The question has two halves and they are answered by two different documents. What to use is more or less settled, and it comes down to foam. How much is set by your climate zone, and then it runs into a fire rule that caps how thick exposed foam is allowed to be. Those two numbers do not always fit together, which is the part nobody mentions.
What to use
The Department of Energy’s insulation guide lists R-values per inch for every common material. These are its figures, and its own caveat is that they are approximate and for general comparison only.
| Material | R per inch | At a rim joist |
|---|---|---|
| Closed cell spray foam | 7 to 9 | Seals and insulates in one pass |
| Polyisocyanurate board | 5.6 to 8 | Cut to the bay, seal all four edges |
| Extruded polystyrene, XPS | 5.0 | Cut to the bay, seal all four edges |
| Expanded polystyrene, EPS | 3 to 4 | Cheapest board, needs the most depth |
| Open cell spray foam | 3.5 to 3.8 | Needs 51⁄2 in to count as an air barrier |
| Fiberglass batt | 2.9 to 3.8 | Not on its own, see below |
Treat the ranges as categories rather than product figures. DuPont rates its own Froth-Pak closed cell foam at an aged R-6.2 per inch, which is below the bottom of the generic range above, and that is the number you would have to design to if you were buying it. Read the label, not the category.
PNNL’s method for an existing rim is the same whichever board you pick: cut a piece of unfaced foam to fit each bay snugly, then seal its perimeter and every penetration with caulk or spray foam. The sealing is not a finishing step, it is the step.
How much: the number the code is asking for
There is no rim joist row in the energy code. PNNL closes that gap directly: insulate the rim joist area to code required levels, and where you are using rigid foam, add enough to meet or exceed the code required insulation level for an exterior wall. So the target is your wall number.
Those come from Table R402.1.3 of the 2021 IECC, which is Table N1102.1.3 in the IRC. Wood frame walls, by climate zone:
| Climate zone | Wood frame wall |
|---|---|
| 1 and 2 | R-13, or 0+10ci |
| 3 | R-20, or 13+5ci, or 0+15ci |
| 4 except Marine | R-30, or 20+5ci, or 13+10ci, or 0+20ci |
| 5 and Marine 4 | R-30, or 20+5ci, or 13+10ci, or 0+20ci |
| 6 | R-30, or 20+5ci, or 13+10ci, or 0+20ci |
| 7 and 8 | R-30, or 20+5ci, or 13+10ci, or 0+20ci |
The ci stands for continuous insulation. Where a row offers alternatives, you pick one. And these are new construction figures: a retrofit in an existing basement is usually not held to them, though they are still the sensible target to aim at.
Where that runs into the fire rule
Foam has to be covered by a thermal barrier, usually half inch gypsum board, because it burns. The rim joist gets a specific exemption from that, and the exemption has a thickness limit on it.
IRC section R316.5.11 lets spray foam sit exposed at a rim joist with no 15 minute thermal barrier over it, provided the foam is less than 31⁄4 inches thick, its density is between 0.5 and 2.0 pounds per cubic foot, and it tests at a flame spread index of 25 or less and a smoke index of 450 or less. Four conditions, and thickness is the one you control.
Now put the two numbers together. At that 31⁄4 inch ceiling, closed cell foam gives you somewhere around R-16 to R-23 depending on whose product it is. Which means:
- Zones 1 to 3. You can hit the wall figure with exposed foam and stop there. R-13 needs about two inches of closed cell, R-20 needs about three.
- Zones 4 to 8. You cannot reach R-30 inside the exemption. Whatever you do next, it is not more spray foam left bare.
Worth asking your inspector which column of that table a rim joist is being held to, because a bay filled with foam is arguably continuous insulation rather than cavity insulation, and the 0+20ci alternative in zone 4 is within reach where the R-30 cavity figure is not. The answer varies and it changes your order.
Flash and batt, and why the batt is allowed this time
The usual answer above zone 3 is the one PNNL describes: a layer of foam against the rim to do the sealing, then batt insulation in front of it to make up the rest of the R-value. Rigid board plus a batt works the same way.
This looks like a contradiction, because the first rule anyone learns about rim joists is that a fiberglass batt there causes condensation. Both things are true, and the difference is what the batt is touching.
A batt stuffed straight into the bay fails because air moves through it, reaches the cold back face of the rim and condenses there. Put foam against the rim first and that cold surface is now on the far side of an air seal. The batt is inboard of the control layer, it never sees the cold face, and all it is doing is adding R. That is the whole distinction, and it is why the sequence matters more than the materials list.
The joint to spend your time on
Of the five joints stacked up at the rim, the one between the concrete and the sill plate is the one Building Science Corporation singles out as a major source of leakage on its own. A sawn board laid on a poured surface never sits flat, so the gap is continuous and it runs the whole perimeter of the house.
On new work that joint gets a sill sealer, a foam gasket rolled out before the plate goes down. On a retrofit it is caulk or canned foam, worked along the line where the wood meets the concrete, and it is worth doing before any insulation goes in front of it because you will not reach it afterwards.
The two walls with no bays
On the two sides where the joists run parallel to the foundation there is no row of pockets. There is one continuous board along the top of the wall, and it is the outermost joist doing the rim joist’s job.
It still leaks and it still wants insulating, but it is a long strip rather than a set of cut pieces, which makes it faster per foot and easier to seal. Count it in your total length and expect the material to go further there.
Turn that thickness into an order
Once you have settled on a thickness, the quantity is a counting problem rather than a judgment one: bays, then how many pieces come out of a sheet, or how many board feet of spray at that depth. If you are still working out what the board actually is and why this detail leaks at all, start with what a rim joist is.
Frequently asked
There is no rim joist row in the energy code, so PNNL’s guidance is to meet or exceed the figure for an exterior wall in your climate zone. Table R402.1.3 of the 2021 IECC puts that at R-13 in zones 1 and 2, R-20 in zone 3 and R-30 in zones 4 through 8, each with continuous insulation alternatives.
IRC section R316.5.11 allows it with no 15 minute thermal barrier over it, on four conditions: the foam is less than 31⁄4 inches thick, its density is between 0.5 and 2.0 pounds per cubic foot, its flame spread index is 25 or less and its smoke index is 450 or less. Thickness is the condition you control.
Closed cell spray foam if you want the sealing and the insulating in one pass, since the Department of Energy puts it at R-7 to R-9 per inch and it needs only 11⁄2 inches of finished thickness to count as an air barrier. Rigid board cut to each bay and sealed round the edges is the practical choice for a homeowner, and polyisocyanurate carries more R per inch than extruded polystyrene.
Yes, and above climate zone 3 it is usually how the R-value gets made up. A batt fails when it sits against the rim, because air passes through it and condenses on the cold wood behind. With foam already sealing that face, the batt is inboard of the control layer and never meets a cold surface, so it is only adding R.
Divide your target R-value by the board’s R per inch. Extruded polystyrene is R-5.0 per inch, so R-13 is about 21⁄2 inches and R-20 is about 4. Note that the exposed foam exemption above is written for spray foam at the rim, so ask locally before leaving thick board uncovered.
Yes, and it is the same detail: a piece cut to each bay with the perimeter and the penetrations sealed. PNNL adds one step for a crawl space, which is to check the floor vapor retarder afterwards and repair any tears you made getting in there.