Spray Foam Insulation Calculator

Foam is sold by the board foot, which is one square foot at one inch thick. That is the unit an installer buys in, prices in and pumps in, and every figure quoted per square foot is that number multiplied by a thickness.

You do not have to take that on trust. Here is a published price table that gives cost per square foot at three thicknesses, with the one inch row multiplied out beside it:

The last two columns are not from the source. They are the first two predicted from its one inch row, and they land on it to the cent at 2 inches and within two cents at 3.

So the table has one row of data in it. Open cell is $0.44 to $1.50 per board foot and closed cell is $1.25 to $2.90, and every other figure on that table is one of those times the inches.

Which is why a headline of "$1.00 to $4.50 per square foot", which appears on the same page with no thickness attached, cannot be applied to a job. It is a board foot price with the unit removed, and you cannot get it back without knowing how deep the foam went.

Ask for a quote in board feet. Any foam contractor works in them, the number is checkable against the rig's own output, and it is the only form in which two quotes for different thicknesses can be compared.

Foam plastic is combustible and the code treats it as such. The default requirement is not a recommendation:

Unless otherwise allowed in Section R316.5, foam plastic shall be separated from the interior of a building by an approved thermal barrier of not less than 1/2-inch (12.7 mm) gypsum wallboard, 23/32-inch (18.2 mm) wood structural panel or a material that is tested in accordance with and meets the acceptance criteria of both the Temperature Transmission Fire Test and the Integrity Fire Test of NFPA 275.

Half inch gypsum, 23/32 wood structural panel, or a material tested to NFPA 275. That is the thermal barrier, and it applies to foam anywhere it faces the interior of the building.

Attics and crawl spaces get relief from it, on conditions:

The thermal barrier specified in Section R316.4 is not required where all of the following apply: 1. Attic access is required by Section R807.1. 2. The space is entered only for purposes of repairs or maintenance. 3. The foam plastic insulation has been tested in accordance with Section R316.6 or the foam plastic insulation is protected against ignition using one of the following ignition barrier materials.

So in an unfinished attic you may use an ignition barrier instead, and the code names which ones. R316.5.3, attics:

1. 1-1/2-inch-thick (38 mm) mineral fiber insulation
2. 1/4-inch-thick (6.4 mm) wood structural panels
3. 3/8-inch (9.5 mm) particleboard
4. 1/4-inch (6.4 mm) hardboard
5. 3/8-inch (9.5 mm) gypsum board
6. Corrosion-resistant steel having a base metal thickness of 0.016 inch (0.406 mm)
7. 1-1/2-inch-thick (38 mm) cellulose insulation
8. 1/4-inch (6.4 mm) fiber-cement panel, soffit or backer board

R316.5.4, crawl spaces:

1. 1-1/2-inch-thick (38 mm) mineral fiber insulation
2. 1/4-inch-thick (6.4 mm) wood structural panels
3. 3/8-inch (9.5 mm) particleboard
4. 1/4-inch (6.4 mm) hardboard
5. 3/8-inch (9.5 mm) gypsum board
6. Corrosion-resistant steel having a base metal thickness of 0.016 inch (0.406 mm)
7. 1/4-inch (6.4 mm) fiber-cement panel, soffit or backer board

Count them. The attic list has 8 entries and the crawl space list has 7. The one that appears in the first and not the second is:

1-1/2-inch-thick (38 mm) cellulose insulation

Adjacent subsections of the same section, otherwise word for word identical, one option dropped and no reason given. Worth knowing before you specify the same detail in both spaces.

Both lists have an escape. Neither barrier is required where the foam has been tested in accordance with R316.6, which is how the intumescent coatings sold for exactly this purpose work. That testing belongs to a specific product and its evaluation report, not to foam in general.

They are sprayed with the same rig by the same crew and they behave nothing alike.

Closed cell is denser and has a much higher R per inch, so it reaches a given R-value in fewer inches. That matters where depth is limited, which is most retrofits: a 2x4 rafter bay is 3 1/2 inches and no amount of open cell will make it perform like a 2x6.

Closed cell also resists vapour and open cell does not. At sufficient thickness closed cell is a vapour retarder in its own right, which changes where the assembly can dry to. That is an assembly design decision rather than a quantity, and getting it wrong traps water in a roof deck.

Open cell is cheaper per board foot and much cheaper per inch. The published figures on this page put it at roughly a third of closed cell for the same board feet, which is why it is normal in an open attic roofline where depth is free.

The calculator prices both at your board feet so you can see the difference, and says on the row that they are not the same R-value, because comparing them at a fixed thickness is the mistake this whole page is about.

Foam arrives as two liquids in two drums and becomes insulation on the wall. That has consequences no bagged product has.

Yield is theoretical. A set of drums has a nominal board foot yield, and it is achieved at the right temperature, the right pressure and the right ratio. Cold drums, a long hose or a worn mixing chamber all cost yield, and the material that comes out off-ratio is not insulation, it is waste that still has to be trimmed off.

Trim is real on any framed cavity. Foam is sprayed proud and cut back flush so drywall can go on, and what comes off the face is gone. On an open roofline there is no trimming and the allowance should drop.

The default here is 10%, it is this page's own figure and it is an input, for the same reason the compaction allowance on the gravel page is. No data sheet publishes one.

And it is why an installed quote is usually the sane way to buy foam. The material, the rig, the operator and the yield risk all sit on the contractor's side of the line, which is not true of a bag of cellulose.

The barrier requirements sit on top of a materials requirement that applies to the foam itself:

Unless otherwise allowed in Section R316.5, foam plastic, or foam plastic cores used as a component in manufactured assemblies, used in building construction shall have a flame spread index of not more than 75 and shall have a smoke-developed index of not more than 450 when tested in the maximum thickness and density intended for use in accordance with ASTM E84 or UL 723.

Flame spread index not more than 75, smoke-developed index not more than 450, tested in the maximum thickness and density intended for use. That last clause is the one to read twice: a foam that passes at two inches has not been shown to pass at six, and the thickness tested is a property of the product's evaluation report.

For comparison, ordinary wall and ceiling finishes are allowed a flame spread index of 200. Foam is held to 75, which is well under half of it.

None of this is a quantity question, which is why no other spray foam calculator mentions it. It is also the part of a foam job that gets failed at inspection, and the barrier is not something you can add afterwards without pulling the foam about.

Open cell and closed cell are different materials sold under one name. They differ in R per inch, in density, in what they do to water vapour and in what they cost per board foot, and the difference in R per inch is roughly a factor of two.

Which makes any comparison at a fixed price per square foot meaningless, and any comparison at a fixed R-value a comparison of two different thicknesses. The Federal Trade Commission solved this by fixing the thickness:

For foamed-in-place insulations, you must show the R-value of your product at 3 1/2 inches. You can also show R-values at other thicknesses.

3.5 inches. Every foamed-in-place product sold to homeowners has to publish its R-value at that one depth, so two fact sheets can be read side by side without arithmetic.

This page will not print an R per inch for either type. It is a property of a specific product, it appears on that product's data sheet and in its evaluation report, and the range across products within one type is wide enough that an average would mislead. The calculator asks for yours, the same way the blown-in page asks for your bag's coverage chart.

The one figure worth carrying between them: the price table above gives a single R-value column covering both open and closed cell, which cannot be right for two materials that differ by a factor of two. Read R-values off the product, not off a cost guide.

Read 2026-09-10.

Seattle Residential Code, Chapter 3, Section R316 Foam Plastic, City of Seattle, adopting the 2018 International Residential Code, Sections R316.3, R316.4, R316.5.3 and R316.5.4. Requires foam plastic to be separated from the interior by an approved thermal barrier, and lists the ignition barrier materials that substitute for it in an attic and in a crawl space. The two lists differ by one entry.

16 CFR 460.13, What must be on the fact sheet, Federal Trade Commission, section 460.13(c)(1). Requires foamed-in-place insulation to show its R-value at 3 1/2 inches, which is the one thickness at which two foams can be compared without doing arithmetic on somebody's marketing.

Seattle is used because codes.iccsafe.org returns HTTP 403 and cannot be fetched, and because a city that publishes its adopted code as a PDF states its edition and jurisdiction on the document. Your jurisdiction may amend R316, and several do. Check the edition your building department enforces.

The price table is quoted with its byline and date and is one publisher's national figures. This site publishes no price of its own. What this page adds to it is the observation that it is a board foot table, which its publisher does not say.