Floors over unconditioned crawlspaces and garages need insulation, and the IECC sets it by zone: R-13 in Zones 1–2, R-19 in Zone 3, R-30 in Zones 4 and above.
Those numbers are reachable. What makes floor insulation different is how reliably it stops working.
The air gap
Insulation only works in contact with the surface it is insulating. A batt that sags away from the subfloor leaves an air space above it, and air in that space circulates — carrying heat from the floor to the cold side, straight past the insulation.
Batts held up by friction alone will sag. Every time, over a few years, with the humidity cycling.
The fix is mechanical: insulation supports (wire "tiger teeth") every 18 to 24 inches, or netting stapled across the joists. Not staples through the facing flanges, which tear.
Which way does the facing go
Kraft facing is a vapor retarder and it goes toward the conditioned space — upward, against the subfloor, in a heating climate.
Getting it wrong puts a vapor barrier on the cold side, where moisture moving out of the house condenses against it. In a cooling-dominated climate the correct direction reverses, which is one of the reasons unfaced insulation plus a separately detailed air barrier is often the better approach.
The better answer is often the crawlspace wall
Insulating the floor treats the crawlspace as outdoors. Insulating the crawlspace walls and sealing the vents brings it inside the thermal envelope.
That approach:
- puts ducts and pipes in conditioned space, which is often a larger saving than the floor insulation itself
- removes the freeze risk from plumbing
- is a smaller area than the floor in most houses
It requires a sealed ground vapor barrier — 6 mil poly across the entire floor, lapped and sealed up the walls — and it needs local code to permit unvented crawlspaces, which most now do.
Where the crawlspace is wet, fix the water first. Insulating a damp crawlspace either way is building a mold incubator.
Garage ceilings
A floor over a garage is the same assembly with an added requirement: it must be an air barrier good enough to keep vehicle exhaust out of the house. Rigid board or spray foam applied tight to the joists, seams sealed, is the usual answer. Batts alone are not.
Rim joists
The rim joist band is a small area with an outsized heat loss, and batts do not seal it. Rigid foam cut to fit and sealed at the edges, or closed-cell spray foam, handles both insulation and air sealing in the one place where they are hardest to separate.