Wall insulation calculator

Wall insulation cost

Exterior walls are the largest surface area in the building envelope, but retrofitting them means opening the wall or dense-packing through drilled holes.

Insulating 1,200 square feet of exterior walls from scratch to the zone 4 recommendation of R-13 costs $1,600 to $2,170 installed at national-average rates, roughly $1.57 per square foot, and saving about $500 a year. Your climate zone changes the target substantially: the same assembly needs R-13 in zone 2 and R-20 in zone 6, so the same house costs meaningfully more to bring up to standard in the north.

Priority: Second priority in cold zones, rarely first anywhere. Retrofit access is the constraint. Dense-pack cellulose through drilled holes is the usual approach in a finished house.

How much wall insulation do you need?

Work from the gap rather than the area. Take your zone's target, R-13 for wall insulation in zone 4, subtract the R-value already in place, and divide what is left by the material's R per inch. Closing an R-13 gap takes 3.7 inches of blown-in cellulose, 4.1 inches of fiberglass batt or 2 inches of closed-cell foam.

Turning depth into material is the last step. Blown-in is sold by the bag against a coverage chart that falls as depth rises, one bag covers 32 square feet at R-30 but only about 20 at R-49, so 1,200 square feet at R-13 works out to roughly 17 bags. Batts are sold by the package at 40 to 49 square feet each, and rigid foam in 32-square-foot sheets.

Enter your square feet in the calculator above and it does all of this from your state, the zone lookup, the target, the gap, the depth and the installed cost, without an email address or a signup.

Best insulation material for exterior walls

Not every material suits every assembly. For exterior walls, the realistic options are fiberglass batt, blown-in cellulose, open-cell spray foam, closed-cell spray foam, rigid foam board.

Fiberglass Batt delivers R-3.2 per inch at about $0.15 per square foot per inch, which puts this job at $1,404. Cheapest material per R-value and available everywhere, though performance collapses if compressed or poorly fitted. It is realistically DIY-installable.

Blown-In Cellulose delivers R-3.5 per inch at about $0.14 per square foot per inch, which puts this job at $1,886. Best R per inch of the affordable materials, though settles roughly 10-20% over time unless dense-packed. It is realistically DIY-installable.

Open-Cell Spray Foam delivers R-3.7 per inch at about $0.45 per square foot per inch, which puts this job at $3,887. Seals air leaks and insulates in a single operation, though roughly three times the material cost of fiberglass. It requires professional installation and equipment.

R-value per inch matters most when depth is constrained. In an open attic you can simply add depth, so cost per R-value wins. In a 2x4 wall cavity or a shallow crawl space you have a fixed 3.5 inches to work with, and closed-cell spray foam's R-6.5 per inch is the only way to reach a high target, which is exactly why it costs what it does.

Insulating walls without opening them

The constraint on wall insulation is access, not material. In a finished house the standard retrofit is dense-pack cellulose blown through holes bored between the studs, from outside under a removed course of siding, or from inside, with the holes patched afterwards. It fills around wiring and plumbing far more completely than cutting the wall open and fitting batts, and it costs about $1,886 for 1,200 square feet of bare wall.

Cavity depth is the other constraint, and it is fixed. A 2x4 stud bay gives you 3.5 inches, which holds R-13 to R-15 in fiberglass. R-19 batts are made for 2x6 framing and deliver only about R-13 once compressed into a 2x4 wall, so you pay for R-19 and get a third less. Closed-cell foam at R-6.5 per inch is the only material that reaches R-22.75 in that space, and continuous rigid foam on the outside of the sheathing is the other way to raise a wall's real performance. It breaks the thermal bridge through the studs, which cavity insulation cannot do.

Set the area selector above to exterior walls and this wall insulation calculator prices the assembly directly, applying your zone's target of R-13 to R-21 depending on where you live.

Can you do this yourself?

Labor is 40-60% of a professional insulation job, so it is the largest single saving on the table: the reference job above drops from $1,886 installed to about $806 doing it yourself, including a $120 allowance for blower rental and protective equipment. Most suppliers lend the blower free with a material purchase, which is why blown-in is the most DIY-friendly option despite needing a machine.

Spray foam is the exception and it is not a close call. Professional equipment costs more than the job, and the chemistry is unforgiving of mistakes in mix ratio and substrate temperature. Badly applied foam either fails to cure or shrinks away from the framing, and both are expensive to remove. Kit foam from a big-box store is fine for rim joists and small patches, not for an assembly.

Whatever you use, budget for a respirator, sealed eye protection and long sleeves, work off boards rather than the ceiling drywall, and do the air sealing before the insulation goes down. Once the material is in place you will not want to move it again to find the leaks.

How we calculated these estimates

Every figure on this page comes from the same open formula the calculator above uses. We start from your state's DOE climate zone, look up the R-value the Department of Energy recommends for the assembly you are insulating, subtract the R-value already in place, and price only the gap that is left, which is why two houses of identical size get very different numbers. Depth comes from the material's published R per inch, material cost from a 2026 national baseline per square foot per inch, and a 10% allowance is added for trimming and irregular framing. Installed labor is then scaled by the construction earnings multiplier the Bureau of Labor Statistics publishes for your state. The energy saving is a steady-state conduction estimate priced against real US Energy Information Administration residential electricity rates and a national natural gas price, with a 70% realization factor applied. It deliberately excludes air leakage, duct losses and solar gain, so it is a conservative planning figure rather than an energy audit. Nothing is hidden behind a lead form and no contractor pays to appear here. What this is not is a quote: attic access, obstructions, existing damage and any air sealing or venting work are priced separately by whoever does the job.