How this insulation calculator works
Insulation sells by the bag, and the bag states its coverage — so the whole estimate is the surface area done honestly, then whole packages over it:
bags = area × (1 + waste%) ÷ ft² per bag, rounded up
At the defaults — 30 × 24 ft walls at 8 ft, 10% openings, 100 ft² bags — that’s 9 bags, about $495; switch to attic mode and the same footprint is 8 bags. The full chain is in the coverage math section below.
Insulation types and where they go
This calculator counts batts and rolls by package coverage — the accessible-cavity product — but the family is worth knowing so you order the right one:
Fiberglass batts and rolls
Pre-cut to stud and joist bays, faced or unfaced, R-13 to R-38. The DIY standard for open walls, floors and accessible attics — what the bag count here assumes. Friction-fit snug, no gaps or compression.
Mineral wool batts
Denser fiber batts with better fire resistance and sound damping, cut and installed like fiberglass at a higher price. Same coverage math, same bag count.
Blown-in (loose fill)
Cellulose or fiberglass blown to depth — the cheap, fast way to hit a high attic R-value, and the only practical retrofit for closed walls. Coverage is bags-per-target-depth off the chart on the bag, not the flat ft² this tool uses.
Spray foam
Open- or closed-cell foam that air-seals as it insulates — the highest performance and highest price, and a board-foot job, not a bag count. Rim joists and irregular cavities are its sweet spot.
Rigid foam board
Continuous foam panels on foundations, under slabs and as exterior sheathing to break thermal bridging — counted by the sheet, not the bag.
Installing batts so they actually work
A batt only delivers its rated R-value when it fills the cavity completely — no gaps at the edges, no compression behind wires and pipes, and split neatly around outlet boxes. A crammed or gapped batt can lose a third of its rating, which is why careful cutting matters more than an extra bag. Friction-fit snug, face the vapor retarder the right way, and don’t stuff.
What R-value insulation by assembly?
Table 1 is the R-value target by assembly; Table 2 shows why the bag coverage is an input, not a constant — it collapses as the batt thickens.
| Assembly | R-value |
|---|---|
| Attic | R-38–60 |
| 2×4 walls | R-13–15 |
| 2×6 walls | R-19–21 |
| Floors (unconditioned below) | R-25–30 |
| Product | ft²/bag |
|---|---|
| R-13 wall batts | ≈ 100–160 |
| R-19 batts | ≈ 75–110 |
| R-30 attic batts | ≈ 55–90 |
| R-38 attic batts | ≈ 40–65 |
How to measure for insulation
Walls are perimeter times height minus openings; attics are the flat footprint. Pick the mode and the bag coverage does the rest.
- Pick the mode: walls for exterior perimeter, attic for a flat overhead area. The height field disappears in attic mode.
- Measure length and width — the building footprint for walls, the attic floor for attic.
- Set the wall height and openings. 10% is a typical door-and-window share; raise it for a glassy or garage-door wall.
- Enter the bag coverage off the package — it swings from 40 to 160 ft² by R-value, so read it, don’t assume.
- Leave waste at 5% for trim cuts and around obstructions; batts waste less than plank flooring.
- Match the R-value to code for your zone and assembly from Table 1 before you pick the product.
Worked example: insulating 30 × 24 exterior walls
The default job, worked line by line — 30 × 24 ft walls at 8 ft, 10% openings, 5% waste, 100 ft² bags:
gross = 108 × 8 = 864 ft²
net = 864 × (1 − 0.10) = 778 ft²
with waste = 778 × 1.05 = 816.5 ft²
bags = 816.5 ÷ 100 = 8.16 → 9 bags ≈ $495
Attic mode is simpler and often the better dollar-for-R spend: the same 30 × 24 footprint is a flat 720 ft², no height, no openings — 8 bags. On most houses the attic is where the first insulation dollar goes, because heat rises and the attic is the cheapest surface to reach.
What this insulation estimate doesn’t include
- Vapor barrier. Poly sheet or a smart membrane where the climate and code require one — separate from faced batts.
- Air sealing. Caulk, canned foam and gaskets at penetrations — the step that makes insulation actually work.
- Attic accessories. Rafter vent baffles, dams around recessed lights, and an insulated hatch cover.
- Rim joists and bandboards. Usually foamed, not batted — a different material.
- Blown-in equipment. Machine rental if you switch the attic to loose fill.
- Safety gear and labor. Respirator, gloves and long sleeves; hanging batts overhead is the slow part.
Insulation FAQ
How much insulation do I need?
Wall mode: perimeter × height minus openings. A 30 × 24 ft building at 8 ft is 864 ft² of wall, 778 ft² after 10% doors and windows — 9 bags at 100 ft² per bag with 5% waste, about $495. Attic mode is the flat area straight up.
What R-value insulation do I need?
By assembly, broadly: attics R-38 to R-60 (colder zones higher), 2×4 walls R-13 to R-15, 2×6 walls R-19 to R-21, floors over unconditioned space R-25 to R-30. Your climate zone’s energy code sets the exact floor — check locally.
How many square feet does a bag of insulation cover?
It is printed on the package and swings hard with R-value — from ~40 ft² a bag for thick R-38 attic batts to 150+ for thin wall rolls. Type the printed figure in and the bag count is exact for your product.
Should I use batts or blown-in insulation?
Open wall cavities and accessible framing suit batts — what this calculator counts. Attics are often cheaper to blow: machine rental plus bags, with the coverage-per-bag chart at your target depth printed on every blown bag.
What is the difference between faced and unfaced batts?
Faced batts have a kraft-paper or foil vapor retarder stapled toward the warm-in-winter side — use them in exterior walls and where a vapor retarder is required. Unfaced batts have no facing and go in interior walls, over existing insulation, and anywhere a separate vapor barrier is used. Both cover the same square feet, so the bag count is identical.
How much of a wall is doors and windows?
Around 10% on a typical house wall, more on glassy living rooms and garage walls with big doors — hence the editable openings figure. Insulation stops at the rough openings; the headers get what fits.
Do I need a vapor barrier with insulation?
It depends on climate and assembly. Cold climates generally want a vapor retarder on the interior (warm) side — kraft-faced batts or poly sheet; hot-humid climates flip the logic and can trap moisture if you add one indoors. Mixed climates often use a "smart" variable-permeance membrane. The energy and building code for your zone governs, so confirm locally before you staple.
Sources and standards
- IECC / IRC N1102: the energy-code R-value tables by climate zone behind Table 1.
- ENERGY STAR recommended insulation levels: the attic and wall targets this page follows.
- ASTM C665: the standard for mineral-fiber batt insulation and its facings.
- Manufacturer package coverage: the printed ft²-per-bag figure — always the governing input.
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