Rim Joist Insulation Calculator

Count the bays in your rim joist, the sheets of foam board or kits of spray foam they take, and the sealant that goes round every piece.

The rim

The framing

For a whole basement this is the perimeter of the house, so add the four walls together. Treat only the walls you are actually doing if you are working in stages. Along the two walls the joists run parallel to, there is no row of bays at all, just one long board, and the count here will be generous.
Measure from the middle of one joist to the middle of the next. Sixteen inches is the common one in a house, and it leaves a clear opening of 14 1/2 inches once the joist itself is taken off.
The nominal size on the lumber, not a measurement. A 2x10 is dressed to 9 1/4 inches deep, and that is the height of the opening you are cutting a piece to fill.

The insulation

Board is cut snug into each bay and sealed round the edge, which is the method PNNL documents for an existing rim. Spray foam does the insulating and the sealing in one pass. Fiberglass is not offered: air moves through it and condenses on the cold rim behind.
Two inches of extruded polystyrene is about R-10, which is a common starting point. Check what your climate zone asks for, and read the thickness note in the answer before going past 3 1/4 inches of spray foam.
Owens Corning gives its FOAMULAR extruded polystyrene R-5 for one inch and R-20 for four, so five per inch. DuPont puts its Froth-Pak closed cell foam at an aged R-6.2 per inch. Polyisocyanurate is usually higher again. Use the figure on the product you are buying.

How it is sold

Four by eight is the usual sheet. Change it if you are buying the smaller 2 ft wide boards, because the number of pieces you get out of one is what decides how many you buy.
The calculator tries the pieces both ways round on the sheet and keeps whichever yields more, which is what you would do on the driveway with a straightedge.
DuPont rates the small Froth-Pak kit at a theoretical 200 board feet and the large one at 630, both measured at an inch thick. Theoretical is the word on the data sheet: real yield is lower, which is what the waste allowance is for.

Prices

The shelf price of one sheet at the thickness you entered above. Thickness changes the price more than anything else on the label.
Price the same kit whose yield you entered above, otherwise the two numbers are describing different products.
Ten percent is sensible for board, because the pieces round pipes and wires come out of somewhere. Spray foam wants at least that, since a kit never delivers its theoretical yield and a part used kit does not keep. 10.00%
The order
90 bays
Foam boards 4 sheets
Spray foam kits 0 kits
Sealant run 356.25 ft
Bay opening
14.5 by 9.25 in
R-value at that thickness
10

The detail

Pieces per sheet 30 pieces
Foam volume 167.66 board ft
Rim face area 83.83 sq ft
Exposed foam Board is covered by the rule too, so check locally before leaving it bare
Material cost $0
Cost per foot of rim $0.00

Every figure here is a planning estimate, not a professional takeoff. Yields, waste factors and code requirements vary by product, supplier and jurisdiction, so confirm quantities against the manufacturer’s data and your local building department before you order.

Generated by Mathterial

The rim joist is not a surface you cover. It is a row of pockets, one between every pair of floor joists, and each pocket is cut, fitted and sealed on its own. That is why a square footage figure is the wrong answer here and a bay count is the right one. This calculator works out how many bays your rim has, how many sheets of foam board they cut out of, or how many board feet of spray foam they take, and how much sealant runs round the edges.

Why bays and not square feet

Two basements with the same length of rim joist do not take the same amount of material, because the material is cut to the opening and the opening depends on the framing. At 16 inch centers the clear gap between joists is 141⁄2 inches. At 24 inch centers it is 22 1/2. A 2x10 makes that opening 91⁄4 inches tall and a 2x8 makes it 7 1/4.

Those four numbers decide how many pieces come out of a sheet, and that is the number that sets your order. The calculator tries each piece both ways round on the sheet and keeps whichever yield is better, which is exactly what you would do with a straightedge on the driveway.

Board and spray are counted differently

Switching the method changes which lines of the answer matter, because the two jobs are not the same job.

  • Rigid foam board. Counted as sheets, from how many pieces each sheet yields. It comes with a second line that people forget to buy: every piece has four edges to seal, so the sealant run is several hundred feet on an average basement.
  • Closed cell spray foam. Counted in board feet, which is square feet at one inch thick, then divided by the kit yield. There is no separate sealant line because the foam seals as it goes, and that is most of the reason people choose it.

DuPont rates its small Froth-Pak kit at a theoretical 200 board feet and the large one at 630, both measured at an inch thick. Theoretical is their word, and it is doing work: real yield is lower, a part used kit does not keep, and that is what the waste allowance is for.

The 31⁄4 inch line

There is a thickness at which this stops being a preference and becomes a code question. Building Science Corporation notes that the IRC lets spray foam sit exposed at a rim joist without a 15 minute thermal barrier over it only while it stays under about 31⁄4 inches. Go thicker and the foam has to be covered.

The answer above says which side of that line your thickness falls on, because it is the kind of thing that is cheap to design around and expensive to discover at inspection. Check it locally either way: the figure is from the model code and your jurisdiction may have amended it.

Fiberglass is deliberately not an option

A batt stuffed into the rim is the most common thing in an older basement and the one detail building scientists are united against. Air moves through fibrous insulation. It reaches the cold face of the rim behind the batt, and in winter that is a surface below the dew point, so moisture condenses there against the wood and stays.

Foam does not have that problem because air does not move through it, which is why both methods here are foam and why neither one wants a batt in front of it afterwards.

Check the wood before you order anything

PNNL’s guidance is to assess the sill plate, the rim and the floor framing for moisture first, and to leave the area uninsulated if the wood cannot dry. Insulating over damp framing traps it, and rot is a worse outcome than a cold rim.

If the rim is dry, the method is the one PNNL documents: a piece cut snug into each opening, then its perimeter and every penetration sealed with foam or caulk. The sealant line in the answer above is counted for that step specifically.

What it does not count

  • The two walls the joists run parallel to. There are no bays there, just one continuous board along the rim, so the bay count is generous on those runs and the area figure is close enough.
  • Cans of sealant. Coverage depends entirely on how fat a bead you run, and no published figure survives contact with a real basement, so the answer gives you the linear feet and leaves the can count to the label.
  • Pipes, wires and dryer vents through the rim. Each one costs a little material and a lot of time, and a bay with three penetrations in it is a different afternoon from a bay with none.

Work a basement in sections if you are doing it in stages, and enter only the run you are treating. The bays do not care which order you fill them in.