Vapor Retarder, Wall Insulation, and Vents

The vapor retarder square footage with the 6 in lap and the 12 in wall lap, the Class I perm rating and the mil choice, the insulation area and the climate-zone R-value, and the sealed-crawl and vent decisions under the IRC.

IRC R316.1 asks for a Class I vapor retarder over the exposed ground: 0.1 perm rating or less, lapped 6 in at the seams, sealed at the laps, and up the walls 12 in. A sealed crawl conditions the space, and a wet crawl gets the moisture source fixed before the plastic goes down.

—Vapor retarder area and rolls
—Mil choice and perm rating
—Wall insulation area and R-value
—Vent area and the sealed decision
—Seams, penetrations, and radon
—Ground prep, water, and mold
How the math works
  • Vapor retarder (IRC R316.1): a Class I vapor retarder with a perm rating of 0.1 or less covers the exposed ground in a crawl space, in strips lapped 6 in at the seams, sealed at the laps and at the penetrations, and extending 12 in up the sides of the foundation walls and piers and sealed. The Class I rating is the perm test, and 15 mil reinforced poly is the field product that carries it; a 6-mil sheet at about 1.6 perms is a Class II material.
  • Area: floor area plus the 12 in lap up the perimeter walls plus 20 percent waste for the seams, the folds, the piers, and the cuts. Rolls ship at 1,000 sq ft in the 10 ft by 100 ft field size, and a 40 ft by 28 ft crawl at 1,120 sq ft of floor is 1,508 sq ft with the 136 sq ft wall lap and the 20 percent waste, which is 2 rolls.
  • Crawl wall insulation (IRC table R402.2.11): crawl wall values are published by climate zone as a continuous insulation layer with a cavity layer: zone 1 and 2 at R-12 plus R-11, zone 3 and 4 at R-11 plus R-11, zone 5 at R-10 plus R-13, zone 6 at R-10 plus R-15, zone 7 at R-13 plus R-21, and zone 8 at R-13 plus R-30. Local code adoption, the climate zone map, the continuous layer product, and the cavity product are verified with the authority having jurisdiction.
  • Vents (IRC R408.2): vented crawl spaces get 1 sq ft of net vent open area per 1,500 sq ft of crawl floor area, with cross ventilation on opposite walls, a screen and a cap, and the vents closable. A conditioned crawl space under IRC R316.2 closes the vents, insulates the wall assembly, conditions the space with the house air with the supply and the return, and the vapor retarder covers the ground.
  • Water first: grade the site away, extend the downspout, keep the footing drain to a daylighted outlet or a sump, cap the sump lid, keep the crawl clearance at the code minimum of 18 in clear under the joists and 12 in over the ground (IRC R408.1), and dry the assembly to the reading before the retarder goes down.
Planning tool on the International Residential Code sections R316, R408, and table R402.2.11 as adopted locally, with the industry encapsulation practice. Climate zone assignment, the code edition your jurisdiction adopted, the crawl wall R-value table in use, the radon zone map, the permit, and the inspection are jurisdiction calls. A crawl with a wood beam, a post, a girder, a pier, a radon system, a duct, a water heater, a sewer line, a gas line, a termite clearance, a flood zone, a frost line, a footing drain, a sump, a mold history, an asbestos pipe wrap, and an unfinished basement wall are all site conditions a calculator cannot see, and the perm rating, the mil spec, and the seam tape are the manufacturer listings.

What plastic goes in a crawl space?

IRC R316.1 asks for a Class I vapor retarder over the exposed ground: a perm rating of 0.1 or less, lapped 6 in at the seams, sealed at the laps and the penetrations, and 12 in up the foundation walls and piers. 15 mil reinforced poly carries the Class I rating in the field product; a 6-mil sheet at about 1.6 perms is Class II.

How much poly does a crawl space need?

Floor area plus the 12 in lap up the perimeter walls plus 20 percent waste: a 40 ft by 28 ft crawl is 1,120 sq ft of floor, 164 sq ft of wall lap, and 1,541 sq ft with the waste, which is 2 rolls in the 1,000 sq ft size. The piers, the posts, the folds, and the cuts are the waste.

Should I close the crawl space vents?

A conditioned crawl space closes the vents, insulates the wall assembly, and conditions the crawl with the house air with a supply and a return (IRC R316.2). A vented crawl keeps 1 sq ft of net vent area per 1,500 sq ft of floor with cross ventilation (IRC R408.2). A wet crawl is the case where the vents are not the fix: the water, the retarder, and the drying come first.

What R-value does a crawl space wall need?

The crawl wall values in IRC table R402.2.11 are published by climate zone as a continuous layer plus a cavity layer: R-12 plus R-11 in zones 1 and 2, R-11 plus R-11 in zones 3 and 4, R-10 plus R-13 in zone 5, R-10 plus R-15 in zone 6, R-13 plus R-21 in zone 7, and R-13 plus R-30 in zone 8. A 3.5 in cavity at R-3.9 per inch is R-13.7, and the continuous layer is the rigid, the board, and the spray foam on the wall face.

Related tools

Mold containmentThe growth a wet crawl grows Water damage dry-outDrying the crawl before the plastic Insulation R-valueThe floor assembly above the crawl Attic ventilation sizingThe 1 in 1,500 venting math Lead RRP work area6 ft area, plastic, and clearance Radon system sizingThe suction pipe that pulls gas under the membrane Foundation drainage slopeThe 18 in clearance and the grade line