Floors & Crawlspace

Floors over crawlspaces and garages need R-13 in Zones 1–2, R-19 in Zone 3, and R-30 in Zones 4 and above. Those numbers are easy to reach. What makes floor insulation different is how reliably it stops working.

Insulation only works in contact

A batt that sags away from the subfloor leaves an air space, and air in that space circulates — carrying heat from the floor past the insulation to the cold side. Friction-fit batts in a floor will sag, every time, over a few years of humidity cycling.

The fix is mechanical: wire insulation supports every 18 to 24 inches, or netting stapled across the joists.

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 envelope — which puts ducts and pipes in conditioned space, removes the plumbing freeze risk, and usually covers less area than the floor.

It needs a sealed ground vapor barrier: 6 mil poly across the entire crawlspace floor, lapped and sealed up the walls. That single measure does more for a crawlspace than any amount of ventilation.

Where the crawlspace is wet, fix the water first. Insulating a damp crawlspace either way builds a mold incubator.

Garage ceilings are an air barrier question

A floor over a garage must keep vehicle exhaust out of the house, not merely slow heat. Rigid board or spray foam tight to the joists with sealed seams. Batts alone do not do it.

Vapor retarders under floors

Kraft facing goes toward the conditioned space — upward, against the subfloor, in a heating climate. In a cooling-dominated climate that reverses, which is one reason unfaced insulation with a separately detailed air barrier is often the more robust approach.

Slabs are a different problem entirely

A slab loses most of its heat at the perimeter, not through the middle. R-10 rigid at the slab edge, extending 2 to 4 ft down or inward, is where the return is.