Do I Need a Vapor Barrier in California? Crawl Space, Walls and Attics

Rule one: the crawl space floor, any zone

Section 150.0(g)1 covers the bare earth under a house. If the crawl space is unvented, the soil is covered with a Class I or Class II vapor retarder. A controlled-ventilation crawl space that uses the exception to Section 150.0(d) must have one too. That exception lets a builder skip raised-floor insulation if the foundation walls are insulated, the vents have temperature-actuated automatic louvers and Appendix RA4.5.1 is met. The appendix wants dampers on the plans, closed near 40 degrees F and open near 70, with vent area spread between opposing walls. Foam that touches the earth must be closed-cell and water resistant.

Rule two: zones 14 and 16 only

Section 150.0(g)2 puts a Class I or Class II retarder on the conditioned-space side of all insulation in all exterior walls, in vented attics, and in unvented attics that use air-permeable insulation. The rule sits in the mandatory list, so it reaches new homes, additions and the altered component in an alteration. A house in zone 8 or 12 has no such rule.

What the class means

The Standards define vapor retarder class by pointing to Section 202 of the California Building Code. They do not list perm ratings themselves, so take the class definition from the Building Code and the product data sheet.

Side effects worth knowing

• Under Residential Code R806.2, attic vent area is 1/150 of the attic space; the compliance manual says it may drop to 1/300 where a ceiling vapor retarder is installed in zones 14 and 16.
• Urea formaldehyde foam goes only in exterior side walls, always with a 4-mil polyethylene retarder or equivalent sheathing between the foam and the interior.

Bathroom moisture and wall assemblies in wet climates are Building Code questions, not Part 6. For the details of each situation, read crawl space insulation, unvented attics and wall insulation. The floor above the crawl space is on raised floor insulation. For a compliance report, see our residential reports.

Common Title 24 Questions

1. When is Title 24 required for a home?
2. Does a remodel need Title 24?
3. Does a room addition need Title 24?
4. What plans are needed for a residential Title 24 report?
5. How much does a residential Title 24 report cost?
6. How long does a residential Title 24 report take?

Frequently Asked Questions

Does an ordinary vented crawl space need one under Part 6?

Do I need one in the walls of a house in zone 10?

Explore Floors and Slabs

1. Rim and Band Joist Insulation: Where Title 24 Wants It
2. Slab Floor Title 24 Requirements for Additions and Conversions
3. Cantilevered Floors and Overhangs: Floor Insulation Under Title 24

View all Floors and Slabs guides → · Windows and Building Envelope · Title 24 guide