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ARMORTHANE PROTECTING YOUR WORLD

PROTECTING YOUR WORLD

ARMORTHANE PROTECTING YOUR WORLD

Rim Joist
Insulation:
Best Way & Code

How to seal and insulate the rim and sill in one pass, with the current code numbers, a foam and kit calculator, a condensation check by city, and honest payback math from the people who make ArmorFoam.
Rim Joist Quotes

Rather have it sprayed? Get a quote from a certified applicator.

Tell us where the rims are (basement, crawl space or both) and roughly how big the house is. We'll connect you with an ArmorThane-certified spray foam applicator who will quote foam type, thickness and R-value in writing.

  • Basement and crawl space rim joists
  • Rims, crawl-space walls and basement walls in one visit
  • Thickness, R-value and board feet on the quote
  • Contractors: ask about ArmorFoam and spray rigs
Rather talk now? Call (417) 831-5090
Step 1 of 3
3¼ in.
Max exposed foam at the rim
IRC R316.5.11 (2024: R303.5.11)
2 in.
One kit’s own rim limit
Froth-Pak, ICC-ES ESR-3228
≈R-4
A bare 2x rim assembly
R-3.97, our math from handbook values
<20%
Wood moisture before you foam
USDA Forest Products Lab margin

The rim joist is the most awkward few inches of a house to insulate, and one of the most worthwhile. It's a band of wood on the foundation with leaky joints above and below it, and in winter it runs close to outdoor temperature. Insulate it wrong and you trade a draft for condensation and rot.

Quick answer

Seal and insulate in one layer, foam first. Spray 1.5–3 in. of closed-cell foam into each bay, or cut 2 in. or more of rigid foam to fit and seal every edge with canned foam or caulk. Never use fiberglass by itself. The IRC lets foam stay exposed at the rim up to 3¼ in. if it meets density and flame/smoke limits, but your product's evaluation report may cap it lower (Froth-Pak: 2 in.). Leave a termite inspection gap where your area calls for one. Jump to the foam calculator or the code checker.

Most pages on this topic stop at "spray foam good, fiberglass bad." True, as far as it goes. What they skip decides whether the job passes inspection and stays dry: the product's own thickness limit, your county's code edition, how much foam keeps a batt from sweating in your climate, and whether the payback is real.

We make ArmorFoam spray foam in Springfield, Missouri, and we've been putting applicators in business for 35 years. So this is written the way we'd explain it to a new applicator. Code text first. Numbers shown. And honest about when a $739 kit or a few sheets of foam board beat hiring a crew.

What's in this guide
  1. 1What a rim joist is, and why it leaks
  2. 2Best way to insulate rim joists: methods compared
  3. 3How thick, how much, what it costs (calculator)
  4. 4What the code requires (checker)
  5. 5Will foam rot my rim joist? (condensation check)
  6. 6Spray foam, step by step
  7. 7Rigid foam board, step by step
  8. 8Pipes, vents, parallel joists and I-joists
  9. 9Is it worth it? (payback estimator)
  10. 10Common mistakes
  11. 11Second-floor rim joists
  12. 12Hire an applicator, or become one
  13. 13Rim joist insulation FAQ
The basics

What is a rim joist, and why does it leak?

The rim joist (also called a band joist, header joist or rim board) is the board that caps the ends of your floor joists and sits on the sill plate, right on top of the foundation. It leaks because three separate joints meet there. It's cold because only about an inch and a half of wood separates it from the outdoors.

Rim joist vs. band joist vs. sill plate vs. rim board

Rim joist, band joist and header all mean the same perimeter framing member; "rim board" usually means the engineered version used with I-joists. The sill plate (mudsill) is the flat board underneath, the member that "rests on the foundation" (IRC R202). Newer homes usually have 2x10 or 2x12 rims; older houses can have 2x6s, 2x8s or even solid timbers (Fine Homebuilding).

1 2 3 4 5 6 7 8 INSIDE (basement)OUTSIDEGRADE
  1. Sill-to-foundation joint. Sealed by the sill gasket, plus foam or caulk.
  2. Rim-to-sill joint. The 2021 IECC requires it air sealed.
  3. Rim-to-subfloor joint. Also required to be air sealed.
  4. Rim (band) joist, with sheathing and siding outside.
  5. Sill plate (pressure-treated in newer homes) on a sill gasket.
  6. Foundation wall and anchor bolt.
  7. Closed-cell foam against the rim and over the sill.
  8. Floor joist (beyond) and subfloor.

The three joints that leak

Air gets in between the sill and the foundation, between the rim and the sill, and between the rim and the subfloor. The 2021 IECC says rims "shall include an exterior air barrier" and that the rim-to-sill and rim-to-subfloor junctions "shall be air sealed." Building Science Corporation calls the rim "a particularly troublesome detail": a chain of workmanship-sensitive joints, plus cold interior surfaces in winter.

Why the rim is the coldest wood in the house

Add up a typical rim's layers (inside air film, 1½ in. of softwood, ½ in. plywood, vinyl siding, outside air film) and you get about R-4: R-3.97 by our math, from standard handbook values. The wall above carries R-13 or more. And floors make up one-fourth to one-third of a house's enclosure area (DOE); the rim is the edge of all of it.

Here's the counterintuitive part. A bare rim loses heat fast, but its inside face stays fairly warm because room air washes over it: about 61°F at Chicago's midwinter average, in our steady-state math. Stuff a fiberglass batt against it and that face drops to about 35°F, well under the 44.6°F dew point of 70°F air at 40% humidity. That one number is the whole fiberglass problem.

Pick a method

The best way to insulate rim joists: methods compared

The best way is one continuous layer of foam against the rim that also seals the joints: closed-cell spray foam, or rigid foam boards cut to fit each bay with every edge sealed. A batt can go over that foam if the foam is thick enough for your climate. Fiberglass alone is the one method we'd rule out.

MethodR per inchAir barrier?VaporLeft exposed at the rim?Climate fitDIYCost basisMain risk
Closed-cell spray foam, pro rigR-6.5 aged (ArmorFoam, ASTM C518)Yes, at 1 in. (ASTM E2178)0.83 perm at 1 in. (Class II)Up to 3¼ in. if it meets density and E84 limits, or the product's report limitAll zonesNo$2,000–$2,500 per rim job (HomeGuide); $800–$2,000 for 150 ft (ArmorThane)Job minimums; foaming wet wood
Closed-cell two-part kitR-6.2 (Froth-Pak, ESR-3228)Yes, at ½ in. or more (ESR-3228)Not stated in the ESRFroth-Pak: 2 in. max per its ESR; DAP kits: 2 in. maxAll zonesModerate$1.22–$1.95 per board foot (2026 kits)Cold kits, wrong respirator
Open-cell spray foamR-3.9 at 1 in. (ArmorFoam 0.5)Only at 5.5 in.+ (DOE)Vapor-open0.5 pcf is inside the density window; check E84 dataZones 1–4NoUsually priced at a job minimumCondensation at the rim in cold zones
XPS, cut-and-cobbleR-5.0Only with sealed edges1.1 perm/in.2015+ IRC covers boards ≤3¼ in. that meet the limits; check the labelAll (3–4 in. in Zone 6+, per FHB)Easy$101–$250 per project (Family Handyman, 2024)Unsealed edges; high-GWP blowing agent
EPS, cut-and-cobbleR-4.0Only with sealed edges2.0–5.8 perm/in. unfacedSame as XPS; check the labelMild to moderateEasySimilar to XPSThin EPS under a batt can sweat
Foil-faced polyisoR-5.0–5.6 (handbook); Thermax HD R-6.9 at 1 in.Only with sealed edgesFacer 0.03 perm (Thermax)Check the label; Thermax is the board FHB names for exposed useAll zonesEasyBoard price variesUnsealed facer seams
Hybrid: foam, then a battFoam R + R-13, 15 or 21Yes (the foam layer)Set by the foamFoam follows the same rules; batt covers itCold zones, finished basementsModerateFoam plus battsFoam too thin for the batt
Fiberglass or mineral wool onlyR-13 or R-15 at 3.5 in.NoVapor-openn/aNot recommendedEasyCheapestCondensation, frost, rot
Canned one-part foam only≈R-5.7, ≈20 bd ft per canSeals gaps only––Air sealing anywhereEasyPer canIsn't an insulation layer
Pre-cut R-10 foam blocksR-10 per 2¼ in. blockOnly with sealed edges–Fire data not on the product pageModerateEasy$129.99 per 10 (Crawl Space Ninja)Bays that don't match 9.5 x 14.5 in.
Exterior rigid foam (re-siding)Board RWith taped seamsKeeps the rim warmCovered by the sidingAll, where termite rules allowRarelyPart of a siding jobTermite limits; drying if foamed both sides
R-values: ArmorThane test data, ICC-ES ESR-3228, the Colorado Energy R-value table, DuPont (Thermax) and Crawl Space Ninja. Canned-foam figures from our closed-cell spray foam guide. Perm ratings: ArmorThane, GreenBuildingAdvisor and DuPont.

Closed-cell spray foam (pro rig or two-part kit)

The only method that seals and insulates in one step. It fills around anchor bolts, wires and the lumpy top of the sill, and bonds to the wood, so there's no edge left to seal. Kits are rated at R-6.2 per inch (Froth-Pak, ESR-3228); ArmorFoam tests at R-6.5 aged, passes as an air barrier at 1 in. (ASTM E2178) and measures 0.83 perm there, a Class II vapor retarder. DOE uses a more conservative 1.5 in. for the air barrier. Downsides: cost per board foot, kit thickness limits, and chemicals that deserve respect.

Rigid foam cut-and-cobble (XPS, EPS, polyiso)

Cut boards to each bay, press them against the rim, and foam or caulk every edge. Cheaper, slower, and only an air barrier if those edges are sealed. Two inches (R-10 in XPS) is the usual start; Fine Homebuilding suggests 3–4 in. in Climate Zone 6 and colder, and notes that green builders avoid XPS for its high-GWP blowing agent. On vapor, XPS runs about 1.1 perm per inch, unfaced EPS 2.0–5.8, and foil-faced polyiso is close to vapor-closed.

Hybrid: foam first, then a batt

Want more R-value than 3¼ in. of exposed foam gives, or finishing the basement anyway? Foam against the rim, batt over it. The catch: the foam's inner face has to stay warmer than the room's dew point, or the batt just moves the condensation inward. Our condensation check sizes it by city.

Open-cell spray foam: where it works and where it doesn't

Open-cell is vapor-open and needs about 5.5 in. to act as an air barrier (DOE). Building Science Corporation says it "may require additional condensation control" in Zone 6 and colder, and Fine Homebuilding would use closed-cell there. In Zones 1–4 it can work. Our take: at a rim, where depth is limited and the wood is cold, closed-cell is the easier call almost everywhere. The open-cell spray foam guide covers where open-cell shines.

Why fiberglass alone fails

Batts are air-permeable. Warm indoor air moves through them, reaches the cold rim and condenses or frosts, and over years that's how rims rot (Fine Homebuilding, GreenBuildingAdvisor). StructureTech, a Minnesota inspection firm, says batts "should never be used at the rim space," and Penn State's PHRC found in 2016 that batts permit convection and condensation. More in spray foam vs. fiberglass.

Two claims you'll read that need an asterisk"ENERGY STAR says batts are fine." Its 2007 DIY guide says that after air sealing you can fill each cavity with rigid foam or fiberglass batts, and DOE's 2024 job aid allows "foam-based or vinyl-faced fiberglass batt insulation" after air sealing. Both assume the rim was sealed first. Our position: a batt only over an air-sealed, foam-first rim, and never as the only layer in a cold climate.

"Rim joists cause 15–25% (or 40%, or 75%) of your heat loss." We couldn't find a primary source for any of those numbers. The sourced figure is ENERGY STAR's modeled average of 15% savings on heating and cooling from air sealing plus insulation in attics, floors over crawl spaces and basement rim joists, all together.

Exterior insulation, when you're re-siding

If the siding is coming off anyway, rigid foam outside keeps the rim warm with no interior condensation question. Two cautions: in "very heavy" termite areas, IRC R318.4 limits foam near grade (see termites), and PHRC found foam on both sides of a rim "dramatically reduces drying potential," so don't sandwich a rim unless it's dry and staying dry.

Interactive tool 1 of 4

Rim joist spray foam: how thick, how much, what it costs

Most rims get 1.5–3 in. of closed-cell foam, about R-9 to R-20. A 30x40 house has 140 linear feet of rim and needs about 300 board feet at 2 in. once you add waste. That's one 600-board-foot kit, or somewhere between $800 and $2,500 if a pro sprays it, depending on whose numbers you use. The calculator runs your house.

Foam & cost estimator

How much foam, how many kits, and what it costs

Enter the footprint over the basement or crawl space. The estimator deducts joist ends, adds waste, prices the cheapest kit combination and checks thickness against the exposure rules.

Bump-outs, cantilevers, garage wall
140 ft
Linear feet of rim
120 sq ft
Net area to cover
299 bf
Board feet, with 25% waste
R-12.4
R-value installed

Perimeter: 2 x (40 + 30) = 140 ft. Height covered: 10.75 in. Deducted 60 joist ends (5.8 sq ft) from 125.4 sq ft. R-value uses a two-part kit at R-6.2/in. (Froth-Pak, ESR-3228).

Cheapest kit plan

≈$739 in kits

  • 1 x HandiFoam QC HFO 605 (605 bf, ≈$739 each)
  • Rated yield 605 bf for the 299 bf you need.
  • At $1.22–$1.95 per board foot, the foam itself is worth $365–$583.

2026 street prices from our closed-cell guide; check the retailer. Yields are theoretical.

Professional

$800–$2,000

275 bf at 15% waste. Small rim-only jobs often price at a minimum charge. HomeGuide reports $2,000–$2,500 as typical for rim joists (Dec 2025); our closed-cell guide cites $800–$2,000 for 150 linear ft. This range uses those as a floor and ceiling (our assumption).

Get a rim joist quote from a certified ArmorThane applicator

Contractors: rims are ~300 bf jobs, and a 55-gallon set yields ~5,000 bf. Add spray foam to your business.

Checks
  • Air barrier: yes. Closed-cell passes ASTM E2178 at 1 in. (ArmorFoam); Froth-Pak’s ESR lists ½ in. DOE uses 1.5 in. as a conservative threshold.
  • Left exposed: 2 in. is within the code’s 3¼ in. limit. OK without drywall if the product meets flame spread ≤25 and smoke ≤450 (ASTM E84) at that thickness and 0.5–2.0 pcf (IRC R316.5.11; 2024 R303.5.11). Your product’s report may set a lower limit.
  • Bays: about 60 joist bays on the two walls the joists land on, plus a long run on each parallel wall.

How we calculated this: area = perimeter x (rim depth + 1.5 in. sill) ÷ 12, minus 1.5 in. x rim depth per joist end on the two walls the joists land on; board feet = area x thickness x (1 + waste). Kit prices are 2026 street prices from our closed-cell guide, not re-checked at retailers. Estimates only; your AHJ and product literature govern.

How many kits do I need? (per-bay table)

Rather count bays? Here's board feet per bay per inch of foam (clear bay width, rim depth plus a 1½ in. sill), and how many bays a 200-board-foot kit covers at 2 in.

Spacing2x82x102x1211⅞ in. I-joist
12 in. o.c.0.64 bf/in (157 / 125)0.78 (128 / 102)0.93 (108 / 86)0.98 (103 / 82)
16 in. o.c.0.88 (113 / 91)1.08 (92 / 74)1.28 (78 / 62)1.35 (74 / 59)
19.2 in. o.c.1.08 (93 / 74)1.32 (76 / 61)1.57 (64 / 51)1.64 (61 / 49)
24 in. o.c.1.37 (73 / 59)1.68 (60 / 48)1.99 (50 / 40)2.09 (48 / 38)
Our calculation. In parentheses: bays per 200-bf kit at 2 in. with no waste / with 25% waste. Kit yields are theoretical, so treat these as ceilings. Our closed-cell guide frames twenty bays on a Saturday as a good use of a 200-bf kit.

DIY kit vs. rigid foam vs. professional: what it costs

OptionPublished figureDateSource
Professional spray foam, rim joists$2,000–$2,500 per projectDec 2025HomeGuide
Professional, 150 linear ft of rim$800–$2,000 (≈300 bf at 2 in.)2026ArmorThane
Two-part kits, 600-bf class≈$739–$919 ($1.22–$1.46 per bf)2026ArmorThane kit table
Two-part kits, 200-bf class≈$377–$389 ($1.87–$1.95 per bf)2026ArmorThane kit table
200-bf kit (older figure)$300–$4002021Fine Homebuilding
DIY rigid foam project$101–$250Jul 2024Family Handyman
Pre-cut R-10 foam blocks$129.99 per 102026Crawl Space Ninja
A contractor's chemical runs roughly $0.35–$0.60 per board foot from a 55-gallon set (our industry estimate); the rest is labor, the rig and job minimums. Our spray foam insulation cost guide breaks it down.

When to call a pro

  • Long runs. Past about 150 linear feet, the job outgrows a Saturday with a kit, and a pro's price (often a minimum charge) gets competitive.
  • Crawl spaces and tight overhead work. The hardest version of this job, and where respirator discipline matters most.
  • Existing water or rot. Get the rim repaired before anyone foams it.
  • Finished basements or inspections. You'll want thickness, product data and barrier details in writing.
  • Bundling. Rims plus crawl-space or basement walls in one visit bury the minimum charge in a bigger job.
Interactive tool 2 of 4

What does the code require for rim joist insulation?

Foam can stay exposed at the rim and sill, with no drywall, if it's 3¼ in. or thinner, between 0.5 and 2.0 lb/ft³, and rated at a flame-spread index of 25 or less with a smoke-developed index of 450 or less. That's IRC R316.5.11 (R303.5.11 in the 2024 IRC). The energy code separately wants the rim air sealed, with insulation touching the rim board.

Code & method checker

Is my rim joist plan code-compliant and moisture-safe?

Specific to the rim and sill. Not sure of your climate zone? The condensation check below shows it for 47 cities.

Acceptable with conditionsClosed-cell SPF · 2 in.
  • Confirm the product’s E84 and density data
Fire

Likely OK exposed, if the data backs it

  • Likely OK if your product’s data sheet or ESR shows flame spread ≤25 and smoke ≤450 at 2 in. or more, and a density of 0.5–2.0 pcf. Some kits, Froth-Pak included, cap exposed foam at 2 in.
Energy

Below the wall table

  • Air barrier at the rim, rim-to-sill and rim-to-subfloor joints sealed, and insulation in permanent contact with the rim board (2021 IECC Table R402.4.1.1).
  • Rims are “peripheral edges of floors,” part of the above-grade wall. Wall target, zone 5, 2021 IECC: R-30 or 20+5ci or 13+10ci or 0+20ci.
  • Your plan: R-13.0, below R-30.
  • More foam, up to 3¼ in., stays exposed: about R-21.1.
  • Options: add a batt over the foam (run the condensation check), go thicker behind a thermal barrier, or ask your code official how the table applies to rims and retrofits.
Moisture

No moisture red flags

  • Dry wood, treated sill and no water signs. Meter the rim before foaming anyway.

Educational summary of model code; your AHJ and local amendments govern.

The 3¼-inch rule: IRC R316.5.11 (2024: R303.5.11)

Foam at the sill and rim can skip the usual thermal barrier when it meets all three conditions:

  • Thickness: not more than 3¼ in. (83 mm).
  • Density: 0.5 to 2.0 pounds per cubic foot.
  • Fire rating: flame-spread index of 25 or less and smoke-developed index of 450 or less, tested to ASTM E84 or UL 723.

The wording matters. The 2009 and 2012 text covered only foam spray applied to the sill plate and header. The 2015–2021 editions added foam "installed in the perimeter joist space," which brings rigid boards in too if they meet the same three conditions. So advice that rigid foam at the rim always needs drywall reflects the pre-2015 text. Check the board's data sheet for density and E84 numbers; we haven't verified which common boards fall inside the density window.

Your product's evaluation report can be stricter

The code sets the ceiling; the product's ICC-ES report can set a lower one. Froth-Pak's ESR-3228 allows a "maximum thickness of 2 inches" on sill plates, band joists and headers without a thermal or ignition barrier, and its E84 result was tested at 2 in. DAP's kit instructions also say 2 in. maximum. So: allowed exposed thickness = the lesser of 3¼ in. or your product's limit. Hiring it out? Ask any installer, us included, for the E84 numbers at the thickness being sprayed.

When you need a thermal or ignition barrier

  • Thicker than the limit: a thermal barrier: ½-in. gypsum board (R316.4; R303.4 in 2024), an NFPA 275-tested equivalent in newer editions, or an intumescent coating listed in the foam's evaluation report. A Class A E84 rating alone isn't a substitute.
  • Crawl spaces entered only for service: an ignition barrier instead: 1½ in. mineral fiber, ¼ in. wood structural panel, ⅜ in. particleboard, ¼ in. hardboard, ⅜ in. gypsum board, or 0.016 in. steel (R316.5.4; R303.5.4 in 2024). ESR-3228's separate conditions for skipping it include no storage and no air circulated to the rest of the house.
  • Finished basements: DOE's weatherization spec calls for a thermal barrier when the space is habitable, and in a finished basement the rim usually ends up behind drywall anyway.
  • Minnesota: the 2014 version of Minn. R. 1309.0314 raised the exposed limit to 5½ in. We couldn't confirm it's unchanged in the current code, so check.

Energy code: air barrier, insulation contact and R-value

The 2021 IECC air-barrier table (R402.4.1.1) has a rim joist row: an exterior air barrier with the rim-to-sill and rim-to-subfloor junctions sealed, and insulation in "permanent contact with the exterior rim board." A loose batt with a gap behind it doesn't meet that.

What R-value? The IECC's above-grade wall includes "peripheral edges of floors," so the rim's target is the wood-frame wall value:

Climate zone2009 IECC2012–2018 IECC2021 IECC
1–2R-13R-13R-13 or 0+10ci
3R-13R-20 or 13+5R-20 or 13+5ci or 0+15
4 (except Marine)R-13R-20 or 13+5R-30 or 20+5ci or 13+10ci or 0+20ci
5 and Marine 4R-20 or 13+5R-20 or 13+5Same as zone 4
6R-20 or 13+5R-20+5 or 13+10Same as zone 4
7–8R-21R-20+5 or 13+10Same as zone 4
Wood-frame wall requirements, DOE/PNNL Building America Solution Center. "13+5" means R-13 cavity plus R-5 continuous insulation. The 2024 IECC location of the rim joist row wasn't confirmed for this guide.

Notice the tension. Under the 2021 IECC in zones 4–8 the cavity-only number is R-30, but 3¼ in. of exposed closed-cell gives only about R-20 to R-21. Your options: a batt over the foam (run the condensation check first), thicker foam behind a thermal barrier, or asking your code official how the table applies to rims and retrofits. The R-13 / R-20 targets in older articles match the 2009 table. And in Springfield, Missouri, like us, Greene County is 4A in the 2018 county table; the 2021 map moved some counties, so verify yours.

Termite zones and inspection gaps

  • IRC R318.4 (R305.4 in 2024): where termite probability is "very heavy," no foam on the exterior face of, or under, foundations below grade, and 6 in. of clearance between above-grade foam and exposed earth. Interior basement walls are excepted.
  • DOE weatherization spec: where termite pressure exists, a 3-in. gap between the top of the insulation and the bottom of any wood.
  • South Carolina (2024 modifications): in crawl spaces, a gap of at least 6 in. along the top of the foundation wall and sill, plus a continuous 6-in. strip centered on the sill.
  • Georgia: reported to require an inspection strip exposing the sill and lower band joist; we haven't confirmed it at the primary source.

Wherever you are, talk to your pest-control company before foaming. A foamed-over sill is hard to inspect.

2024 IRC section renumbering

Topic2015–2021 IRC2024 IRC
Sill plates and headers (the 3¼-in. rule)R316.5.11R303.5.11
Thermal barrierR316.4R303.4
Crawl space ignition barrierR316.5.4R303.5.4
Foam plastic and termitesR318.4R305.4
Decay protection (treated wood)R317.1R304.1
Numbering per ICC-ES ESR-3228 and the 2024 IRC text on UpCodes.
Interactive tool 3 of 4

Will spray foam rot my rim joist?

Not if the wood is dry, the water stays out and the foam is thick enough. Decay needs wood moisture above the fiber saturation point (about 30%), and wood at 20% or less gives "a reasonable margin of safety" (USDA Forest Products Laboratory). The real risk is condensation on a cold surface, and that's a sizing question you can check.

The first condensing surface, in plain English

Cool humid air past its dew point and water comes out. The question is which surface indoor air reaches first, and whether it's colder than the room's dew point. With a batt alone, air goes straight to the cold rim. With closed-cell foam, air stops at the foam's inner face, which stays warm. With a hybrid, the batt cools that foam face, so the foam must be thick enough to keep it above the dew point.

R-13 batt only2 in. closed-cell2 in. closed-cell + R-13 battfoambattfoam 35.0°Fat the rim faceBelow dew point 68.3°Fat the foam faceDry 50.5°Fat the foam faceDry, 6.0°F margin
Inside is on the left. Chicago's three coldest months average 28.2°F (NOAA 1991–2020 normals); room air at 70°F and 40% RH has a 44.6°F dew point. Dashed line = the first surface indoor air can reach. Steady-state, our calculation.
Condensation check

How much foam do I need before I add a batt?

Winter temperatures are NOAA 1991–2020 normals; climate zones are from the 2018 IRC county table, so verify your county.

Used when city = custom
44.6°F
Indoor dew point
50.5°F
Foam face temperature
+6.0°F
Margin over dew point
0.94 in.
Minimum closed-cell under R-13

Stays dry Chicago, IL (zone 5A; verify your county): design temperature 28.2°F, dew point of 70°F / 40% air 44.6°F. Your 2 in. clears the 0.94 in. needed under R-13. Never go under 1 in. of closed-cell, since it’s also the air barrier. Steady-state screening, not a hygrothermal model. Want it done to spec? Find an ArmorThane applicator.

How we calculated this: dew point by the Magnus formula (Alduchov-Eskridge coefficients); layer R-values of 0.68 inside film, batt, foam, rim, 0.63 sheathing, siding and 0.17 outside film; surface temperature from the R-value ratio on either side. Steady-state screening: it ignores moisture stored in the wood and solar drying, and isn't a WUFI model. Closed-cell at 1 in. or more is itself a Class II vapor retarder (0.83 perm, ArmorThane).

How much foam you need under a batt, by city

City (zone)Rim face behind R-13 batt aloneClosed-cell under R-13, 40% RHUnder R-13, 50% RHUnder R-21, 40% RH
Houston (2A)58.0°F0 in.0 in.0 in.
Atlanta (3A)50.7°F0 in.0 in.0 in.
Springfield, MO (4A)42.2°F0.23 in.1.07 in.0.61 in.
St. Louis (4A)40.8°F0.37 in.1.25 in.0.83 in.
Kansas City (4A)38.1°F0.64 in.1.60 in.1.25 in.
Boston (5A)38.6°F0.59 in.1.53 in.1.17 in.
Chicago (5A)35.0°F0.94 in.2.00 in.1.73 in.
Detroit (5A)35.2°F0.93 in.1.98 in.1.71 in.
Minneapolis–St. Paul (6A)27.9°F1.65 in.2.93 in.2.86 in.
Duluth (7)23.7°F2.07 in.3.47 in.3.52 in.
Fairbanks (8)8.0°F3.62 in.5.48 in.5.97 in.
Our calculation: 70°F indoors, mean of the three coldest months, 1½ in. sawn rim, ½ in. sheathing, siding ignored, closed-cell at R-6.5 per inch. Dew point is 44.6°F at 40% RH and 50.5°F at 50%. Red rim-face values are below the 40% RH dew point. In practice, never go under 1 in. of closed-cell, because the foam also has to be the air barrier. Amber values exceed the 3¼ in. exposure cap, so they'd need a thermal barrier or a thinner batt.

Older homes: untreated sills, no capillary break and wet rims

Current code (IRC R317.1; R304.1 in 2024) requires naturally durable or treated wood where framing sits on a foundation less than 8 in. from exposed ground. Houses built before roughly the 1980s may have untreated mudsills and no capillary break, per Fine Homebuilding and GreenBuildingAdvisor, and interior foam cuts their ability to dry inward. In an older house, work through this first:

  1. Meter the rim and sill with a pin moisture meter. Under 20% is the target.
  2. Look for stains, efflorescence or drips after rain.
  3. Fix grading, gutters and splash-back first. Our basement waterproofing page and foundation waterproofing guide cover the water side.
  4. Probe the sill for soft spots. Rot gets repaired, not foamed over.
  5. No capillary break? Fine Homebuilding notes a house can be jacked about ¼ in. to slip in flashing. That's a job for a pro.

Crawl spaces: vented vs. sealed

In a sealed, conditioned crawl space the rim is part of the envelope, so treat it like a basement rim; our crawl space encapsulation guide covers the full sequence. In a vented crawl space the floor above is usually the insulated boundary, so the rim isn't doing the same job. Either way, crawl spaces bring in the ignition-barrier rules, and they're the worst-ventilated place to spray kit foam.

Step by step

How to insulate rim joists with spray foam

Confirm the wood is dry, clear the bays and warm the kit. Then picture-frame each bay: seal the sill joints, run the bay's perimeter, and fill the middle in lifts no thicker than the product allows. Wear an organic-vapor/P100 respirator, ventilate hard, and follow the kit's re-entry instructions.

  1. Inspect and meter

    Pin-meter the rim and sill (under 20%), look for water stains and termite activity, and note flues or vents through the rim. Fix water first; foam over a leak just hides it.

  2. Clear and clean the bays

    Bag the old batts and vacuum dust and cobwebs. ESR-3228 says not to spray onto a wet or frosted substrate.

  3. Mask and protect

    Keep foam off heaters, furnaces, chimneys, vents and recessed lights (ESR-3228). If your area needs a termite inspection gap, mark it with tape now.

  4. Warm the kit

    DuPont wants Froth-Pak chemicals at 70–120°F, best at 75°F or above, with the air and substrate at 65°F or more. DAP's range for its kits is 70–90°F in the tanks and 60–90°F for surfaces and air. A cold kit makes poor foam. If you're working in winter, read applying spray foam insulation in cold weather first.

  5. Test spray, then picture-frame

    Test on cardboard until the foam looks right. Seal the sill joints, then outline each bay before filling it, as DAP describes. Froth-Pak sets in 30–60 seconds; DuPont says not to reuse a nozzle idle for 30 seconds.

  6. Fill in lifts

    Build thickness in passes of 1.5–2 in. at most (ESR-3228: spray to a maximum of 2 in.). Check depth as you go, and don't pass your product's exposed limit.

  7. Inspect and clean up

    Touch up voids at joist ends and penetrations. DAP says a partly used kit keeps for 30 days.

PPE, ventilation and re-occupancy

ESR-3228 quotes DuPont's call for a respirator with an organic-vapor sorbent and a P100 particulate filter, especially in confined areas like crawl spaces; DuPont's how-to adds goggles, nitrile gloves and hooded Tyvek coveralls. DAP's page lists an N95 mask. An N95 is a particulate filter with no vapor cartridge, so we'd follow the ESR.

Ventilate at 10 or more air changes per hour during application and for at least an hour after (ESR-3228); anyone entering in that hour needs a respirator. Keep kids and pets out and follow the kit's re-entry instructions. For pro-sprayed foam, get the manufacturer's re-occupancy time in writing.

One more, unrelated to foam: air sealing a basement can make a chimney-vented water heater or furnace backdraft (StructureTech). If you have one, get a combustion-safety test afterward.

Spray foam applicator in a hooded protective suit and respirator spraying foam between wall studs
Professional crews spray in full suits and respirators. Kit users need the same respect for the chemistry.
The budget method

How to insulate rim joists with rigid foam board

Measure each bay, cut 2 in. (or thicker) board slightly undersize, press it against the rim, and seal all four edges with canned foam, or caulk for joints of ¼ in. or less. The board is only an air barrier if every edge is sealed.

  1. Seal the sill first

    Foam or caulk the sill-to-foundation joint and the rim-to-sill joint before the boards go in, since the boards will hide them.

  2. Measure every bay

    Bays vary. Write each one's size on the joist.

  3. Cut ⅛–¼ in. undersize

    Family Handyman cuts strips ⅛ in. undersize so they slide in without crushing. Notch around anchor bolts and pipes.

  4. Fit it tight to the rim

    Friction-fit, use a foam-safe adhesive, or hold with cap nails. The code wants insulation in permanent contact with the rim board, so no air gap behind the board.

  5. Seal every edge

    ENERGY STAR's DIY guide suggests caulk for gaps up to ¼ in. and spray foam for gaps from ¼ in. to about 3 in. Canned one-part foam runs about R-5.7 per inch and roughly 20 board feet per can.

  6. Layer or add a batt if you want more R

    If 1-in. boards are easier for you to cut, stack two layers and seal both. Adding a batt over the foam? Run the condensation check first. If the batt is faced, DOE's job aid puts the facing toward the conditioned side. Skip poly sheeting over foam: it can trap moisture.

Fire rules: under the 2015–2024 IRC, boards 3¼ in. or thinner that meet the density and E84 limits can stay exposed at the rim. Under 2009–2012 text, or for any board you can't document, add a thermal barrier or use a board listed for exposed use.

Details

Tricky rim joist spots: pipes, vents, parallel joists and I-joists

Every basement has a few bays that aren't simple rectangles. Here's how to handle them without burying something you'll need later.

Pipes and wires through the rim

Foam seals around them well. DOE's job aid says penetrations bigger than ¼ in. get backing or pest-proof fill before sealing. Don't bury junction boxes, and leave wiring accessible.

Dryer vents, flues and exhausts

ESR-3228 says keep foam off heaters, furnaces, chimneys and vents. Seal around them without foaming the duct or flue itself, and follow the appliance maker's clearances.

Parallel joist walls

Where joists run parallel, the bay is one long, narrow slot between rim and first joist. Spray foam handles it easily; boards need ripping. The estimator counts these runs.

Cantilevers and bump-outs

Add their rims to "extra rim runs." The cantilevered floor over outdoor air is a separate insulation job.

I-joists and engineered rim boards

Engineered rim board is thinner than sawn lumber (often 1⅛ in.), which the condensation check accounts for. Deeper rims mean more square feet per foot of wall.

Anchor bolts and lumpy sills

Spray foam fills around bolts and uneven sill tops. With boards, notch around the bolt and foam the notch.

Interactive tool 4 of 4

Is rim joist insulation worth it? The honest payback math

Yes, though not mainly for the reason people think. Conduction savings on a typical rim are modest: about $23–$82 a year with a 95% gas furnace, Atlanta to Duluth, by our math. The bigger payoffs are air leakage you can't model without a blower door, no more frost or damp wood, and warmer floors.

Savings & payback estimator

What will insulating the rim save, and how long to pay back?

Conduction math on the rim area only, using NOAA heating degree days and 2025 EIA average prices. Assumes the basement or crawl space is inside the heated envelope.

Used when city = custom
From tool 1, or 2 x (L + W)
$739 = one 605-bf kit
Blower-door numbers
3.47
MMBtu saved per year
$54
Saved per year
14 yr
Simple payback
77%
Rim conduction loss eliminated
1 in. (R-6.5)
$44/yr
2 in. (R-13.0)
$54/yr
3 in. (R-19.5)
$59/yr
3¼ in. (R-21.1)
$59/yr

Conduction through 125.4 sq ft of rim and sill, R-3.97 before, R-16.97 after, 5,974 heating degree days. Air sealing isn’t in these numbers; if you have blower-door results, tick the box above. Bars show yearly savings by thickness; bar length is the share of the rim’s conduction loss eliminated. It flattens fast. Assumes the basement or crawl space is inside the heated envelope.

How we calculated this: area = rim length x (rim depth + 1.5 in.) ÷ 12; heat saved = (1/R before − 1/R after) x area x 24 x HDD65. Gas at $1.48/therm is derived from EIA's 2025 residential average of $15.34 per Mcf, assuming about 1,036 Btu per cubic foot; electricity is EIA's 2025 average of 17.30¢/kWh. The heat-pump COP of 2.5 and blower-door N factor of 20 are assumptions.

CityHDD65Heat savedGas furnace (95%)Heat pump (COP 2.5)Electric resistance
Atlanta2,5311.47 MMBtu$23/yr$30/yr$75/yr
Springfield, MO4,2772.48 MMBtu$39/yr$50/yr$126/yr
Chicago5,9743.47 MMBtu$54/yr$70/yr$176/yr
Minneapolis–St. Paul7,3994.30 MMBtu$67/yr$87/yr$218/yr
Duluth9,1155.29 MMBtu$82/yr$107/yr$268/yr
Our calculation: 140 ft of 2x10 rim plus sill (125.4 sq ft), bare rim (R-3.97) to 2 in. closed-cell at R-6.5 (R-16.97). Conduction only.

Two things jump out. Going from 2 in. to 3 in. of closed-cell adds only about 8% (Chicago: $59 a year instead of $54), and 2 in. of XPS captures about 93% of what 2 in. of closed-cell does ($51). On conduction alone, DIY rigid foam pays back in a few years and a professional spray job takes decades.

What changes the math is what this estimator can't see: air. Three leaky joints meet at the rim, which is why ENERGY STAR bundles basement rim joists into its modeled 15% average heating and cooling savings for air sealing plus insulation. Add no more frost, dry wood and a warmer floor edge, and it's one of the cheapest envelope fixes per dollar, especially DIY or bundled into a crawl-space or basement job. One caveat: this assumes the basement or crawl space is inside the heated envelope. If the floor above is insulated and the basement is cold by design, the rim isn't your boundary.

Avoid these

Common rim joist insulation mistakes

  • Fiberglass as the only layer. It moves the condensation onto the rim.
  • Foaming over wet wood. Meter first; fix leaks and grading first.
  • Lifts that are too thick. Stay within the product's per-pass limit (Froth-Pak: 2 in.).
  • Spraying a cold kit. Warm the tanks to the maker's range before you start.
  • Leaving board edges unsealed. An unsealed board is insulation, not an air barrier.
  • Ignoring termite inspection gaps where your state or pest-control company requires them.
  • Going past the product's exposed limit because "the code says 3¼ in."
  • An N95 instead of an organic-vapor/P100 respirator.
  • Skipping the combustion-safety check when there's a chimney-vented appliance nearby.
  • Burying wiring, junction boxes or vents you'll need to reach later.
Upstairs

Second-floor and between-floor rim joists

Rims between floors count as above-grade wall too; the IECC definition includes "between-floor spandrels." They're just rarely reachable. GreenBuildingAdvisor notes that in a finished house the drywall ceiling acts as the air barrier there, so the practical moments are a remodel with the ceiling open, or re-siding, when foam can go outside. With the ceiling open, treat it like a basement rim: seal the joints, then foam.

ArmorThane applicators

Hire an applicator for rim joist spray foam, or become one

If the calculator says kit, buy the kit. If the job is long, in a crawl space, in a finished basement, or bundled with other work, an ArmorThane-certified applicator can spray ArmorFoam closed-cell to a stated thickness and R-value. Our closed-cell tests at R-6.5 per inch aged, passes as an air barrier at 1 in., and measures 0.83 perm at that thickness.

Because we make both spray foam and polyurea, an applicator with the right rig can seal the rims, spray the crawl-space walls and apply a polyurea liner in one visit. That's how a rim job stops being a minimum charge. More in our closed-cell spray foam guide, the spray foam R-value guide and spray foam wall insulation.

Get a rim joist quote →Spray foam insulation
ArmorThane Mobile Coatings Unit spray trailer with contractors standing beside it
An ArmorThane Mobile Coatings Unit.
R-6.5 / in.

ArmorFoam closed-cell

Aged, ASTM C518. Two inches is R-13; 3¼ in. is about R-21.

1 in.

Air barrier

Passes ASTM E2178 at 1 in. and measures 0.83 perm (ASTM E96).

≈5,000 bf

Per 55-gallon set

A 140-ft rim at 2 in. is about 300 board feet of it.

For contractors

Rim jobs are small. Bundle them.

A rim is a ~300 board-foot job; a 55-gallon ArmorFoam set yields about 5,000. Pair rims with crawl spaces, basements and polyurea work and the rig earns its keep. We supply ArmorFoam, matched spray equipment and training at your location. Not a franchise: no franchise fee, no royalties.

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Straight answers

Rim joist insulation FAQ

What is the best way to insulate rim joists?

Seal and insulate in one continuous layer, with foam against the rim. Closed-cell spray foam does both at once. Rigid foam boards cut to fit each bay work too, if every edge is sealed with canned foam or caulk. Fiberglass alone lets indoor air reach the cold wood, where moisture condenses (Fine Homebuilding; StructureTech).

Can I use fiberglass insulation on rim joists?

Not by itself. Air-permeable batts let warm, moist indoor air reach a rim that sits near outdoor temperature, which causes condensation or frost. In our steady-state check, a rim behind an R-13 batt in Chicago averages about 35°F in midwinter, below the 44.6°F dew point of 70°F air at 40% RH. A batt is fine over a sealed foam layer that's thick enough; the condensation check sizes it.

How thick should spray foam be on rim joists?

Most rims get 1.5–3 in. of closed-cell, about R-9 to R-20 at R-6.2–6.5 per inch (ESR-3228; ArmorThane). Closed-cell is an air barrier at ½–1 in. under ASTM E2178, and DOE uses 1.5 in. as a conservative threshold. IRC R316.5.11 allows up to 3¼ in. exposed, but a product's evaluation report may cap it lower: Froth-Pak's ESR-3228 says 2 in.

Does spray foam on rim joists need to be covered with drywall?

Usually not, if it meets IRC R316.5.11 (2024: R303.5.11): 3¼ in. or less, 0.5–2.0 pcf density, and a flame-spread index of 25 or less with a smoke-developed index of 450 or less. It also has to be within your product's report limit. Thicker foam needs a thermal barrier such as ½-in. gypsum (R316.4), or an ignition barrier in a crawl space entered only for service (R316.5.4).

Do rim joists need a vapor barrier?

Not a separate sheet. One inch of ArmorFoam closed-cell measures 0.83 perm, which makes it a Class II vapor retarder (0.1–1.0 perm under IRC R202). Foil-faced polyiso and XPS also limit vapor flow. Poly sheeting behind or over foam can trap moisture, so let the foam do the vapor control.

What R-value do rim joists need?

The energy code treats rims as part of the above-grade wall ("peripheral edges of floors"). The 2021 IECC wood-frame wall requirement is R-13 in Zones 1–2, R-20 (or 13+5ci) in Zone 3, and R-30 (or 20+5ci, 13+10ci or 0+20ci) in Zones 4–8, per DOE/PNNL. Older editions were lower. Exposed foam at a rim tops out around R-20 to R-21 at 3¼ in., so ask your code official how the table applies to rims and retrofits.

Is closed-cell or open-cell spray foam better for rim joists?

Closed-cell, in most of the country. It's an air barrier at 1 in. or less, a vapor retarder, and about R-6.2 to R-6.5 per inch. Open-cell needs about 5.5 in. to act as an air barrier (DOE), and Building Science Corporation says it may need extra condensation control in Zone 6 and colder. Open-cell is reasonable in Zones 1–4.

Should I use 1-inch or 2-inch rigid foam in rim joists?

Two inches is the common minimum: R-10 in XPS, and more in polyiso depending on the product. In Climate Zone 6 and colder, Fine Homebuilding recommends 3–4 in. If 1-in. boards are easier for you to cut, stack two layers and seal every edge. Adding a batt? Run the condensation check first.

Can I insulate rim joists with canned spray foam like Great Stuff?

Use one-part canned foam to seal gaps and board edges; it isn't a practical way to fill whole bays. ENERGY STAR suggests spray foam for gaps from ¼ in. to about 3 in. and caulk below that. Our closed-cell guide puts canned foam at about R-5.7 per inch and roughly 20 board feet per can, and it isn't equivalent to a two-part kit.

How much does it cost to insulate rim joists?

DIY rigid foam runs about $100–$250 (Family Handyman, 2024). Two-part kits cost about $1.22–$1.95 per board foot; a 140-ft perimeter at 2 in. needs about 300 board feet, which is one 600-bf kit at roughly $739–$919 (2026 prices). HomeGuide reports $2,000–$2,500 for professional rim joist spray foam (Dec 2025). The estimator runs your house.

Will spray foam rot my rim joist?

Not if the wood is dry and water is controlled. Serious decay needs wood moisture above the fiber saturation point (about 30%), and wood at 20% or less has "a reasonable margin of safety" (USDA Forest Products Laboratory). Fix leaks, grading and splash-back first, meter the rim below 20% before foaming, and be cautious with older, untreated sills that have no capillary break.

Do I need a termite inspection gap?

Where termite pressure exists, often yes. DOE's weatherization spec keeps a 3-in. gap between the top of the insulation and any wood. South Carolina requires a 6-in. gap at the foundation top and sill in crawl spaces, plus a 6-in. strip on the sill. IRC R318.4 (2024: R305.4) also restricts foam below grade in "very heavy" termite areas.

Can I spray foam rim joists myself?

Yes, with a two-part kit, if you follow its temperature, thickness and safety instructions. Warm the chemicals (Froth-Pak: 70–120°F, best at 75°F or above), spray in lifts no thicker than the product allows, wear an organic-vapor/P100 respirator, and ventilate at 10 air changes per hour during spraying and for an hour after (ESR-3228). Crawl spaces, long runs and wet or rotted rims are better left to a pro.

AT
ArmorThane Technical Team · Springfield, Missouri
Written from manufacturer data sheets, code text and 35+ years of spraying and supporting applicators. Last updated September 26, 2026.

Methodology and sources

Code text is quoted or summarized from the 2021 IRC and IECC, with 2024 IRC numbering from ICC-ES reports. Climate data: NOAA 1991–2020 normals; climate zones from the 2018 IRC county table; energy prices: EIA 2025 residential averages. Calculations are ours and steady-state: screening tools, not a substitute for your code official or product literature.

  1. IRC R316.5.11, sill plates and headers (2021 text, UpCodes): up.codes/s/sill-plates-and-headers
  2. IRC R318.4, foam plastic protection from termites (UpCodes): up.codes/s/foam-plastic-protection
  3. IRC R317.1, wood required to be protected from decay (UpCodes): up.codes/s/location-required
  4. 2024 IRC R304, protection of wood against decay (UpCodes): up.codes/s/protection-of-wood-and-wood-based-products-against-decay
  5. IRC R202, sill plate definition (UpCodes): up.codes/s/sill-plate
  6. IRC R202, vapor retarder class (UpCodes): up.codes/s/vapor-retarder-class
  7. IECC, above-grade wall definition (UpCodes): up.codes/s/above-grade-wall
  8. IECC climate zone criteria (UpCodes): up.codes/s/international-climate-zones
  9. 2018 IRC climate zones by county (UpCodes): up.codes/s/climate-zones
  10. ICC-ES ESR-3228, Froth-Pak spray foam (Jan. 2026, PDF): icc-es.org/wp-content/uploads/report-directory/ESR-3228.pdf
  11. DOE/PNNL Building America Solution Center, 2009–2021 IECC insulation requirements: basc.pnnl.gov/information/2009-2021-iecc-and-irc-minimum-insulation-re
  12. 2021 IECC insulation and air barrier checklist (Bernards Township, NJ): bernards.org/government/forms/construction/forms/1156-insulation-check
  13. U.S. DOE WAP job aid 12-1, air seal sill plate and rim joist (June 2024, PDF): energy.gov/sites/default/files/2024-07/12-1_air-seal-sill-plate-and-ri
  14. U.S. DOE WAP job aid 12-2, insulate rim joist (June 2024, PDF): energy.gov/sites/default/files/2024-07/12-2_insulate-rim-joist.pdf
  15. U.S. DOE Building Science Education, insulating and sealing rim joists: bsesc.energy.gov/energy-basics/insulating-and-sealing-rim-joists
  16. Building Science Corporation Info Sheet 408 for DOE Building America (PDF): www1.eere.energy.gov/buildings/publications/pdfs/building_america/crit
  17. Penn State PHRC, durability evaluation of insulated rim joists (2016, PDF): phrc.psu.edu/assets/docs/Publications/Durability-Evaluation-of-Insulat
  18. Penn State PHRC, IRC spray foam thermal and ignition barriers (PDF): phrc.psu.edu/assets/docs/Webinars/2009IRCSprayFoamThermalBarriersIgnit
  19. ENERGY STAR, seal and insulate savings methodology: energystar.gov/saveathome/seal_insulate/methodology
  20. ENERGY STAR, A Do-It-Yourself Guide to Sealing and Insulating (PDF): energystar.gov/ia/partners/publications/pubdocs/DIY_Guide_May_2008.pdf
  21. USDA Forest Products Laboratory, Wood Handbook ch. 14 (2021, PDF): fpl.fs.usda.gov/documnts/fplgtr/fplgtr282/chapter_14_fpl_gtr282.pdf
  22. South Carolina 2024 IRC modifications (PDF): llr.sc.gov/bcc/Forms/2024%20Combined%20Continued%20Modifications%20wit
  23. Minnesota Rule 1309.0314 (2014 version): revisor.mn.gov/rules/1309.0314/version/2014-01-18%2000:47:46+00:00
  24. NOAA NCEI 1991–2020 climate normals data service: ncei.noaa.gov/access/services/data/v1
  25. U.S. EIA, natural gas prices: eia.gov/dnav/ng/ng_pri_sum_dcu_nus_a.htm
  26. U.S. EIA, Electric Power Monthly table 5.3: eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_3
  27. DuPont Froth-Pak, how to air seal rim joists: frothpak.dupont.com/how-to/air-seal-rim-joists.html
  28. DAP, how to insulate your rim joists: dap.ca/resources-support/how-to-s-tips/insulating-home-sealing/how-to-
  29. DuPont Thermax Heavy Duty: dupont.com/products/thermax-heavy-duty.html
  30. Colorado Energy, R-value table: coloradoenergy.org/procorner/stuff/r-values.htm
  31. Fine Homebuilding, insulating rim joists: finehomebuilding.com/project-guides/insulation/insulating-rim-joists-2
  32. GreenBuildingAdvisor, insulating rim joists: greenbuildingadvisor.com/article/insulating-rim-joists
  33. StructureTech, rim joist insulation: structuretech.com/rim-joist-insulation/
  34. Family Handyman, insulate basement rim joists (2024): familyhandyman.com/project/insulate-basement-rim-joists/
  35. Crawl Space Ninja, pre-cut rim joist insulation: supply.crawlspaceninja.com/products/rim-joist-insulation
  36. HomeGuide, spray foam insulation cost (Dec. 2025): homeguide.com/costs/spray-foam-insulation-cost
  37. ArmorThane, closed-cell spray foam guide: armorthane.com/closed-cell-spray-foam/
  38. ArmorThane, ArmorFoam 0.5 open-cell spray foam: armorthane.com/products/armorfoam-products/open-cell-spray-foam/

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