How we calculate

These are planning estimates, not engineering specifications. They are accurate enough to budget a project and buy materials with confidence. They are not a substitute for an energy audit, and they do not account for your specific construction.

Area

You can enter square footage directly, measure length × width, or — for walls — give a perimeter and a height. All three resolve to one net figure before anything else happens. In perimeter mode, windows and doors default to 15% of gross wall area if you have not measured them, because assuming zero would over-order material on every job.

Framing: when we deduct it and when we must not

This is the single most common way an insulation estimate goes wrong, and the answer is not the same for every material.

• Batts: we do not deduct framing. The coverage printed on the package is already net cavity coverage — an R-15 package stating 67.81 sq ft is seven 15-inch batts. Those seven cavities at 16 inches on center span about 72 sq ft of gross wall, so dividing gross area by the stated coverage over-orders by roughly 7%, which is the cuts allowance you want. Deducting framing on top would leave you short.
• Loose fill blown over attic joists: we do not deduct framing either. It covers the joists as well as the space between them.
• Blown or sprayed into a cavity: we do deduct it. Here the material fills true void volume, so we use 75% of gross wall area at 16 inches on center and 78% at 24 inches. The code assumption for thermal calculations is 25% and 22% framing respectively.

Walls, garages, and crawl spaces

Wall area is perimeter × height less openings. Garage surfaces are derived from the footprint: ceiling is width × depth, the shared wall is the wall opposite the door, and exterior walls are the remaining perimeter less the garage door. Crawl space geometry serves both approaches from the same inputs — floor area less a 10% joist fraction for insulating between joists, and perimeter × wall height for insulating the perimeter. Crawl space perimeter is taken as twice length plus width, which understates an L-shaped or stepped foundation by 10 to 25%.

Target R-value

Targets follow current ENERGY STAR and US Department of Energy recommended insulation levels for existing homes, by IECC climate zone. These are recommendations for retrofits, not code minimums for new construction — your local building code may require more.

How we guess your climate zone

If you enter a ZIP code, we map it to a state and then to that state's most populous climate zone. This is deliberately coarse. Several states — California, Texas, Nevada, Arizona, Oregon, Washington, New York, New Mexico, and Idaho — span three zones, and we flag those as low confidence and ask you to confirm.

A precise lookup needs county-level IECC data. We would rather tell you the guess is rough than present a county-accurate answer we do not actually have.

Existing insulation

If you know your R-value, we use it. Otherwise we estimate from depth × a per-inch rate for the material you identify:

These rates are below the nominal ratings for new material, deliberately. Insulation that has been in an attic for twenty years has settled, been walked on, and has gaps. Assuming it performs as new would over-estimate what you have and under-specify what you need — the more expensive mistake.

If you say you are unsure whether there is insulation, we assume none. That produces a conservative estimate rather than a guess.

Depth

We use these rates:

Material quantity

Loose fill (cellulose and blown fiberglass)

Manufacturer coverage charts are linear in R-value: a bag covering 31.8 sq ft at R-38 covers 24.6 sq ft at R-49. Multiply coverage by R-value and you get a constant —R-square-feet per bag — which reproduces the entire chart from one number.

We use 1,200 R·sq ft per bag for cellulose, calibrated against published GreenFiber coverage data, and 1,450 for blown fiberglass. Bags vary by manufacturer, so check the coverage chart on the bag you actually buy.

Batts

We pick the single batt product closest to your required R-value, then divide your area by that product's package coverage. We deliberately avoid recommending stacked mismatched batts — that is how DIY jobs end up with compressed insulation performing well below its rating.

Installed depth for a batt is the actual product thickness, not the R-value divided by a nominal R-per-inch. High-density batts break that arithmetic: an R-15 batt is 3.5 inches thick so it fits a 2x4 cavity, where dividing R-15 by a nominal R-3.2 per inch would claim 4.7 inches and imply it does not fit.

Dense-pack retrofits

Blowing insulation into a finished wall is a different calculation from blowing it loose over an attic floor. Dense-pack is installed at 3.5 lb per cubic footor more specifically to stop it settling, where attic loose fill sits nearer 1.5. Using the attic coverage chart for a drill-and-fill job understates the bag count by a factor of four, so we compute it by mass instead.

We also say plainly that this is not a DIY job. Dense-pack needs around 2.9 psi of static pressure at the hose, which rental hoppers cannot produce; an underfilled cavity settles and leaves an uninsulated strip at the top of every bay.

Spray foam

Spray foam is quoted in board feet, not bags, so it uses its own model. The Spray Polyurethane Foam Alliance defines a board foot as a volume of 12 by 12 by 1 inches, which makes the arithmetic exact.

We ask for thickness rather than a target R-value, because working backwards from R-value produces impossible answers — R-49 of open-cell foam needs 13 inches, which fits in no rafter bay. Each application presets the thickness installers actually apply, and we show the resulting R-value alongside the board feet so neither figure can mislead on its own.

R-values are the aged figures: R-6.5 per inch for closed-cell rather than the day-one R-7.0 commonly advertised, because the blowing agent diffuses out over the first five to ten years. Open-cell foam is water-blown and does not drift, so its aged and initial values are both R-3.7. Set yields are derated to 80% of nameplate — rated yields assume a warm substrate, an exact mix ratio, and thick passes, and real jobs do not get all three.

Waste and rounding

We add 10% for loose fill and 12% for batts. Loose-fill waste covers settling and uneven depth; batt waste covers offcuts around obstructions, which is higher because cutting waste is real and running short mid-job means another trip.

Cost

Material cost is expressed per R per square foot rather than per square foot, because cost scales with how much insulation you install, not just floor area. Doubling your R-value target roughly doubles material cost. A flat per-square-foot model gets this wrong.

DIY totals include:

• Material, including the waste allowance
• Air sealing at $0.05–$0.10 per sq ft — included by default, because doing it after insulating is impossible
• Blower rental $0–$100, since most retailers waive it above a minimum bag purchase
• Safety gear $45–$95, assuming you own none of it

Professional estimates use installed cost per R-square-foot, adjusted by a regional labor multiplier (0.95× to 1.15× by climate zone), and floored at a minimum job charge of $400–$700. Small attics do not scale down linearly — mobilisation and minimum crew time dominate on a small job.

Where cost does not scale with R-value

The per-R model is right for an attic, where more depth is more material and more blowing time. It is wrong for a wall retrofit: drilling a line of holes in every stud bay and patching afterwards is the same work whether the cavity ends up at R-13 or R-21. So walls, garages, crawl spaces, and spray foam are priced per square foot (or per board foot) instead.

Those rates apply to the whole assembly, not the cavity. A contractor stages, drills and patches the entire elevation, not the 75% of it that ends up holding insulation — so the framing deduction changes how much material you buy, and does not change the labor.

On a multi-surface project — a garage wall plus its ceiling, or crawl space walls plus the rim joist — the minimum charge is applied once to the whole job, not once per surface. It represents one visit, and charging it per surface would quote several call-out fees for a single trip. Equipment rental and safety gear work the same way: you rent one blower and buy one respirator however many surfaces you do.

Costs that are not insulation

Some of what a job costs is not insulation at all, and we show those as their own lines rather than hiding them inside a material figure. A garage needs its gypsum separation put back — half inch on the shared wall, five-eighths Type X under living space, per IRC Table R302.6 — at $1.50–$3.50 per square foot installed. An encapsulated crawl space needs a ground liner at $1.85–$3.00 per square foot and a dehumidifier at $1,200–$3,000, because IRC R408.3 requires humidity control rather than suggesting it. Exposed spray foam usually needs a covering under IRC R316, commonly an intumescent coating at $0.75–$1.50 per square foot.

What a cavity can actually hold

Where a code target cannot physically be reached, we cap the plan at what fits and say so rather than quoting a buildable-looking number that is not. A 2x4 cavity tops out near R-15 with a high-density batt and near R-12 with dense-pack cellulose, against a 2021 IECC figure of R-30 in climate zone 4 and above. The compliance path for that figure is cavity insulation plus continuous exterior foam, written in code as something like 20+5ci, and we name the cheapest path your cavity can support. Filling the cavity is still worth doing on its own — it is most of the available benefit.

We never show live retailer prices. Pricing changes constantly and varies by store, so presenting a specific figure would imply an accuracy we do not have. Costs are rounded to the nearest $10 below $1,000 and $50 above, for the same reason.

What this does not account for

Being direct about the limits:

• Air leakage. We include a budget for air sealing materials, but we cannot estimate how leaky your house is. In many homes air sealing matters more than insulation depth.
• Thermal bridging. Heat moves through framing faster than through insulation. Real whole-assembly R-value is typically lower than the nominal figure.
• Local pricing. Ranges are national. High-cost metros can exceed the top of our professional range.
• Your attic's specifics. Access, obstructions, knob-and-tube wiring, existing damage, and ventilation all affect real cost and are invisible to a calculator.
• Energy savings. We deliberately do not estimate them. Credible savings figures need your actual energy use, fuel prices, equipment efficiency, and local weather data. Anyone quoting a savings number from a square-footage form is guessing.

Sources

• US Department of Energy and ENERGY STAR recommended insulation levels by climate zone
• IECC climate zone definitions
• Published manufacturer coverage charts for loose-fill products
• Published material R-values from manufacturer technical data
• National average material and installed pricing ranges

Found something wrong? Tell us — corrections to the underlying assumptions are genuinely welcome.