You need the surface area of the assembly multiplied by the number of inches of depth it needs, and that answer is expressed in board feet. Measure the square feet, divide your target R-value by the product’s R per inch to get the depth, multiply the two together, and you have the number every spray foam quote in this market is actually built from.
That is the whole calculation. The rest of this page is the part nobody hands you: how to measure each kind of space without climbing into it, how to work out the depth (which is the step people get wrong), the one constraint that decides most Seattle jobs, and four worked examples on the houses we actually see around the Puget Sound.
The two lines of arithmetic
square feet of surface × inches of depth = board feet
target R-value ÷ R per inch of the product = inches of depth
Step one: the board foot, and why it is the only unit that matters
A board foot is one square foot of surface sprayed one inch thick. Foam is sold and installed by volume, so a quote per square foot is an incomplete number until you know the thickness behind it.
Here is the arithmetic worked through on a real surface. Say you measure 480 square feet of crawl space wall, and the spec is 2 inches of closed-cell:
- 480 square feet × 2 inches = 960 board feet
- 960 board feet × $1.00 to $2.00 per board foot = about $960 to $1,920 installed
Change nothing except the depth and watch what happens. The same 480 square feet at 4 inches is 1,920 board feet, and it costs roughly twice as much. That is why two bids on the same house can differ by thousands of dollars without either contractor being dishonest: one of them quoted more foam. For the full pricing breakdown and what moves the per board foot rate, read spray foam insulation cost in Seattle.
Step two: measuring the areas that actually matter
You can get most of this done in twenty minutes with a tape measure and a notepad, without going into the crawl space. Measure the surface being sprayed, not the floor area of the house.
Crawl space walls and the rim joist perimeter
Walk the outside of the house and measure the footprint, then add the sides together twice for the perimeter. A 28 by 36 foot footprint gives you 2 × (28 + 36) = 128 linear feet.
Now you need two heights. The stem wall height is the distance from the dirt or vapor barrier up to the underside of the floor framing, commonly 2.5 to 4 feet in a Seattle crawl space. The rim joist band is the depth of the floor joist plus the sill plate on top of that wall, usually 10 to 12 inches. Multiply each by the perimeter and add them.
128 linear feet × 3 feet of wall = 384 square feet. 128 linear feet × 1 foot of rim band = 128 square feet. Total sprayed surface: 512 square feet. Subtract nothing for the vents; they get sealed and sprayed over.
An attic floor
This one is easy, because the attic floor is the footprint of the heated space directly beneath it. Use the exterior dimensions of the upper storey and subtract any area that is not over living space, such as an unheated garage or a covered porch. Do not subtract for the joists themselves.
A roof deck, where the slope catches people out
If you are conditioning the attic, the foam goes on the underside of the roof sheathing, and that sloped surface is always bigger than the footprint underneath it. A 26 by 38 foot footprint is 988 square feet of ground. The roof over it is not.
Multiply the footprint by the slope factor for your pitch:
| Roof pitch | Multiply footprint by | 988 sq ft footprint becomes |
|---|---|---|
| 4:12 (shallow) | 1.05 | About 1,040 sq ft |
| 6:12 | 1.12 | About 1,110 sq ft |
| 8:12 (common on a Seattle bungalow) | 1.20 | About 1,190 sq ft |
| 10:12 | 1.30 | About 1,280 sq ft |
| 12:12 (steep) | 1.41 | About 1,390 sq ft |
On a steep old bungalow roof, ignoring the slope factor understates the job by a third. To find your pitch without getting on the roof, hold a level out from a rafter in the attic, mark 12 inches along it, and measure straight down to the rafter. That drop in inches is the first number of the pitch.
Wall cavities
Measure the length of each exterior wall by its height, add them up, then subtract the windows and doors. A 9 by 5 foot picture window is 45 square feet you are not paying to insulate. One thing worth knowing first: spray foam goes into a wall that is open, either during a remodel or after the siding or drywall comes off. If your walls are closed and staying closed, foam is not usually the product, and we will say so rather than sell you a wall job.
Step three: working out the depth, which is where it goes wrong
Most homeowners guess the depth or accept whatever the bid says. It is a division problem. Take the R-value you are aiming at, divide it by the product’s R per inch, and round up.
The figures we work to are closed-cell at about R-6 to R-7 per inch and open-cell at about R-3.6 to R-4 per inch. Use the middle of the range to plan, then confirm the actual product data on the quote. Worked through, using R-6.5 for closed-cell and R-3.8 for open-cell:
| Target | Closed-cell | Open-cell |
|---|---|---|
| R-21 | 21 ÷ 6.5 = 3.2 in, call it 3.5 | 21 ÷ 3.8 = 5.5 in |
| R-30 | 30 ÷ 6.5 = 4.6 in, call it 5 | 30 ÷ 3.8 = 7.9 in, call it 8 |
| R-38 | 38 ÷ 6.5 = 5.8 in, call it 6 | 38 ÷ 3.8 = 10 in |
| R-49 | 49 ÷ 6.5 = 7.5 in | 49 ÷ 3.8 = 12.9 in, call it 13 |
Always round up, never down. Foam is sprayed to a depth range rather than a precise number, and the low spots are what you feel in February. More on what the R number does and does not tell you in the R-value guide for climate zone 4C.
The constraint that decides most Seattle jobs: how many inches you actually have
You can do all that division correctly and still be unable to buy the answer, because the cavity is only so deep. This is the single most useful thing on this page.
A 2×4 stud wall gives you 3.5 inches. That is it. Not 4, not 3.75. Run the products through that fixed 3.5 inches:
- Closed-cell: 3.5 × 6 to 7 = about R-21 to R-24
- Open-cell: 3.5 × 3.6 to 4 = about R-13 to R-14
Same cavity, nearly double the performance, and no amount of anything cheaper closes that gap because the inches are not there to close it with. That is the core argument for closed-cell in older Seattle housing stock, where 2×4 framing is the norm and there is no depth to spare. In a 2×6 wall you get 5.5 inches and the argument softens. In an open attic, where depth is effectively free, it often goes away entirely and a cheaper material is the better buy. The full comparison is on open-cell vs closed-cell, and the case for the premium product on closed-cell spray foam insulation.
Two other depths worth writing on your notepad: a 2×8 rafter gives 7.25 inches, and a 2×10 floor joist gives 9.25 inches. If the roof is staying vented, part of that rafter depth has to be given back as an air channel above the foam.
Four worked examples on Puget Sound houses
Each one runs measurement, depth, board feet, then the cost range derived from our published rates. These are estimates for sanity-checking a quote, not quotes. Your house has its own numbers and nobody can produce them from a keyboard.
1. Crawl space walls and rim joist, 1920s Craftsman
Footprint 28 by 36 feet, so 128 linear feet of perimeter. Three feet of stem wall plus a one foot rim band gives 512 square feet of surface. Closed-cell at 2 inches, which is the usual spec here because it handles ground moisture as well as heat.
512 sq ft × 2 in = 1,024 board feet. At $1.00 to $2.00, about $1,000 to $2,050. Add removal and disposal on top if there are wet batts or rodent damage down there, which in a vented Seattle crawl space there often is. See crawl space insulation.
2. Attic floor, 1,150 square foot rambler
The floor is the footprint, 1,150 square feet. If the target you settle on is R-38, open-cell needs 10 inches: 38 ÷ 3.8 = 10.
1,150 sq ft × 10 in = 11,500 board feet. At $0.50 to $0.70, about $5,750 to $8,050.
This is the example where we most often talk people out of foam. Watch what happens if a quote offers 5 inches instead: 5,750 board feet, roughly $2,900 to $4,000, landing near R-19, about half the target. Neither number is wrong, they are answers to different questions. In an accessible vented attic, sealing the floor properly and blowing in cellulose to full depth usually buys more comfort per dollar, and we will say so. More on attic insulation.
3. Roof deck, 1.5 storey bungalow with a conditioned attic
Footprint 26 by 38 feet = 988 square feet. The roof is 8:12, so 988 × 1.20 = about 1,190 square feet of sloped deck. Closed-cell at 5 inches to land near R-30 to R-35.
1,190 sq ft × 5 in = 5,950 board feet. At $1.00 to $2.00, about $6,000 to $11,900.
This is the expensive one, and it is the job people underestimate most, usually because they measured the footprint and forgot the slope. Budget for it honestly before you commit to a conditioned attic.
4. Bonus room over a garage
The cold room over the garage is never one surface, it is five or six, and the one everybody forgets is the floor. A 14 by 20 foot room: flat ceiling 8 by 20 = 160, two slopes at 4.5 by 20 = 180, two knee walls at 3 by 20 = 120, and the floor over the garage 14 by 20 = 280. Total 740 square feet. Closed-cell at 3 inches.
740 sq ft × 3 in = 2,220 board feet. At $1.00 to $2.00, about $2,200 to $4,450.
If you leave the garage ceiling out you save about 840 board feet and the room stays cold, because that floor is sitting over an unheated space with air moving under it. Expect the per board foot rate to sit toward the top of the range on this job: it is a lot of setup, a lot of masking, and awkward angles for not very much foam.
Why your number and our number will not match
Run your arithmetic, then expect the quote to come in above it. That is normal, and here is exactly why rather than a shrug.
Overspray and trim
Foam that rises past the face of the framing gets shaved off so the drywall sits flat. You paid for that foam and it went in a bag.
Irregular framing
Old Seattle houses are not square. Bay depths vary, rafters are hand-cut, and the sprayer fills the real cavity rather than the one on paper.
Depth built in passes
Closed-cell goes on in controlled lifts, not one thick coat. Hitting an average of 5 inches means some spots land at 5.5.
Nothing is a rectangle
Ductwork, plumbing, chimney chases, knee walls, and blocked joist bays all add surface and time that your tape measure never saw.
So expect the sprayed volume to come in somewhat above the paper calculation. If a quote is wildly above your number, ask which of those four is driving it. A contractor who will show you the square footage, the thickness, and the product for each area is doing it right. One who will not show you those three inputs is the warning sign.
About R-value targets
You will notice we used R-21, R-30, R-38, and R-49 as examples rather than telling you which one applies to your house. That is deliberate. Recommended and required levels vary by assembly and by climate zone, and Seattle sits in zone 4C marine, which is mild, damp, and not much like the climates most insulation advice is written for.
We are also not going to print Washington State Energy Code figures here and tell you they are current for your project. Code gets amended, jurisdictions adopt amendments on their own schedule, and whether a minimum applies at all depends on your scope of work. Confirm the current requirement for your project with your local building department before you sign anything. We confirm it per job, in writing, for the specific assembly we are proposing to touch.
Common questions about how much foam you need
How do I convert a quote written per square foot into board feet?
Ask the contractor for the thickness, then multiply. If a bid says 1,200 square feet at $4.50 per square foot, that is $5,400, and you still do not know what you are buying. If the thickness is 3 inches of closed-cell, that is 3,600 board feet, which works out to $1.50 per board foot, squarely inside the normal Seattle range. If the same $5,400 covers only 2 inches, that is 2,400 board feet at $2.25, and you should ask what about the access or the prep is driving it up. The thickness is the missing number in almost every confusing quote.
Do I need to subtract the studs and joists from my square footage?
No, and you should not. Foam is usually sprayed across the framing faces as well as into the bays, especially in a crawl space or on a roof deck, so the framing is part of the sprayed surface rather than a gap in it. Covering the framing is also what reduces thermal bridging, which is heat taking a shortcut through the wood. Measure the gross surface, do not deduct for framing, and do deduct for windows and doors.
My contractor’s board foot number is higher than mine. Is that a red flag?
Usually not. Overspray and trim waste, irregular old framing, depth built up in passes, and surfaces you could not see from the hatch all push the real volume above a paper calculation. What matters is that the contractor can show you the inputs: square feet, inches, and product, area by area. If the inputs are shown and the gap is modest, that is a normal job. If there are no inputs at all, or the number is double yours with no explanation, ask for the breakdown before you ask for a discount.
Can I just get more R-value by asking for more inches?
Only up to the depth you physically have. A 2×4 wall stops at 3.5 inches, a 2×8 rafter at 7.25, and in a vented roof you give some of that rafter depth back as an air channel. Where depth is unlimited, such as an open attic floor, there is a point of diminishing returns instead: nothing to R-20 is a big change, R-40 to R-60 is a small one for similar money.
Does this calculation tell me what my job will cost?
It gets you into the right ballpark so you can judge a quote, which is genuinely useful. It does not account for removing old wet or rodent damaged insulation, containment and ventilation, difficult crawl space access, or the fixed setup a spray rig needs whether the job is small or large. Those are real line items, and they are why a small job costs more per board foot than a big one. Use your number to check whether a bid is in the right neighbourhood, not to predict it to the dollar.
The honest close
Everything above gets you into the right ballpark, which is exactly what you need to look at a quote and know whether it is reasonable. It is not a spec. The actual spec comes from somebody getting into the crawl space or up through the attic hatch with a tape measure and a flashlight, finding out what is already in there, whether it is wet, where the air is moving, and how deep the cavities really are.
We do that for free, we show you the square feet and the inches and the board feet on the estimate rather than one round number, and if the honest answer is that air sealing solves your problem for a fraction of the price, that is what the estimate will say.