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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Insulating a floor over an unheated space is the fastest fix for cold feet. Enter the footprint and joist layout for bay counts and batt quantities.
With these inputs:
| Result | Value |
|---|---|
| Floor area | 720 sq ft |
| Joist bays | 18 bays |
| 8 ft batts required | 82 pieces |
| R-value achieved | R-29.6 |
| Recommended for your zone | R-30 |
| Support rods needed | 246 rods |
| Material cost | $952 |
Floor area = length × width · Bays = width in inches ÷ joist spacing · Batts = area × (1 + waste %) ÷ area per batt
Facing toward the heated side — that means up, against the subfloor. In humid southern climates unfaced batts are often the better choice.
Wire support rods (lightning rods) every 16 inches, or house wrap stapled across the joists. Never rely on friction — batts sag and lose their R-value once there's a gap.
Any vapour retarder faces the heated side, so in a floor over an unheated crawlspace or garage the facing goes up, toward the room. Getting it upside down puts the retarder on the cold side, where moisture moving up from below can condense against it inside the assembly.
Support them mechanically rather than by friction. Wire insulation supports sprung between the joists every couple of feet are the usual method; netting or lath also works. Batts that rely on friction alone will sag within a few seasons, and a sagging batt with an air gap above it is doing very little.
It will stop the floor pulling heat out of the room, which is what most people are actually feeling. It will not make the floor warm the way radiant heating does. If the complaint is specifically cold feet on tile, insulation helps but underfloor heating is the thing that solves it.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.