# §150.0(d): Floor insulation
What it requires
When a residence has a floor that does not sit on grade — over a crawl space, over an unconditioned basement, over a garage, or cantilevered over outside air — the floor assembly must be insulated:
- Raised floor over crawl space — R-19 cavity mandatory minimum.
- Floor over unconditioned garage — R-19 cavity mandatory (sometimes higher with fire-rated assembly stack-ups).
- Cantilevered floor over outside air (overhangs, bay-window floors) — R-19 cavity mandatory, with continuous insulation under the cantilever recommended in colder climate zones.
- Floor over conditioned basement — no §150.0(d) requirement; the basement is conditioned space.
Floors on grade (slab-on-grade) are covered by §150.0(e), not this section.
When it applies
- Newly constructed residential, on every floor that meets the descriptions above.
- Additions, on the addition's new floor assembly.
- Alterations whenever the floor cavity is opened up (a sub-floor replacement triggers it; a new finish-floor layer does not).
How we use it
The Confirm step asks for the floor type when the climate zone is one where R-19 isn't the prescriptive default — CZ1, CZ2, and CZ16 each call for R-30 on the §150.1(c)1 prescriptive path, which stacks on top of the §150.0(d) floor. For all other zones, R-19 cavity satisfies both.
The CF1R lists the floor construction used in the compliance model. The AHJ field inspector verifies installed batts at framing inspection — and crawl- space access is one of the most common late-stage delays because the inspector physically crawls under the house.
Common gotchas
- Crawl spaces with vapor barriers but no insulation — common in older houses that get an addition; the addition triggers §150.0(d) on the new floor area but not the existing.
- Garage ceilings that share a floor with conditioned space above — the assembly must be fire-rated (typically Type-X drywall both sides) AND R-19 insulated, two separate requirements that get value-engineered into one.
- Cantilevered bay-window floors that the framer treats as "interior" because the bay is finished space — the floor below the bay still vents to outside air and must be insulated.