Insulation Calculator
Batt-roll count for walls, attics, and crawlspaces
What is insulation calculator?
Insulation roll count is cavity area ÷ coverage-per-roll × (1 + waste factor), ceiling to a whole number. Coverage per roll drops as R-value rises: an R-13 roll covers 88 sqft, R-19 covers 75 sqft, R-30 covers only 40 sqft — because the denser, thicker batt fills the same roll package with less face area. The 5% default waste is low because batt rolls cut with a utility knife and straightedge, and offcuts from one bay often fill the next shorter bay with almost no loss. Heavily obstructed cavities with lots of cross-blocking, pipes, and wiring want 10%.
R-value selection is driven by climate zone and framing depth. R-13 fills a 3.5 in (2×4) wall cavity — the minimum for most residential exterior walls and the Energy Star baseline for Climate Zones 1-3. R-19 fills a 5.5 in (2×6) cavity and is required for walls in Climate Zones 4-7. Attic floors are the highest-ROI insulation upgrade in most homes — stepping from R-19 to R-38 in an attic cuts heating and cooling load by 15-25% and pays back within 3-5 years in most US climates. R-30 is the minimum; cold-climate zones (5-7) should target R-49 to R-60 with a second perpendicular layer of unfaced batts.
Faced vs unfaced is a vapor control decision. Faced batts (kraft paper or foil backing) have a Class II vapor retarder built in — install them with the facing toward the warm side (living space side in cold climates, exterior side in hot-humid climates). Never install two faced layers; the vapor retarder traps moisture between them. Unfaced batts are used when adding a second layer over existing insulation, in basement walls where a separate 6-mil poly sheet will serve as the vapor barrier, and in hot-humid climates where inward vapor drive makes a retarder counterproductive.
Coverage per roll varies by manufacturer and stud spacing. The presets use Owens Corning EcoTouch coverage at 16 in on-center framing — the most common residential spacing. At 24 in on center, fewer studs means slightly higher net coverage per roll. Use Custom if your supplier’s packaging states a different sqft rating, and always verify the roll label before buying in quantity. A project that requires 16 rolls at 40 sqft each and gets 35-sqft rolls by mistake will run 13% short.
When to use a insulation calculator
- Wall cavities of an addition — A 320 sqft wall area insulated with R-13 batts (88 sqft per roll) at 5% waste requires 4 rolls. R-13 fills a standard 2×4 stud cavity at 16 in on center — the most common residential wall framing. Buy an extra roll for the headers and blocking that get odd-sized cuts.
- Attic floor — A 600 sqft attic floor insulated with R-30 batts (40 sqft per roll) at 5% waste requires 16 rolls. R-30 is the Energy Star minimum for attic floors in most US climate zones; colder zones (5-7) warrant R-49 to R-60, which may require two layers of batts laid perpendicular.
- Crawlspace floor or basement walls — A 240 sqft crawlspace floor insulated with R-19 batts (75 sqft per roll) at 5% waste requires 4 rolls. R-19 fills a 2×6 floor joist cavity — standard for floors over unheated crawlspaces in most climate zones. Secure with wire support rods or insulation hangers between the joists.
How to use the Insulation Calculator
- Measure the cavity area — Surface area of the wall, floor, or ceiling being insulated. Subtract major openings (door, window) only if they're large — the waste factor covers small studs and headers.
- Pick the R-value batt roll — R-13 for 2×4 walls, R-19 for 2×6 walls or floors, R-30 for attics. Coverage per roll falls as R-value rises — denser batts cover less area per package.
- Apply a waste factor — 5% is the default. Long uninterrupted runs (open attics) waste almost nothing; cut-up cavities with lots of studs and obstructions want 10%.
Worked examples
Wall area 320 sqft, R-13
Input: Total area 320 sqft, R-13 roll (88 sqft)
Output: 4 rolls (320 sqft + 5% waste)Attic 600 sqft, R-30
Input: Total area 600 sqft, R-30 roll (40 sqft)
Output: 16 rolls (600 sqft + 5% waste)Floor joists 240 sqft, R-19
Input: Total area 240 sqft, R-19 roll (75 sqft)
Output: 4 rolls (240 sqft + 5% waste)