Rim and Band Joist Insulation: Where Title 24 Wants It

The baseline: match the wall beside it

Reference Residential Appendix RA3.5 repeats one sentence for batts, loose fill, rigid board, panelized walls and spray foam: all rim joists are insulated to the same R-value as the adjacent walls. Each version bans gaps and voids; batts and loose fill also limit compression. A rim joist packed with R-11 scraps beside an R-21 wall does not meet it.

Three junctions with extra wording

• Floor over a garage, or any floor with an air space between the insulation and the subfloor: the rim joist is insulated. Where living space sits above, rim and band joists next to it must also be airtight.
• Garage roof meeting an attic above living space: the band joist gets an air barrier touching the edge of the attic insulation.
• Joists between floors of a multistory building: the compliance manual says they satisfy the mandatory wall rule with R-15 on the inside of the joist, fitted between the intersecting joist members.

The R-15 sentence is written for the mandatory wall requirement, while the QII text is the adjacent-wall rule, so ask the rater which figure your report relies on.

With spray foam

Closed-cell foam need not fill every framed cavity if the installed thickness matches the compliance documents. The condition is that plates and framing ends, band and rim joists included, are sprayed to fill the cavity beside the framing.

Where QII does not reach

QII is prescriptive for new homes and additions over 700 square feet. The alteration section lists no QII item. Adding insulation to an existing floor over a garage is an alteration that brings in the raised-floor rule of Section 150.0(d), but whether the department expects the joist ends insulated at the same time is its call.

See raised floor insulation for the floor itself, ADU over a garage and bonus room over a garage for the garage cases, cantilevered floors for overhangs and the QII page for inspection. Our residential reports list the QII requirement.

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

Which R-value goes in the rim joist?

Does the R-15 rim joist note apply to every home?

Explore Floors and Slabs

1. Slab Floor Title 24 Requirements for Additions and Conversions
2. Cantilevered Floors and Overhangs: Floor Insulation Under Title 24
3. Do I Need a Vapor Barrier in California? Crawl Space, Walls and Attics

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