A 2×4 wall stuffed with R-13 batts does not perform anywhere near R-13 — once you account for the wood framing slicing through it every 16 inches, the whole-wall number lands closer to R-10. Continuous insulation (ci) fixes that by wrapping the outside of the structure in an unbroken layer of rigid foam or mineral wool over the sheathing, covering studs, plates, headers, and rim joists alike. It is now baked into the energy code for most of the northern U.S., and it is the single biggest thermal upgrade available when a house gets new siding. This article stays on the exterior-ci lane: the thermal bridging math, the code paths, and the attachment and detailing that make or break the assembly. (Cavity insulation itself — batts, dense-pack, spray foam — is covered in separate guides.)
Thermal Bridging: Why R-13 Cavity ≠ R-13 Wall
Wood is a mediocre insulator at about R-1.25 per inch. In a standard 16-inch on-center wall, studs, plates, headers, and window framing add up to a “framing factor” of roughly 23-25% of the wall area. Every one of those wood paths is a thermal short circuit around your batts. Run the parallel-path math on a 2×4 R-13 wall and the effective whole-wall value comes out around R-10; a 2×6 R-20 wall lands near R-15. Infrared cameras make it obvious — the stud grid glows through the drywall on a cold night, and those cold stripes are also where condensation and ghosting stains show up first.
Continuous insulation attacks the bridge itself. Add just R-5 of exterior foam to that 2×4 wall and the whole-wall value jumps from ~R-10 to ~R-15 — the ci layer is uninterrupted, so every inch of it counts at full value. This is why codes write ci as its own number instead of letting you swap it for more cavity fill: R-5 continuous does more work than R-5 added to the cavities.
What the Code Asks For
The IECC prescriptive wall table (2021 edition, R402.1.3) sets combined requirements by climate zone, written as “cavity + ci”:
- Zones 4-5 (mid-Atlantic through southern New England, lower Midwest): 2×6 walls at R-20 + R-5ci, or 2×4 at R-13 + R-10ci, or R-0 + R-15ci all-exterior.
- Zones 6-8 (northern tier): R-20 + R-5ci or R-13 + R-10ci in zone 6; colder zones push toward R-20 + R-5ci minimum with many jurisdictions requiring more.
- Zones 1-3 (South): cavity-only paths like R-13/R-20 still satisfy prescriptive code, so ci is optional — though it still pays in comfort and in hot-humid cooling loads.
You can also comply via performance modeling (REScheck) or ERI paths, which is how some builders in ci zones still avoid exterior foam. For remodels, most jurisdictions trigger the requirement only when you expose the sheathing — which is exactly why re-siding is the moment to act.
Material Choices for the CI Layer
- Polyiso — R-5.6-6.5/inch, foil-faced versions double as a WRB when taped. Cheapest R per inch; derates somewhat in cold temperatures (design at R-5/inch in zones 6+). Roughly $0.70-1.00/sq ft per inch.
- EPS (expanded polystyrene) — R-3.8-4.2/inch, most vapor-open of the foams, cheapest per board, stable R-value. About $0.40-0.60/sq ft per inch. Graphite-infused GPS (Neopor) hits R-4.7/inch.
- XPS — R-5/inch, familiar pink/blue board. Higher embodied carbon (improved with newer HFO blowing agents) and drifts toward R-4.5/inch long-term. $0.70-0.90/sq ft per inch.
- Mineral wool board (Rockwool Comfortboard 80) — R-4.2/inch, noncombustible, vapor-open, drains and dries. The premium pick at $1.30-1.80/sq ft per inch; the go-to where fire codes or vapor-open assemblies matter.
One critical design rule for cold climates: with foil-faced polyiso or thick XPS, the ci layer is a vapor retarder on the cold side of the sheathing, so it must be thick enough to keep the sheathing above the dew point — in zone 5 that means at least roughly R-7.5 ci over a 2×6 R-20 wall (IRC R702.7 ratios). Too thin is worse than none. Vapor-open EPS or mineral wool sidesteps that trap by letting the wall dry outward.
Attachment: Long Screws and Furring
Up to about 1 inch of foam, siding can often be nailed through the foam into studs with extended fasteners per the siding manufacturer’s tables. Beyond that, the standard detail is vertical furring strips — 1x4s or plywood strips — run over the foam at each stud and screwed through everything into the framing with structural screws (GRK RSS, Headlok, or FastenMaster), typically #10-#14 screws penetrating studs 1.25-1.5 inches, spaced 12-24 inches vertically depending on siding weight. The furring does three jobs: it clamps the foam, gives the siding solid attachment, and creates a 3/4-inch rainscreen gap that dramatically improves drying and siding paint life. Foam up to 4 inches hangs fine this way — the screws work in bending and the foam itself braces them; engineering tables from FastenMaster and the Foam Sheathing Committee cover the spacing. Compressible mineral wool board needs care torquing screws straight and a fender-washer touch, but crews get the rhythm within a wall.
WRB, Windows, and the Details That Matter
Decide where your water-resistive barrier lives before the first board goes up. Two clean options: housewrap over the sheathing with foam outside it (WRB stays in the familiar place; tape foam seams as a bonus layer), or taped foil-faced foam as the WRB itself (fewer layers, but every seam, penetration, and screw pattern now matters). Either way, flash window openings to whichever layer is the WRB, and slope-flash the bottom of the foam to daylight.
Windows are the fussy part. With 1.5+ inches of ci, windows either sit deep as “innie” units flashed back to the WRB, or move out to the foam plane as “outies” on plywood buck extensions screwed to the framing. Outies keep exterior trim proportions normal; innies give deep, European-style interior sills. Neither is wrong — pick one and detail every opening the same way. Budget extra for jamb extensions, deeper trim, thicker mounting blocks for hose bibs and lights, and electrical box extensions. These accessories, not the foam, are where retrofit budgets slip.
Siding Compatibility
Vinyl, fiber cement, LP SmartSide, and wood lap siding all install happily over furred ci — manufacturers publish fastening tables for foam-backed assemblies (James Hardie allows direct attachment over up to 1-inch foam, furring beyond). Heavy claddings are the exception: brick veneer needs its ledge and ties engineered around the foam thickness, and three-coat stucco over thick foam requires a drainage mat and lath engineering. Vinyl over unfurred thick foam can telegraph waviness, so keep foam at 1 inch or less for direct-nailed vinyl or add furring.
Cost and the Retrofit Moment
Installed as part of a re-side, expect the ci layer to add roughly $1.50-3.50 per sq ft of wall — about $2,500-6,000 on a typical 1,800 sq ft-wall two-story — on top of the siding job itself. Material is the smaller half; labor for furring, screws, and window extensions is the bigger. As a standalone project it rarely pencils, but at re-siding time the scaffolding, tear-off, and trim labor are already paid for, and the increment often returns 10-20% heating/cooling savings plus a warmer-feeling house (interior surfaces run several degrees closer to room temperature, which reads as comfort no thermostat measures). It also currently qualifies as insulation material for the 25C federal tax credit. If your siding has 5 years left in it, wait and do it right; if the siding is coming off next summer, put continuous insulation in the spec now — you get one shot at that wall every 30-40 years.
One last hiring note: ci is still unfamiliar to plenty of siding crews, and the failure mode is a contractor who agrees to “throw some foam on” without furring math, window bucks, or a WRB plan. Ask bidders directly how they handle screw engagement, rainscreen gaps, and flashing at the ci plane — a crew that answers in specifics has done it; a crew that shrugs will improvise on your wall. Two or three extra bid calls is cheap insurance on an assembly that has to stay dry for four decades.
