Free Contractor Tool
Insulation Calculator
Enter the area and the R-value you are targeting, pick a material, and get the exact thickness, board feet, and material cost — with credit for insulation that is already there.
Built by SubcontractorHub — the software contractors use to quote, finance, and run every job.
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Calculate Insulation Thickness and Cost
Set your target R-value, subtract whatever is already installed, and the calculator works out how deep the new layer has to be. Results update as you type.
Determines how thick the install has to be to hit your target.
R-49 to R-60 for most attics
0 if starting bare
Areas to insulate
Leave blank to use the typical rate for blown-in cellulose.
Insulation needed
Thickness
13.2"
Area
1,200
Board feet
15,892
Est. material
$1,112
R-value to add
R-49
Material
Blown-In Cellulose
R per inch
R-3.7
Rate used
$0.07/sq ft-in
Recommended R-Values by Climate Zone
US Department of Energy guidance for uninsulated assemblies. Local code may require more — always check before quoting.
How thick is R-49?
Entirely a function of material. R-49 takes about 15.3 inches of fiberglass batt, 19.6 inches of blown-in fiberglass, 13.2 inches of cellulose, 12.3 inches of mineral wool — or 7.5 inches of closed-cell spray foam.
That spread is why depth-limited assemblies like cathedral ceilings and rim joists so often get spray foam: it is the only way to reach code R-value in the cavity you actually have.
Topping up existing insulation
R-values add, so an attic already at R-19 only needs R-30 more to reach R-49. Enter the existing value in the calculator and it sizes the top-up layer instead of the whole assembly.
Two rules: lay new batts unfaced and perpendicular to the old ones, and never stack a second faced batt — the extra vapour barrier traps moisture between layers. Air-seal penetrations first, or you are just insulating over the leaks.
R-Value per Inch by Material
When spray foam pays for itself
Closed-cell runs roughly fifteen times the cost per square foot-inch of blown-in fiberglass. What you buy for that is air sealing and double the R-value per inch — both irrelevant in an open attic with unlimited depth.
Where it wins is rim joists, cathedral ceilings, knee walls, and any assembly where air leakage rather than conduction is driving the energy loss.
Why insulation belongs in the HVAC conversation
Envelope improvements change the load calculation. An attic taken from R-19 to R-49 can cut the design heating and cooling load enough to drop the replacement system a half-ton or more.
Run the Manual J load calculation after the insulation upgrade, not before — sizing to the old envelope is how oversized, short-cycling systems get installed.
From Material Takeoffs to a Signed Proposal
Envelope work and equipment replacement sell far better as one package than as two visits. SubcontractorHub lets you build a single branded proposal covering both, priced and presented on a tablet at the house.
Show good-better-best options, put monthly financing next to the total, take a signature on the spot, and send the signed job straight to operations.
See How It WorksBundle envelope and equipment in one proposal
Materials, labor, and markup applied automatically
Good-better-best options side by side
Financing and monthly payments shown on site
Signed job flows straight to operations
Insulation quantities and costs are estimates based on the area, target R-value, and material entered. R-value per inch varies by product and installed density, and settling reduces the effective value of loose fill over time. Confirm rated R-values with the manufacturer and check local energy code before quoting.
Frequently Asked Questions
- How much insulation do I need?
- Start with the target R-value for your climate zone and the area you are insulating — most US attics need R-49 to R-60. Divide that target by the R-value per inch of your chosen material to get the required thickness, then multiply your square footage by that thickness to size the order. This calculator does all three steps and estimates material cost.
- What R-value do I need for my attic?
- The US Department of Energy recommends R-30 to R-49 for attics in the far south (zone 1), R-49 to R-60 across most of the southern and central states (zones 2 and 3), and R-60 for zone 4 and colder. Walls typically call for R-13 to R-21 depending on cavity depth, and floors over unconditioned space run R-13 to R-38.
- How thick does insulation need to be for R-49?
- It depends entirely on the material. R-49 needs about 15.3 inches of fiberglass batt at R-3.2 per inch, 19.6 inches of blown-in fiberglass at R-2.5, 13.2 inches of cellulose at R-3.7, 12.3 inches of mineral wool at R-4.0, or just 7.5 inches of closed-cell spray foam at R-6.5 per inch.
- What is R-value per inch for common insulation?
- Blown-in fiberglass is roughly R-2.5 per inch, fiberglass batt R-3.2, blown-in cellulose and open-cell spray foam both about R-3.7, mineral wool batt R-4.0, and closed-cell spray foam about R-6.5. Closed-cell delivers the most R-value per inch, which is why it is used where cavity depth is limited.
- Is spray foam worth the extra cost?
- Closed-cell spray foam costs several times more per square foot than batts or blown-in, but it air-seals as it insulates and delivers roughly twice the R-value per inch. It earns its price where cavity depth is tight, in rim joists and cathedral ceilings, and where air leakage is the real problem. In an open attic with unlimited depth, blown-in cellulose or fiberglass usually gives better value per dollar.
- Can you add new insulation over old insulation?
- Yes in most attics, and R-values add together. Lay unfaced batts perpendicular over existing insulation, or blow loose fill straight on top. Never add a second faced batt over an existing one — the second vapour barrier traps moisture between the layers. Air-seal penetrations before topping up, or you are insulating over the leaks that cause most of the loss.