A board foot is a square foot one inch deep
That single definition explains most of the confusion around foam kits. A kit rated at 200 board feet does 200 square feet at one inch, 100 at two inches, and 67 at three. The box usually leads with a square footage, and the depth that figure was quoted at is somewhere further down in smaller type.
| Depth | Area a 200 bd ft kit covers | Kits for 480 sq ft |
|---|---|---|
| 1 in | 200 sq ft | 3 |
| 2 in | 100 sq ft | 5 |
| 3 in | 67 sq ft | 8 |
| 4 in | 50 sq ft | 10 |
Those kit counts are at the full rated yield with no waste. The calculator defaults to 80 percent yield and a 15 percent waste allowance, which is why it asks for more than this table does. Whether 80 percent is right for your day is not something anyone can tell you in advance — cold canisters, a cold substrate and a long hose all take from it, and the tail of a kit that will not push out is a real loss rather than a rounding one.
Cans are rated in feet, at a bead you will not run
A can of gun foam quotes a headline like 1,000 feet. Read the small print and that is at a quarter inch bead. Model the bead as a cylinder and its cross-sectional area is pi over four times the diameter squared, so the coverage scales with one over the diameter squared:
| Bead diameter | Feet from a can rated 1,000 ft at 1/4 in | Fraction of the rating |
|---|---|---|
| 1/4 in | 1,000 ft | 100% |
| 3/8 in | 444 ft | 44% |
| 1/2 in | 250 ft | 25% |
| 3/4 in | 111 ft | 11% |
Half inch is an ordinary gap bead and it is a quarter of the rating. Three quarters, which is what a rough opening around a window takes, is a ninth. On the default gap list — 378 feet of run, plus 15 percent waste, which is 435 feet — a half inch bead needs 2 cans against the 1 the headline rating implies. The teaching point is not that the label lies; it is that the label is quoting a different job.
Which gaps are worth a can and which are not
Foam belongs in gaps that never move: the space between a window frame and its rough opening, the hole a cable passes through a top plate, the joint between a rim joist and the subfloor above it. It does not belong on joints that move, which is what weatherstrip and sweeps are for, and it does not belong on joints that are meant to flex, which is what a movement joint is for. Around a window in particular the wrong foam under too much expansion pressure bows the jamb and stops the sash working, which is why low-expansion products for window and door openings exist as a separate line.
For the surfaces rather than the gaps, the insulation calculator handles batts and blown, rim joist insulation covers that specific detail, and the assembly R-value calculator stacks a wall up layer by layer once you know what is going in it. The wall condensation risk calculator is the one to run before putting an impermeable layer inside an existing wall.
The part with no arithmetic
Two-component kit foam is an exothermic reaction. Sprayed too thick in one pass it holds its own heat, and thick uncontrolled pours have gone on to smoulder and burn. The lift thickness on the data sheet is a safety limit rather than a productivity note. The vapour and aerosol while it cures are an exposure hazard with a re-entry time that is a product figure. Every one of those numbers lives on the safety data sheet for the specific product, and none of them appears on this page. Machine-applied foam and anything in a closed cavity is professional work.
Questions people ask
How many board feet do I need for a 2 inch layer over 480 square feet?
Board feet are square feet times inches, so 480 times 2 is 960 board feet before any allowance. With a 15 percent waste allowance that is 1,104. At a 200 board foot kit rating and an 80 percent real yield, each kit gives 160 usable board feet, so the job takes 7 kits. Nearly all of the gap between the naive answer and that one comes from the yield assumption, which is why this page asks for it as an input rather than burying it.
Why does a can of foam never go as far as the label says?
Because the label is quoting a small bead and you are running a bigger one. Coverage scales with the square of the bead diameter, so a can rated at 1,000 feet at a quarter inch bead gives about 250 feet at a half inch and about 111 feet at three quarters. That is a factor of nine between the two ends. The other losses are real but smaller: the plug that sets in the nozzle, the trigger tests, and the part of a can that will not come out once it has stood.
Can I spray a 4 inch layer in one go?
That is a product question with a safety answer, not a preference. Every two-component foam has a maximum lift thickness on its data sheet, and it exists because the reaction generates heat that thick foam cannot shed. Thick uncontrolled applications have scorched and caught fire. If the product says two inches a lift, four inches is two passes with the cooling time the sheet specifies between them, and that cooling time is on the sheet rather than here.
Does foam need to be covered once it is in?
Foam plastic in a building generally has a covering requirement and clearances from heat sources, flues and some electrical equipment, but what applies to your wall comes from the code your jurisdiction adopted and from the manufacturer instructions for that specific product. Neither is something to take from a calculator. The practical version: do not assume that because a foamed cavity looks finished it is finished, and ask the building department what the covering has to be before the drywall goes up.
Should I use foam around a window or something else?
Around a window the risk is expansion pressure. Ordinary gap-filling foam expands hard enough to bow a jamb and stop the sash sliding, which is why low-expansion window and door products are sold separately and why the run is done in a thin bead rather than a full fill. For a new opening the foam is only one item on a list that also includes flashing tape, a sill pan and a perimeter sealant joint, and the flashing calculator on this site works that whole take-off out from the rough opening sizes.