Insulating a 15 ft Wall - How Much Material to Buy?

Same wall, four insulation choices - which one fits?

A 2x6 exterior wall accepts fiberglass batts, mineral wool boards, rigid foam sheathing or spray foam. Each material covers a different number of square feet per package, hits a different R-value per inch, and costs a different amount per square foot. This calculator takes your wall or attic dimensions, subtracts windows and doors, adds a waste margin, and produces a materials list with package counts and an optional cost estimate - for whichever insulation type you pick.

How to use - step by step

Length - total length of the wall run or attic floor in feet. For all four walls of a room, add them: 2 x length + 2 x width.
Height/width - wall height (usually 8 or 9 ft) or attic width.
Windows & doors - subtract openings. Standard window ~15 sq ft, entry door ~21 sq ft.
Insulation type - fiberglass, mineral wool, rigid foam or spray foam.
Thickness - from 2 in to 12 in. The calculator shows the approximate R-value for your combination.
Waste allowance - 10% for standard framing, 15% for irregular spaces.
Read the results - area, volume, package count, R-value and cost if prices entered.

Six scenarios - from a single wall to a whole attic

Scenario 1: One bedroom wall - 14 x 8 ft, fiberglass 4 in, minus 15 sq ft window, 10% waste
Area: 106 sq ft | R-value: R-13 | Packages: 2 rolls

Scenario 2: Full house perimeter - 140 x 8 ft (2x4 walls), fiberglass 4 in, minus 120 sq ft openings, 10% waste
Area: 1,122 sq ft | R-value: R-13 | Packages: 18 rolls

Scenario 3: Attic floor - 40 x 30 ft, fiberglass 10 in, no openings, 5% waste
Area: 1,260 sq ft | R-value: R-32 | Packages: 40 rolls

Scenario 4: Basement wall - 80 x 8 ft, rigid foam 2 in, minus 10 sq ft, 10% waste
Area: 693 sq ft | R-value: R-10 | Packages: 22 boards

Scenario 5: Rim joist - 140 x 1 ft, spray foam 2 in, no openings, 5% waste
Area: 147 sq ft | R-value: R-12 | Packages: 1 kit

Scenario 6: Cathedral ceiling - 24 x 16 ft, mineral wool 8 in, no openings, 15% waste
Area: 442 sq ft | R-value: R-30 | Packages: 17 packages

Insulation type comparison

R-value per inch, typical cost and best application for each material.

Type	R-value/in	Cost ($/sq ft)	Best for	Drawbacks
Fiberglass batts	3.2	$0.50-0.80	Standard walls, attic floors	Sags over time, needs vapor barrier
Mineral wool	3.7	$1.00-1.50	Fire-rated walls, soundproofing	Heavier, more expensive
Rigid foam	5.0	$0.80-1.50	Exterior sheathing, basements	Not flexible, needs sealing at joints
Spray foam	6.0	$1.50-3.00	Rim joists, irregular cavities	Requires pro installation, highest cost

R-value requirements by climate zone (IRC 2021)

Climate zone	Wall cavity	Wall continuous	Attic / ceiling	Floor
Zone 1-2 (South FL, TX coast)	R-13	-	R-30	R-13
Zone 3 (GA, SC, AZ, NM)	R-13	R-5	R-38	R-19
Zone 4 (NC, TN, VA, OR)	R-20	R-5	R-49	R-19
Zone 5 (NY, OH, PA, WA)	R-20	R-7.5	R-49	R-30
Zone 6-7 (MN, WI, ME, MT)	R-20	R-10	R-49	R-30

FAQ - Frequently asked questions

Can I stack two layers of insulation for a higher R-value?

Yes. R-values are additive. Two layers of R-13 fiberglass batts give R-26. In attics, a common approach is R-19 batts between joists plus R-19 unfaced batts laid perpendicular on top = R-38. Just make sure only the bottom layer has a vapor barrier - the top layer must be unfaced.

How much waste should I expect with fiberglass batts?

In standard 16 in or 24 in on-center framing, 5-10% waste is typical. Trimming around outlets, pipes and irregular framing creates scrap. Cathedral ceilings with variable rafter depths or rooms with many windows can push waste to 12-15%. Spray foam has the least waste (3-5%) because it fills the cavity precisely.

What R-value do I actually need for my walls?

It depends on your climate zone. Most of the US falls in zones 3-5, which require wall cavity insulation of R-13 to R-20. The table above shows IRC 2021 minimums. Local codes may be stricter. For a 2x4 wall, R-13 fiberglass fills the cavity. For 2x6 walls, R-19 or R-21 batts fit perfectly.

Is spray foam worth the extra cost?

Closed-cell spray foam delivers R-6 per inch and acts as both insulation and air/vapor barrier. For rim joists, irregular cavities and cathedral ceilings where air sealing matters, it often pays for itself in energy savings. For standard wall cavities with good air sealing, fiberglass or mineral wool at 1/3 the cost performs well enough.

How do I calculate insulation for an irregularly shaped attic?

Break the attic into rectangles. Calculate each section separately and add the results. For a hip roof, measure the rectangular center and the triangular hip ends separately. The calculator handles one rectangular area at a time - run it once per section and sum the package counts.

Do I need a vapor barrier with insulation?

In climate zones 4-7 (cold winters), a vapor barrier goes on the warm side (interior face) of wall insulation. In zones 1-2, it goes on the exterior or is omitted entirely. Kraft-faced fiberglass batts have a built-in vapor retarder. Unfaced batts need a separate poly sheet. Spray foam and rigid foam are their own vapor barriers - no additional sheet needed.

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How much insulation does a 15 ft wall take?

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