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ARMORTHANE PROTECTING YOUR WORLD

PROTECTING YOUR WORLD

ARMORTHANE PROTECTING YOUR WORLD

Injection Foam
Insulation:
Honest 2026 Guide

What injection foam costs, what R-value you really get, and when dense-pack cellulose is the smarter fill for closed walls. We make spray foam, and we’ll tell you where it isn’t the answer.
Spray Foam Quotes

Opening walls, or insulating the rest of the house? Talk to a certified applicator.

ArmorThane-certified applicators spray ArmorFoam in open walls, attics, rim joists and crawl spaces. Tell us about your house and we'll connect you with one who puts thickness and R-value in writing.

  • Remodels and additions with open studs
  • Attics, rim joists and crawl spaces
  • Thickness, R-value and board feet on the quote
  • Contractors: ask about ArmorFoam rigs and training
Rather talk now? Call (417) 831-5090
Step 1 of 3
$1.50–$4.50
Injection foam, per sq ft of wall
Installed, Angi (July 2026)
≈R-4.5
Per inch, tripolymer-type foam
Handbook value R-4.48
R-13
Dense-pack cellulose in a 2x4
Greenfiber sidewall chart
≈10x
Less leakage through a dense-packed cavity
Building Science Corp. tests

Plenty of older houses have walls that are nothing but studs, air and siding. You can fix that without tearing off a sheet of drywall: drill a hole into each stud cavity, fill it, plug it. The real questions are what goes in the hole, and whether it's worth the money.

Quick answer

Injection foam is foam pumped into closed wall cavities through drilled holes, so the drywall stays put. Expect about $1.50–$4.50 per square foot of wall installed (Angi, 2026) and roughly R-4.5 per inch, about R-15 in a 2x4 wall. Dense-pack cellulose does nearly the same job for less: R-13 in a 2x4 at about $2.00–$2.30 per square foot. Either way, rule out knob-and-tube wiring, active leaks and unblocked balloon framing first. Jump to the cost estimator or the wall checker.

Injection foam is one good answer. Dense-pack cellulose is another, and it's the one building scientists usually reach for first. This guide puts them side by side with prices, R-values and payback math, and covers the things that should stop any wall job before the first hole: old wiring, wet walls, lead paint and framing that lets the fill run away.

Who's writing: we make ArmorFoam spray foam in Springfield, Missouri, and have been in this business for 35+ years. Our foam is sprayed onto open framing. It isn't an injection product, and we'll explain why nobody should pump regular spray foam into a closed wall. That leaves us free to tell you where injection foam wins, where cellulose wins, and where spray foam belongs in an older house.

What's in this guide
  1. 1What injection foam is (and isn't)
  2. 2Insulating existing walls without removing drywall
  3. 3What it costs (estimator)
  4. 4Is your house ready? (wall checker)
  5. 5Moisture and mold
  6. 6How it's installed
  7. 7Is it worth it? (payback)
  8. 8Is your quote fair? (checker)
  9. 9Where spray foam fits
  10. 10Injection foam insulation FAQ
The basics

What is injection foam insulation?

Injection foam is a foam mixed on site and pumped through a hose into closed wall cavities. The crew drills one hole per cavity, from outside after a course of siding comes off or from inside through the drywall, fills the bay around the wires and pipes, and plugs the hole. The foam cures in place. That's the whole idea: insulation for walls you don't want to open.

The three kinds of foam sold as "injection foam"

The name covers different chemistry. Ask which one you're being quoted, because R-value, pressure on the wall and price all follow from it.

TypeExampleHow it worksR per inchPressure on the wall
Tripolymer (resin) foamRetroFoam and similarResin, a foaming agent and water are mixed in the rig; it goes in with the consistency of shaving cream≈4.5 (urea terpolymer, R-4.48 handbook); published claims about R-4 to R-4.6Low. RetroFoam says it doesn't expand
Slow-rise polyurethaneHandiFoam Slow RiseTwo-part polyurethane that flows before it rises and cures5.9 initial (ASTM C518)Real. Its data sheet warns drywall can crack or buckle if overfilled
Cementitious foamClimaVaultMineral (magnesium oxide) foam mixed on site and pumped3.5+Low; maker reports zero shrinkage (ASTM C951)
Sources: Colorado Energy R-value table (handbook values), Angi (2026), Ecotelligent Homes, RetroFoam, ICP technical data sheet TD014 (2025) and ClimaVault. Angi also prices "open-cell" and "closed-cell" injection foam; those labels come from spray foam, so ask for the product name and data sheet.

Watch the fine print on R-value. One injection product is advertised at R-4.92 per inch "at 25°F mean temperature." The FTC's R-value rule tests at a 75°F mean, and a number measured cold isn't comparable to the label on a bag of cellulose. Ask for the 75°F figure.

Injection foam vs. spray foam: why spray foam can't go in a closed wall

Spray foam goes onto open framing in thin passes you can see. Our closed-cell ArmorFoam is sprayed in lifts of about 1.5–2 in. because the reaction gives off heat; a lift much thicker than 2 in. can scorch the core, and in bad cases smolder. More on that in our closed-cell spray foam guide.

A closed 2x4 cavity is 3½ in. deep and 8 ft tall. Fill it in one shot with a fast-rise foam and you trap that heat, push against drywall that can't move, and can't see the voids. That's why injection products are different chemistry: they barely expand, or they flow before they rise. If someone offers to "spray foam" your closed walls, ask for the exact product and its data sheet.

Applicator in a white protective suit and respirator spraying foam into an open wood stud wall
Spray foam belongs on open framing, where the applicator can see every pass.
Compare your options

How to insulate existing walls without removing drywall

Drill and fill. From outside, a crew pulls one course of siding, drills a hole into each stud cavity and fills it with dense-pack cellulose, dense-pack fiberglass or injection foam. From inside, the holes go through the drywall and get patched. If the siding or the drywall is coming off anyway, you have better choices, so check that first.

MethodR in a 2x4 (3½ in.)Installed cost, publishedAir leakageMain risksBest for
Dense-pack celluloseR-13 (R-3.7/in.)$2.00–$2.30/sq ft (Angi, Sep 2026)Cavity leakage cut about 10x at ≈3.5 lb/ft³Low density leaves voids; K&T wiring; wet wallsMost empty walls; old plaster
Dense-pack fiberglass≈R-13 to R-15 (3.7–4.3/in.)Inside HomeGuide's $1.10–$3.70 blown-in bandSimilar with some productsDensity and product chart matter; same wiring and water limitsWhere cellulose isn't wanted
Injection foam (tripolymer)≈R-15.7$1.50–$4.50/sq ft (Angi, Jul 2026)Fills the bayCost; cracked drywall or plaster; missed fire stops; not with K&TOwners who want foam and accept the price
Slow-rise polyurethaneR-20.7 initialKits alone ≈$5.76/sq ft of wall (our calc)Air-sealing foamFoaming pressure; professional use only; cavity must be emptyPros filling small, empty cavities
Cementitious foam≈R-12No published price foundFills the baySpecialty installers; cost unknownFire-focused retrofits
Fill + exterior foam at re-sidingCavity fill plus R-5 to R-10 outsidePart of the siding jobContinuous outer layerWindow and trim detailsAny house getting new siding
Open the wall + spray foamClosed-cell R-6.5/in.; open-cell R-3.7/in.See our spray foam cost guideAir sealsDrywall demo and replacementGut remodels only
R-values: Greenfiber and CIMA (cellulose), Colorado Energy handbook table (blown fiberglass in walls, urea terpolymer foam), ICP TD014 (slow-rise, initial value), ClimaVault, ENERGY STAR (exterior sheathing), ArmorThane (spray foam). Leakage: Lstiburek, Building Science Corp. BSI-043 (2010).

Dense-pack cellulose: the default for good reason

Building Science Corporation's Joe Lstiburek calls blowing cellulose into empty frame walls one of the most effective ways to deal with them. His numbers: empty walls leaked 0.4–4 cfm per sq ft at 75 Pa, and dense-packing to about 3.5 lb/ft³ cut that to 0.04–0.2. It isn't an air barrier on paper, he notes, but in a retrofit it gets close enough that you can usually live without one.

CIMA, the cellulose trade group, rates dense-pack at R-3.7 per inch and says properly installed cellulose won't settle in sidewalls. Greenfiber's chart gives R-13 in a 2x4 and R-21 in a 2x6 at 3.5 lb/ft³ minimum. The National Park Service, which has every reason to protect old houses, says dense-packed cellulose or fiberglass does the least damage to historic materials, and recommends a borate-only grade in historic buildings.

Dense-pack fiberglass

Same method, different fiber. BSC saw similar leakage cuts with some blown fiberglass products. Handbook values for blown fiberglass in walls run R-3.7 to R-4.3 per inch, but the real number comes from the product's dense-pack chart. Attic loose-fill fiberglass is much fluffier and rates only R-2.2 to R-2.9 per inch (HomeGuide), so don't accept attic numbers on a wall quote.

Injection foam: where it wins and where it doesn't

It wins on R per inch (about R-4.5 vs. R-3.7 for cellulose), no dust, and it flows around obstructions. RetroFoam says old fiberglass batts can stay in the wall and get compressed. It loses on price, on a small but real chance of bowed or cracked drywall or plaster (RetroFoam's own FAQ says it can happen), and it's off the table with knob-and-tube wiring. GreenBuildingAdvisor's Martin Holladay called injection foam tricky in 2016 and pointed owners to dense-pack instead. Fair, as a default. Injection foam is still a reasonable pick when a local installer you trust backs it with a data sheet and a damage policy.

One more thing to ask about: shrinkage. An energy-audit firm, Ecotelligent Homes, warns that injection foam mixed incorrectly can shrink and leave cold gaps. We couldn't find independent shrinkage figures for current resin foams, so ask for the product's dimensional-stability test and how the crew checks its fill.

When the siding is coming off anyway

Then do both. ENERGY STAR's advice for an uninsulated wood-frame wall getting new siding: drill the sheathing, blow the empty cavities, and add R-5 insulating sheathing (zone 3) or R-5 to R-10 (zones 4–8) under the new siding. Holladay called re-siding a "once-every-50-years opportunity." It's also the moment to add sheathing and a water-resistive barrier if the house never had them.

When the wall is coming open anyway

Then it isn't an injection job. With the studs exposed, closed-cell spray foam gives R-6.5 per inch aged, and 2 in. of it plus a batt is the value play in a 2x6. Open-cell fills a 2x4 at about R-13 for less. Our spray foam R-value guide has thickness tables, and spray foam wall insulation covers ArmorFoam in walls. Tearing out good drywall just to spray foam rarely pays.

Interactive tool 1 of 4

How much does injection foam insulation cost?

About $1.50–$4.50 per square foot of wall installed, or $4,300–$8,500 for a typical project (average $6,200), per Angi's July 2026 data. Dense-pack cellulose runs about $2.00–$2.30 per square foot (Angi, September 2026). A 30x40 one-story with 12 windows has about 900 sq ft of net wall. Run your house below.

Wall retrofit estimator

Wall area, holes, fill volume, R-value and cost

Enter the house footprint and the walls you're filling. The estimator nets out windows and doors, counts cavity segments and holes, and prices the fill at published 2025–2026 rates.

Measure at an outlet or a test hole
Price level
900 sq ft
Net wall to fill
≈131
Holes to drill
R-15.7
Cavity R-value
$2,700
Installed cost, typical

Wall area: 2 x (40 + 30) x 8 ft x 1 story = 1,120 sq ft, minus 220 sq ft of openings (15 per window, 20 per door) = 900 sq ft. Fill volume: 238 ft³, assuming 1½ in. studs. Estimates only; prices are published ranges, not quotes.

Price range

Injection foam (tripolymer resin)

  • Published band for injection foam: $1.50–$4.50 per sq ft of wall (Angi, Jul 2026; $3.00 is the midpoint).
  • For your 900 sq ft: $1,400–$4,100.
Material

R-15.7 in the cavity

  • Tripolymer-type foam: about R-4.5 per inch in handbook tables (urea terpolymer, R-4.48). Published claims run from about R-4 (Angi) to R-4.6.
  • Ask for the product’s data sheet with R-value at a 75°F mean, the temperature the FTC R-value rule requires for labels.
Checks
  • Holes: about 105 stud bays x 1 segment, plus the short cavities above and below 12 windows and over 2 doors. Our estimate; blocking you don’t know about adds more.
  • Access: a course of siding comes off around the house, one hole per cavity, then the siding goes back on.

How we calculated this: net wall = 2 x (length + width) x height x stories, minus 15 sq ft per window and 20 per door. Holes = stud bays x (blocking rows + 1), times 3 for brick, plus 2 per window and 1 per door for the short cavities above and below openings. Fill volume uses 1½ in. studs. Prices: Angi (injection foam, July 2026; dense-pack and removal, September 2026) and HomeGuide (blown-in walls, November 2025); kit price from a retailer listing, September 2026. Estimates, not quotes.

1 2 3 4 5 6 OUTSIDE VIEW, ONE COURSE OF SIDING OFF
  1. Siding course removed. The holes run in a band around the house (RetroFoam, Greenfiber).
  2. One hole per cavity. The hose or fill tube reaches up and down from it.
  3. Fire blocking splits the cavity, so the part below needs its own hole.
  4. Windows and doors: the short cavities above headers and below sills each get a hole.
  5. Balloon framing: no top plate at the floor line, so the cavity runs on unless it's blocked (NPS).
  6. Outlets and wiring: fill flows around modern cable. Knob-and-tube is a hard stop.

Published prices, with dates

ItemPublished figureDateSource
Injection foam, installed$1.50–$4.50 per sq ftJul 2026Angi
Injection foam, typical project$4,300–$8,500 (average $6,200)Jul 2026Angi
Injection foam, 1,000 sq ft$2,750–$7,500Jul 2026Angi
"Proprietary" retrofit foams$2.25–$4.00 per sq ftJul 2026Angi
Dense-pack cellulose$2.00–$2.30 per sq ftSep 2026Angi
Replacing insulation in an older house$1.75–$3.50 per sq ftSep 2026Angi
Blown-in wall insulation$1.10–$3.70 per sq ft; $1,900–$7,800 for all exterior wallsNov 2025HomeGuide
Removing old insulation$1.50–$3.00 per sq ftSep 2026Angi
Wall repairs and repainting$200–$2,000Jul 2026Angi
Slow-rise PU kit, 41 ft³$909 (≈$5.76 per sq ft of 2x4 wall at full yield, our calc)Sep 2026Retailer listing; ICP TD014
Angi's per-foot range is for injection foam specifically. None of these include fixing water damage, rewiring knob-and-tube or lead-safe work practices.

What moves the price

  • Stories and access. RetroFoam lists the number of stories, the siding type and prep work as the big variables. Ladders and lifts cost time.
  • Siding type. Vinyl and aluminum come off and go back. Brick means holes in the mortar joints at the top, middle and bottom of each cavity, then re-mortaring. Wood siding that won't come off gets drilled and plugged.
  • Inside vs. outside. Inside work means covering floors and furniture, plugging holes and mudding them. Paint is usually on you.
  • What's in the wall. Old cellulose gets vacuumed out before injection foam, a separate line item.
  • Blocking. Every fire block or brace adds a hole to that cavity.
  • Where you live. Dealers and contractors set prices locally.

DIY cavity-fill kits: cheaper? Not really

Two-part slow-rise kits are sold online. A 41 ft³ kit at $909 fills about 158 sq ft of 2x4 wall on paper, or about $5.76 per square foot in chemical alone, and the maker notes that foam packed into a cavity runs denser, so real yield is lower. That's above Angi's entire installed range for injection foam. The same data sheet says professional use only, wants cavities with no existing insulation, and warns that drywall can crack or buckle if you overfill or plug the relief holes. For DIY, the better targets are open areas like rim joists. Our DIY spray foam guide covers kits.

Interactive tool 2 of 4

Is your house ready for injection foam?

Probably, if the wiring is modern, the walls are dry, the framing is blocked at the floors and there's sheathing behind the siding. Knob-and-tube wiring and active leaks are hard stops. Pre-1978 paint, plaster, balloon framing and old cellulose in the cavities are all workable, but they change who you hire and how they drill.

Wall checker

Can my walls take injection foam, and what should go in first?

Answer what you know. "Not sure" is fine; the checker tells you what to look for.

Workable, after a few fixesBest value: dense-pack
  • Lead paint: anyone you pay to drill through painted siding, trim or plaster must be lead-safe certified under EPA’s RRP rule. Ask for the certificate.
  • Asbestos: EPA lists textured paint and patching compounds on walls, siding shingles and vermiculite wall insulation as possible sources. Test suspect material before drilling.
Possible, with conditions

Injection foam

  • Framing unknown: if the studs run past the floor framing (balloon framing), ask the installer how they’ll block the cavities.
  • Plaster: the NPS warns foam pressure in a wall can break plaster keys or crack plaster. Work from outside and ask how they relieve pressure.
Good fit

Dense-pack cellulose or fiberglass

  • Framing unknown: if the studs run past the floor framing (balloon framing), ask the installer how they’ll block the cavities.
  • Plaster: the NPS says dense-packed cellulose or fiberglass does the least damage to old materials, with access from outside where the plaster is worth keeping.
Situational

Exterior insulation at re-siding

  • Only when the siding is coming off. Then it’s the best upgrade you can make to a wall.
Situational

Open the wall and spray foam

  • Only if the drywall or plaster is coming off anyway. Tearing out good walls to spray foam rarely pays.

A screening checklist from the NPS, EPA, ENERGY STAR and Washington’s electrical rules. Your inspector, electrician and local code govern.

Knob-and-tube wiring: stop here first

Knob-and-tube was built to shed heat into open air. The National Electrical Code (Article 394) restricts concealed knob-and-tube in cavities filled with loose, rolled or foamed-in-place insulation that surrounds the wires. A few states allow exceptions. Washington is one, and even there, loose or rolled insulation is allowed only after a licensed electrical contractor surveys and certifies the wiring. Its rule then says flatly that foam may not be used with knob-and-tube (WAC 296-46B-394). Look for porcelain knobs and tubes in the attic and basement before anyone drills.

Lead paint and asbestos

EPA estimates about three-quarters of U.S. homes built before 1978 still have some lead paint. Drilling through painted siding, trim or plaster disturbs it, and EPA's RRP rule requires anyone paid for that work in a pre-1978 home to be lead-safe certified. Ask to see the certificate. EPA also lists textured paint and patching compounds on walls, siding shingles and vermiculite wall insulation as places asbestos may turn up. If you're not sure, test before drilling.

Balloon framing, missing sheathing and plaster

  • Balloon framing. Studs run from the sill toward the roof, and the NPS notes the cavity is continuous between floors except where fire stops were added. Unblocked, the fill runs into floor framing, the basement or the attic.
  • Missed fire stops. The opposite problem: RetroFoam lists a missed fire stop as a way a cavity ends up only partly filled. Probe every cavity.
  • No sheathing. Some old houses have siding nailed straight to the studs. The NPS calls filling those walls more problematic, since wind-driven rain wets the insulation against the back of the siding.
  • Plaster. The NPS warns that the pressure of foam in a wall can break plaster keys or crack plaster, and suggests working from outside where the plaster is worth keeping.

What's already in the wall

Old fiberglass batts: RetroFoam says injection foam compresses them in place. Old loose cellulose: crews vacuum it out first, and cellulose that has hardened or been glued in can block injection foam entirely. Don't know what's in there? A small test hole somewhere hidden answers it in two minutes.

Moisture

Will injection foam cause moisture or mold in my walls?

Not by itself, but any cavity fill changes how a wall dries. A filled wall keeps heat inside, so the sheathing and siding run colder in winter and stay wet longer after rain, as the NPS puts it. A dry, sheathed wall with sound siding handles that. A wall that already leaks doesn't.

So the rule is simple: fix water first. The NPS calls dealing with existing moisture problems essential before insulating, and admits there are no reliable prescriptive measures that prevent moisture problems in old buildings. Walk the house after a hard rain. Check gutters, window flashing, trim caulk, and paint that peels in the same spots every year.

  • Skip the plastic. Vapor retarders have to be continuous to work, which is hard to do in an existing house, and the NPS generally doesn't recommend adding them there. Air sealing from the attic side and at outlets does more good.
  • Hot, humid climates. The NPS ties summer moisture trouble mainly to heavy air conditioning and interior finishes that block vapor, like vinyl wallcovering or thick paint buildup.
  • Mold claims. RetroFoam says its foam isn't a food source for mold. Fine, but the thing that grows mold in a wall is water getting in, whatever fills the cavity.
Solid masonry is a different animalThis page is about wood-frame walls. For solid brick or stone walls insulated from inside, the NPS says the long-term effects of foam haven't been adequately documented, and foam should be avoided where the masonry is poor or rising damp isn't controlled. Get a specialist.
Step by step

How injection foam is installed, step by step

Most crews work from outside: pull a course of siding, drill one hole per cavity, fill, plug and put the siding back. A typical house takes one to two days, according to Angi. Inside installs go through the drywall instead and end with plugged holes and a rough coat of mud.

  1. Inspect before drilling

    Wiring type, water stains, paint age, what's in the cavities and how the house is framed. The checker above lists what to look for.

  2. Mark what's in the wall

    Greenfiber's drill-and-fill guidance says to locate wiring, outlets, plumbing and gas lines and mark them on the outside so nobody drills into them.

  3. Open a path

    Siding course off, or holes through brick mortar at the top, middle and bottom, or through the drywall inside (RetroFoam). Greenfiber's dense-pack instructions call for 2½ in. holes centered in the stud bays.

  4. Probe each cavity

    A rod or the hose finds blocking and obstructions. Every blocked cavity needs a second hole.

  5. Fill it

    RetroFoam's crews run the hose to the top of the cavity, fill down to the middle, then fill from the middle up. Dense-pack installers listen for the blower's pitch to rise as the cavity packs tight.

  6. Plug, patch, replace

    Plugs go in, the siding goes back or the mortar gets repointed. Inside, holes are plugged and mudded, and you paint.

From outside vs. from inside

AccessWhere the holes goMess insideFinish workBest when
Outside, vinyl or aluminumUnder a removed course, one per cavityLittleSiding goes back onMost sided houses
Outside, brick veneerMortar joints, top, middle and bottomNoneRe-mortar and match colorBrick homes
Outside, wood sidingBehind a pulled board, or through the sidingNonePlugs and paint if drilledBoards that come off cleanly (NPS prefers this on historic houses)
Inside, drywall or plasterThrough the wall finish, one per cavityDust, moved furniturePlugs, mud, paintStucco, historic exteriors, one room at a time
RetroFoam install descriptions; NPS Preservation Brief 3.
Interactive tool 3 of 4

Is injection foam worth it? Savings and payback

Filling empty walls pays almost anywhere with a real winter. In Chicago, 1,000 sq ft of empty 2x4 wall filled with injection foam saves about $381 a year on gas heat, by our math. The catch: dense-pack cellulose saves about $365. The foam premium buys you roughly $16 a year.

Savings & payback estimator

What will filling my walls save, and which fill pays back fastest?

Heating math through the wall, framing included, using NOAA heating degree days and 2025 EIA energy prices.

Used when city = custom
From tool 1
25% is our assumption for 16 in. o.c.
Project cost
R-4.0
Whole wall today
R-12.9
Whole wall filled
$343
Saved per year
8 yr
Simple payback
Injection foam (R-15.7)
$343/yr
Dense-pack fiberglass (R-14.0)
$335/yr
Dense-pack cellulose (R-13.0)
$329/yr
Cementitious foam (R-12.3)
$324/yr
Slow-rise PU kit (R-20.7)
$361/yr

Chicago, IL: 5,974 heating degree days. Whole wall goes from R-4.0 to R-12.9 (25% framing), saving 22.0 MMBtu a year. Injection foam beats dense-pack cellulose here by about $15 a year, for roughly $770 more up front at typical prices. Cost uses the typical published price; tick the box to enter your quote. Heating conduction only: cooling and air-leakage savings aren’t counted.

How we calculated this: a parallel-path wall. Cavity path = 0.68 inside air film + 0.45 drywall + fill + sheathing + siding + 0.17 outside film; stud path uses R-1.25 per inch of softwood instead of the fill (Colorado Energy handbook table). Heat saved = (U before − U 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 at about 1,036 Btu per cubic foot; electricity is EIA's 2025 average of 17.30¢/kWh. The 25% framing share, the heat-pump COP and the ¾ in. board value are our assumptions. Plaster is treated like drywall.

CityHDD65Injection foamDense-pack celluloseDense-pack fiberglass
Atlanta2,531$162/yr$155/yr$158/yr
Springfield, MO4,277$273/yr$261/yr$266/yr
Chicago5,974$381/yr$365/yr$372/yr
Minneapolis–St. Paul7,399$472/yr$452/yr$461/yr
Duluth9,115$582/yr$557/yr$568/yr
Our calculation: 1,000 sq ft of net wall, 2x4 at 16 in. o.c. (25% framing), ½ in. drywall, ½ in. plywood, vinyl siding, empty cavity to filled, gas at $1.48/therm in a 95% furnace, NOAA 1991–2020 normals. Whole-wall R goes from 4.0 to 12.9 (foam), 11.8 (cellulose) or 12.3 (fiberglass). Heating conduction only.

Run the payback. At Angi's midpoint of $3.00 per square foot, injection foam on 1,000 sq ft costs about $3,000 and pays back in about 8 years in Chicago. Dense-pack at about $2.15 costs $2,150 and pays back in about 6. The extra $850 for foam, though, earns about $16 a year. That's the honest math behind the default recommendation.

What the tool can't see: air leakage (both fills cut cavity leakage sharply, which conduction math ignores), cooling, and comfort, like a wall that no longer feels cold to the touch. What it won't back up: brand-site claims of cutting bills in half, or an installer's promise of payback within a year. For scale, ENERGY STAR's modeled average savings from air sealing plus insulation is about 15% of heating and cooling costs, and that covers attics, crawl-space floors and rim joists together. Walls usually come after the attic; the NPS puts it in that order too. Start with attic foam insulation if yours isn't done.

Interactive tool 4 of 4

Is your injection foam quote fair?

Divide the quote by the square feet of wall it covers. Between $1.50 and $4.50 is inside Angi's published band for injection foam; dense-pack usually lands lower. Then check what the price includes: patching, paint, siding work, old insulation removal, and what happens if a wall cracks.

Quote checker

Price per square foot, price per R, and what's missing

Use the net wall area from the estimator, or the square footage on the quote.

In writing on the quote
$4.00
Per sq ft of wall
R-15.7
Cavity R-value
25.5¢
Per sq ft, per R-1
Inside the published band

Your quote: $4.00 per sq ft

  • Published injection foam band: $1.50–$4.50 per sq ft (Angi, Jul 2026; $3.00 is the midpoint). Your quote: $4.00.
  • For comparison, dense-pack cellulose at a typical $2.15 per sq ft would run about $1,940 for the same wall, at about R-13.0 instead of R-15.7.
  • Not in writing yet: removing old cellulose, if any is in the walls ($1.50–$3.00 per sq ft, Angi); a written plan if drywall or plaster cracks or bows; the product data sheet (R-value at 75°F mean, flame spread and smoke index).
Ask before you sign
  • Which product, exactly? Get the data sheet. The FTC R-value rule tests at a 75°F mean; a number tested cold isn’t comparable.
  • Is it Class 1 (flame spread 25 or less, smoke 450 or less, ASTM E84)?
  • How do you confirm each cavity is full, and how do you find blocking?
  • Are you lead-safe certified? (Required for paid work disturbing paint in pre-1978 homes.)
  • Did you check for knob-and-tube wiring?
  • Who patches, who paints, and what happens if a wall cracks?

Bands: Angi injection foam, July 2026; HomeGuide blown-in walls, November 2025. Price per R divides price per square foot by the cavity R-value; it ignores framing, so use it only to compare quotes against each other.

ArmorThane applicators

Where ArmorThane spray foam fits, and where it doesn't

If your walls are closed and staying closed, ArmorFoam isn't the product. Get dense-pack and injection foam quotes from local insulation contractors, and run the wall checker before anyone drills.

Where we fit in an older house is everything around those walls. Attics, rim joists and crawl spaces are open, so spray foam goes on in controlled lifts you can inspect, and the NPS puts attic and basement work ahead of walls anyway. When a wall is open for a remodel or an addition, an ArmorThane-certified applicator can spray ArmorFoam to a stated thickness and R-value. Details in our spray foam insulation overview and the open-cell spray foam guide.

Find an ArmorFoam applicator →Spray foam vs. fiberglass
Applicator in a respirator and protective suit spraying foam into open stud bays on a sheathed wall
Open framing: where spray foam belongs.
R-6.5 / in.

ArmorFoam closed-cell

Aged, ASTM C518. Lifts of about 1.5–2 in. on open framing; 2 in. is R-13.

R-3.9 at 1 in.

ArmorFoam 0.5 open-cell

Low-density, vapor-open foam for deep open cavities and attic roof decks.

Not an injection foam

Closed walls

None of our ArmorFoam insulation foams is made to be pumped into a closed wall. We'd rather say so than sell you the wrong fix.

For contractors

Filling walls already? Add the open cavities.

Crews that drill and fill walls walk past attics, rim joists and crawl spaces on nearly every old house. We supply ArmorFoam, matched spray equipment and training at your location. Startup packages from $25,000. Not a franchise: no franchise fee, no royalties.

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Straight answers

Injection foam insulation FAQ

How much does injection foam insulation cost?

Angi's July 2026 data puts it at $1.50–$4.50 per square foot of wall installed, with typical projects at $4,300–$8,500 (average $6,200). Brick, extra stories, interior access and removing old cellulose push the price up. Dense-pack cellulose usually costs less, about $2.00–$2.30 per square foot (Angi, September 2026).

What is the R-value of injection foam insulation?

About R-4.5 per inch for tripolymer-type foam (the handbook value for urea terpolymer foam is R-4.48), or roughly R-15.7 in a 2x4 cavity. Published claims run from about R-4 to R-4.6. Slow-rise polyurethane cavity fill rates R-5.9 per inch initially, and dense-pack cellulose R-3.7.

Is injection foam the same as spray foam?

No. Spray foam is sprayed onto open framing in thin passes, and closed-cell spray foam goes on only about 1.5–2 inches at a time because the reaction gives off heat. Injection foams are made for closed cavities: they barely expand, or they flow before they rise. ArmorFoam is a spray foam, not an injection product.

Is injection foam better than dense-pack cellulose?

It has a higher R per inch (about R-15.7 vs. R-13 in a 2x4) and no dust. In our payback math that edge is worth about $16 a year per 1,000 sq ft of wall in Chicago, while foam typically costs more to install. Dense-pack also cut cavity air leakage about tenfold in Building Science Corporation tests, so it's our default pick unless you have a reason to want foam.

How do you insulate existing walls without removing drywall?

Drill and fill. A crew drills one hole per stud cavity from outside, after pulling a course of siding, or from inside through the drywall, fills it with dense-pack cellulose, dense-pack fiberglass or injection foam, and plugs the hole. If you're re-siding, ENERGY STAR recommends filling the cavities and adding R-5 to R-10 insulating sheathing too.

Can injection foam crack drywall or plaster?

It can. RetroFoam's own FAQ calls it rare but possible, and the National Park Service warns that foam pressure can break plaster keys or crack plaster. Drywall glued only to the studs, old patches and water damage raise the risk, so agree in writing on who fixes a bowed or cracked wall before work starts.

Does injection foam shrink?

Some can. Ecotelligent Homes, an energy-audit firm, warns that incorrectly mixed injection foam can shrink and leave cold gaps. We couldn't find independent shrinkage figures for current products, so ask for the product's dimensional-stability test and how the crew checks its fill.

Can injection foam be installed from inside the house?

Yes. The crew drills through the drywall or plaster into each cavity, fills it, plugs the hole and applies a rough coat of joint compound; painting is usually yours. It's messier than working from outside but avoids visible patches in stucco, brick or historic siding.

Do I have to remove old insulation first?

Old fiberglass batts can usually stay; RetroFoam says the foam compresses them. Old loose cellulose has to be vacuumed out before injection foam, and hardened or glued-in cellulose can block the foam. Angi puts insulation removal at about $1.50–$3.00 per square foot.

Can you insulate walls that have knob-and-tube wiring?

Not with foam. The National Electrical Code restricts insulation around concealed knob-and-tube wiring, and even Washington, which allows loose insulation after an electrician certifies the wiring, says foam may not be used with it (WAC 296-46B-394). Have an electrician retire or replace those circuits first.

Can I install injection foam myself?

Not realistically. Resin injection foams are mixed in an installer's rig, and the two-part slow-rise kits sold online are labeled for professional use, need empty cavities and can crack drywall if overfilled. They also cost about $5.76 per square foot of 2x4 wall in chemical alone, more than most installed quotes. DIY effort pays better on rim joists and air sealing.

Does injection foam contain formaldehyde?

It depends on the product. RetroFoam, a tripolymer resin foam, says its dry product contains trace formaldehyde (0.06 ppm) that bonds during mixing, and calls the installed foam formaldehyde-free. Ask any maker for its emissions data in writing.

Is injection foam insulation worth it?

Filling empty walls usually is. By our math, 1,000 sq ft of empty 2x4 wall filled with injection foam in Chicago saves about $381 a year on gas heat, roughly an 8-year payback at $3.00 per square foot. Dense-pack cellulose pays back in about 6. If the attic isn't insulated yet, do that first.

AT
ArmorThane Technical Team · Springfield, Missouri
Written from manufacturer data sheets, building-science and preservation guidance, and 35+ years of making spray foam. Last updated September 26, 2026.

Methodology and sources

Prices are published ranges with their dates, not quotes. R-values come from handbook tables, trade associations and manufacturer data sheets; product claims are labeled as the maker's. Code and safety points come from the NEC as applied in Washington's WAC 296-46B-394, EPA, ENERGY STAR and the National Park Service. Savings are our steady-state calculations: screening numbers, not an energy audit.

  1. Angi, injection foam insulation cost (Jul. 2026): angi.com/articles/making-rooms-cozy-injected-foam-insulation.htm
  2. Angi, blown-in insulation cost (Sep. 2026): angi.com/articles/blown-in-insulation-cost.htm
  3. HomeGuide, blown-in insulation cost (Nov. 2025): homeguide.com/costs/blown-in-insulation-cost
  4. Building Science Corp., BSI-043 Don't Be Dense (J. Lstiburek, 2010): buildingscience.com/documents/insights/bsi-043-dont-be-dense
  5. Cellulose Insulation Manufacturers Association, homeowners: cellulose.org/homeowners/
  6. Cellulose Insulation Manufacturers Association, contractors: cellulose.org/contractors/
  7. Greenfiber, SANCTUARY cellulose product data: greenfiber.com/products/sanctuary
  8. Greenfiber, cellulose for retrofitting walls: greenfiber.com/documents-and-tools/blog/why-choose-cellulose-insulatio
  9. Colorado Energy, R-value table: coloradoenergy.org/procorner/stuff/r-values.htm
  10. National Park Service, Preservation Brief 3 (rev. 2011, PDF): nps.gov/orgs/1739/upload/preservation-brief-03-energy-efficiency.pdf
  11. Washington WAC 296-46B-394, knob-and-tube wiring: app.leg.wa.gov/wac/default.aspx?cite=296-46B-394
  12. U.S. EPA, lead renovation, repair and painting program: epa.gov/lead/lead-renovation-repair-and-painting-program
  13. U.S. EPA, learn about asbestos: epa.gov/asbestos/learn-about-asbestos
  14. ENERGY STAR, recommended home insulation R-values: energystar.gov/saveathome/seal_insulate/identify-problems-you-want-fix
  15. ENERGY STAR, seal and insulate savings methodology: energystar.gov/saveathome/seal_insulate/methodology
  16. U.S. DOE Energy Saver, types of insulation: energy.gov/energysaver/types-insulation
  17. FTC R-value rule, 16 CFR 460.5: ecfr.gov/current/title-16/chapter-I/subchapter-D/part-460/section-460.
  18. ICP HandiFoam Slow Rise technical data sheet TD014 (Oct. 2025, PDF): cdn.shopify.com/s/files/1/0842/2729/3495/files/TD014-HandiFoam-HFO-Slo
  19. Energy Efficient Solutions, slow-rise cavity fill kit listing (Sep. 2026): energyefficientsolutions.com/products/handi-flow-cavity-fill-slow-rise
  20. RetroFoam, FAQ: retrofoam.com/faq
  21. RetroFoam, buyer's guide: retrofoam.com/buying-retrofoam-insulation
  22. RetroFoam, drywall cracking or bowing: retrofoam.com/blog/will-retrofoam-insulation-cause-drywall-crack-bow
  23. ClimaVault, cementitious foam insulation: climavault.com/
  24. Ecotelligent Homes, injection foam insulation: ecotelligenthomes.com/what-you-should-know-about-injection-foam-insula
  25. Comfort to Life, injection foam (Applegate): comforttolife.com/injection-foam.aspx
  26. GreenBuildingAdvisor, injection foam Q&A (2016): greenbuildingadvisor.com/question/injection-foam
  27. NOAA NCEI 1991–2020 climate normals data service: ncei.noaa.gov/access/services/data/v1
  28. U.S. EIA, natural gas prices: eia.gov/dnav/ng/ng_pri_sum_dcu_nus_a.htm
  29. U.S. EIA, Electric Power Monthly table 5.3: eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_3
  30. ArmorThane, closed-cell spray foam guide: armorthane.com/closed-cell-spray-foam/
  31. ArmorThane, ArmorFoam spray foam products: armorthane.com/products/armorfoam-products/

latest news & bLOGS

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Closed wall or open wall, get the right fix.

For closed walls, get dense-pack and injection foam quotes from local insulation contractors. For open walls, attics, rim joists and crawl spaces, ArmorThane-certified applicators spray ArmorFoam with thickness and R-value in writing. Contractors: we make the foam, build the rigs and train your crew, with no franchise fee.

Find an ArmorFoam applicator →Start a spray foam business
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