the walls, wet soil after rain and efflorescence (white mineral deposits) on block.
• If groundwater comes up through the floor, install a drain and sump before the liner.
• Let wet walls dry. Spray foam needs a clean, dry surface and won't bond to wet masonry.

If there is mold on the framing, deal with it separately. Foam does not fix mold.

A note on radon

About 40% of Pennsylvania homes have radon above EPA's action level of 4.0 pCi/L, according to the Pennsylvania DEP. A sealed liner and foamed walls close many soil-gas paths, but they are not a mitigation system. Test after the work, ideally in the colder months on the lowest floor of the house. If the result is 4.0 pCi/L or higher, a state-certified mitigator can install a fan system that draws soil gas from under the crawl liner. Our radon case study explains where foam helps.

When a vented crawl is the better call

Sealing is not always worth the cost. A small, dry crawl with no ducts or water lines, under a floor that is already well insulated and air sealed, can stay vented. So can a seasonal camp that is drained and unheated in winter, where a sealed crawl would just be a cold, closed box.

Want a number for your crawl space? Price My Project or call (814) 208-3478.