Understanding your result
The headline is the bags of batts or the sheets of foam board, with the product, area and joist spacing beneath it. The stats give the R-value, the area between the joists and the wire supports; the table repeats them with the floor area.
Area between the joists is the floor area less the share taken by the joists themselves: a 1 1/2 in joist on 16 in centres takes 1.5 ÷ 16 of the floor, about 9%. Batts fill only the bays. Bags are that area with 5% for trimming, divided by the bag’s coverage. Bag coverage for floor batts is usually printed per bag of a given width, so if your bag says “covers 49 sq ft” for 16 in centres, the arithmetic here matches it.
Foam board is counted on the full floor area, because it is often fixed across the joists rather than between them. Wire supports are counted one every 24 in along each bay; they spring between the joists and hold the batts against the subfloor. The joists themselves are counted by the floor joist calculator, the panels above by the subfloor plywood calculator, and the crawl space ground cover by the moisture barrier calculator.
How we calculate this
The bag coverages are typical figures for floor batts sized for 16 and 24 in joist spacing; check the label of the product you buy and enter its figure. The R-value requirements quoted are from the 2021 IECC, Table R402.1.2, which many states have adopted with amendments. The subfloor prep guide covers the floor above the insulation, and the radiant heating guide explains why insulation under a heated floor matters more than under an ordinary one.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Batts | R-30, 10 in, about 58 sq ft per bag | Typical fiberglass floor batt packaging; R-19 bags cover about 49 sq ft |
| Joists | 1 1/2 in thick at 16 in centres | Standard 2× lumber framing |
| Trimming | 5% | Cuts around pipes, ducts and blocking |
| Wire supports | one every 24 in per bay | Insulation maker guidance (18 to 24 in) |
| Code R-values | R-13 to R-30 by climate zone | 2021 IECC Table R402.1.2 |
| Facing | toward the heated space | Insulation maker instructions |
Assumptions last reviewed October 8, 2026.
The calculator does not identify your climate zone or local code; it flags low R-values as a reminder. It does not count air sealing, which should come first at plumbing and wiring holes in the subfloor, or the thermal barrier exposed foam may need. It does not judge whether your crawl space should be vented or sealed, which depends on the house and the climate.
Two worked examples
A 30 × 24 ft floor over a crawl space in R-19
16 in joists, bags of 49 sq ft, wire supports.
- Between joists: 720 × (1 − 1.5 ÷ 16) = 653 sq ft
- Bags: 653 × 1.05 ÷ 49 = 14.0, rounded up to 14 bags
- Wire supports: 23 bays × 12 = 276
R-19 suits climate zones 3 and 4. In a colder zone the code asks for R-30, which in 2 × 8 joists is a problem: a 10 in batt does not fit a 7 1/4 in cavity, which is where high-density R-30 or foam board comes in.
A 20 × 14 ft room over a garage in R-30
24 in joists, bags of 58 sq ft.
- Between joists: 280 × (1 − 1.5 ÷ 24) = 263 sq ft
- Bags: 263 × 1.05 ÷ 58 = 4.8, rounded up to 5 bags
- Wire supports: 10 bays × 7 = 70
A room over a garage is one of the coldest floors in a house, and the garage ceiling below usually needs fire-rated drywall as well; the insulation goes in before it.
Where to find your inputs
Floor. The area of the floor being insulated, from below.
Joist spacing and thickness. Measure from below; 2× lumber is 1 1/2 in thick, I-joist flanges wider.
Coverage per bag. On the bag, for the joist spacing it is cut for.
R-value. From your local energy code for floors over unconditioned space.
Common mistakes
- Batts sagging away from the subfloor. The gap lets cold air in. Support them against the floor.
- Facing toward the crawl space. It goes toward the warm side.
- Crushing a thick batt into a shallow cavity. It loses R-value. Choose a batt that fits.
- Insulating a sealed crawl space at the floor. Insulate its walls instead.
- Skipping air sealing. Seal holes in the subfloor first.
- Exposed foam with no fire barrier. Check the code for crawl spaces and garages.
Questions people ask
- How much insulation for a floor over a crawl space?
- A 30 × 24 ft floor on 16 inch joists has about 653 sq ft of space between the joists. In R-19 batts at about 49 sq ft per bag, that is 14 bags with 5% spare. Wire supports every 24 inches add about 276 pieces.
- What R-value do I need under a floor?
- The 2021 International Energy Conservation Code asks for R-13 in the warmest zones, R-19 in zones 3 and 4, and R-30 (or insulation filling the cavity, at least R-19) in zone 4 marine and zones 5 to 8. Local codes may differ; check with your building department.
- Which way does the kraft facing go?
- Toward the heated space, so up against the subfloor, when the floor is over an unheated crawl space or garage. Unfaced batts are also common under floors. Never add a second vapor barrier on the other side.
- How are floor batts held in place?
- With wire supports (often called lightning rods or tiger teeth) sprung between the joists every 18 to 24 inches, or with mesh or strapping nailed across the joists. The batts must touch the subfloor without gaps and without being crushed.
- Can I use foam board under a floor?
- Yes, rigid foam (XPS or polyiso) can be cut to fit between joists and sealed with foam, or fixed across the underside of the joists, which also covers the joists. Exposed foam in a crawl space or garage may need an ignition or thermal barrier under the fire code.
- Should I insulate the floor or the crawl space walls?
- A vented crawl space is insulated at the floor; a sealed, conditioned crawl space is insulated at the walls and its ground covered with a barrier. Mixing the two approaches wastes money and can cause moisture problems.