Calculator 11 / Insulation
Insulation Calculator
Batts are sold by coverage per bag and spray foam by the board foot, so the two are ordered in completely different units. Figure bags/rolls needed for batt insulation, or board feet and kits needed for spray foam.
How this calculator works
A "board foot" of spray foam is 1 square foot covered at 1 inch of thickness — the standard unit the industry sells foam kits by. Batt/roll coverage is left as a field you fill in from the product label rather than a hardcoded assumption, since coverage per bag varies by R-value, batt width, and manufacturer.
About the R-value figure
<strong>The R-value shown with your result is a rough arithmetic estimate, not a performance claim.</strong> It multiplies your thickness by a nominal per-inch figure for the foam type you picked. Real installed performance depends on the specific product, its age, the installed density, the temperature it was sprayed at, and how completely the cavity was filled. <strong>Only the tested R-value printed on the product's own label counts for building-code compliance or for any energy calculation</strong> — use that number, not this one, and have an installer or energy assessor confirm anything you are relying on.
| Foam type | Nominal R per inch | Character |
|---|---|---|
| Open-cell | ~3.5 | Vapour-permeable, lower cost, expands a long way, stays flexible |
| Closed-cell | ~6.5 | Rigid, adds stiffness, acts as a vapour retarder at sufficient thickness |
The practical consequence of that gap is depth. To reach a given R-value, open-cell needs roughly 1.8 times the thickness of closed-cell — which is why open-cell is common in generous roof and attic cavities and closed-cell wins where the depth simply is not there. Note that published per-inch values vary between manufacturers and that some products are rated on an aged rather than an initial basis; the numbers above are round figures for orientation only.
Worked example: 600 sq ft of closed-cell
A 30 ft × 20 ft area at 2 in of closed-cell foam, 15% overspray allowance, 600 board-foot kits:
- Area = 30 × 20 = 600 sq ft
- Board feet = 600 × 2 = 1,200 bf
- With 15% overspray = 1,380 bf
- Kits = 1,380 ÷ 600 = 2.3, rounded up = 3 kits
- Nominal R-value at 2 in = about R-13
Three kits deliver 1,800 nominal board feet for a job needing 1,380 — the round-up is steep because kits are large and indivisible. Before buying the third, note that kit yields are nominal: real output is commonly below the number on the box, and falls further if the chemicals or the substrate are cold. Plan for the round-up rather than hoping to squeeze the job out of two.
What actually fits in a cavity
Framing depth is a hard limit, and it is the first thing to check before choosing a product. These are dressed lumber dimensions, so they do not change with supplier.
| Framing | Actual cavity depth | Commonly insulated to |
|---|---|---|
| 2×4 wall | 3½" | R-13 to R-15 batt |
| 2×6 wall | 5½" | R-19 to R-21 batt |
| 2×8 joist | 7¼" | R-25 to R-30 |
| 2×10 joist | 9¼" | R-30 to R-38 |
| 2×12 joist | 11¼" | R-38 and up |
The classic trap is a nominal R-19 batt, which is manufactured around 6¼ inches thick, pushed into a 2×6 cavity that is only 5½ inches deep. Compressed like that it does not deliver its rated value — the assembly performs closer to R-18, and every published figure assumes the batt is at full loft and completely filling the space. Gaps, compression around wiring, and batts cut short at the ends all cost more performance than most people expect. Required R-values themselves depend on your climate zone and on which code edition your jurisdiction has adopted, so look yours up rather than assuming.
Where insulation estimates go wrong
- Taking kit yield at face value. Board-foot ratings on foam kits are nominal and generous. Cold chemicals, cold substrate, or a slow trigger all cut real output.
- Measuring the floor instead of the surface. A sloped roof or a gable end has more surface area than its footprint. Insulate by the area you are actually covering.
- Ignoring framing area. Studs and joists are perhaps 15% of a wall and they conduct heat around your insulation. That is thermal bridging, and it is why an assembly never performs at the number on the batt.
- Forgetting ventilation and clearances. Attic soffit vents need baffles kept clear, and recessed lights, flues and chimneys have required clearances. Insulation packed against them is a fire risk, not a saving.
- Skipping air sealing. Insulation slows conduction; it does not stop air movement. Sealing the leaks first is usually the cheaper half of the job and makes the insulation work as intended.
Where these numbers come from
R-value is a regulated claim in the United States, not a marketing figure. The number that counts for code compliance is the product's tested, labeled value.
- FTC R-Value Rule, 16 CFR Part 460 — The federal rule governing how R-value may be stated, tested and advertised.
- ENERGY STAR Seal and Insulate — Recommended insulation levels by climate zone, and where air sealing matters more than R-value.
- Insulation Institute (NAIMA) — Manufacturer association guidance on installation quality, which drives real-world performance.
Open-cell vs. closed-cell — does it change the math?
Not the board-foot math itself — that's pure area × thickness. The foam type matters for how much R-value you get per inch (closed-cell roughly doubles open-cell's R-value per inch), which affects what thickness you should target in the first place.
Why is waste higher for spray foam than batts?
Spray foam overspray, trimming proud foam flush with framing, and uneven cavity fill typically account for more material loss than cutting batts to fit — 15% is a reasonable starting point versus 10% for batts.
Can I use the R-value shown here for my building permit?
No. It is thickness multiplied by a nominal per-inch figure, which is fine for deciding roughly how deep to spray and useless as evidence. Building officials and energy assessors work from the tested R-value on the product's label, determined under standard test methods. Installed performance also depends on density, temperature and how completely the cavity was filled. Use the label figure for anything official, and get an installer or assessor to confirm it.
Why did my kit not go as far as the box said?
Kit yields are nominal and most people get less. Output drops when the chemicals or the surface are below the temperature range on the instructions, when the tank pressure falls near the end, and whenever foam is sprayed thicker than the recommended pass depth. That is why the overspray factor here defaults to 15% rather than the 10% used for batts — and why it is worth planning for the round-up instead of hoping to finish on the last kit.
Does a compressed batt still give its rated R-value?
No. Rated values assume the batt is at full loft and filling the cavity completely. The common case is a nominal R-19 batt, made around 6¼ inches thick, squeezed into a 5½-inch 2×6 cavity — it performs closer to R-18. Gaps at the edges, compression around wiring and batts cut short cost more than most people expect, which is why a carefully fitted lower-rated batt often beats a compressed higher-rated one.
Can I save, share, or print a calculation?
Yes — use the Save, Copy link, and Print buttons under your results. Saved calculations can be grouped into a named project on the My Projects page for a combined job estimate with cost totals. Everything is stored only in your browser, never uploaded — see the Privacy Policy for details.
Estimates only. Confirm exact coverage and yield figures against your specific product's data sheet, and consult a licensed insulation contractor for code-required R-values in your climate zone. The R-value shown with spray-foam results is a nominal arithmetic estimate for planning, not a tested performance rating — only the R-value printed on the product's own label is valid for code compliance or energy calculations.