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★ The ArmorThane Reference
Everything contractors, homeowners, and builders need to know about open cell spray foam insulation — R-values, costs, applications, installation, and how ArmorFoam systems outperform the competition.
◆ On This Page
■ Fundamentals
Open cell spray foam is a type of spray polyurethane foam (SPF) insulation characterized by its soft, spongy texture and open-cell structure. Unlike closed-cell foam — where individual cells are sealed and rigid — open cell foam intentionally leaves its cells incomplete, or “open,” allowing air to fill those spaces. The result is a lightweight, flexible, highly expansive material that excels at sealing air leaks, dampening sound, and providing consistent thermal performance across complex building geometries.
Open cell spray foam is a two-component system: an isocyanate (A-side) and a polyol resin blend (B-side). When these components are heated and mixed at the spray gun, a rapid chemical reaction causes the liquid mixture to expand — often up to 100 times its original volume — and cure in place within seconds. This dramatic expansion is what allows open cell foam to fill every crack, gap, and irregular void in walls, attics, crawlspaces, and rim joists that other insulation types simply cannot reach.
Open cell spray foam has a density of approximately 0.5 lb per cubic foot, compared to closed-cell foam at roughly 2 lb per cubic foot. This lower density contributes to its affordability, its flexibility, and its impressive sound attenuation. Because the foam is permeable to vapor, it “breathes” more than closed-cell foam — a property that can be a strength or a consideration depending on the climate and building assembly.
Open cell spray foam is a soft, lightweight spray-applied insulation that expands to fill building cavities, creates an airtight seal, reduces noise transmission, and delivers an R-value of R-3.5 to R-3.8 per inch. It is the preferred insulation for interior walls, attic floors, soundproofing applications, and any space where large-volume expansion and acoustic performance are priorities.
The chemistry behind open cell spray foam is polyurethane, the same polymer platform used in ArmorThane’s high-performance protective coatings. When the A-side and B-side materials react, carbon dioxide gas is generated as a byproduct. In open cell foam, this gas is allowed to escape through the cell walls as the foam cures, creating the open, air-filled structure. The foam matrix sets around an interconnected network of burst cells, giving it both its characteristic softness and its ability to absorb and deaden sound.
On a molecular level, this open-cell architecture means the foam’s thermal performance comes primarily from its ability to block air movement — what building scientists call convective heat transfer. The still air trapped within the foam matrix acts as the insulating medium. This is why open cell foam, despite a lower R-value per inch than closed cell, dramatically outperforms fiberglass batts in real-world installations: batts allow air movement within the cavity; open cell foam does not.
■ Thermal Performance
The R-value of open cell spray foam is approximately R-3.5 to R-3.8 per inch. This places it on par with fiberglass batts and blown-in cellulose on a per-inch basis — but the comparison is misleading. In real-world building assemblies, open cell foam consistently delivers greater energy savings than any other material in its R-value class because it eliminates air infiltration entirely.
The U.S. Department of Energy estimates that air leakage accounts for 25 to 40% of the energy used for heating and cooling in a typical home. Fiberglass batts and blown-in cellulose have no meaningful ability to block airflow. Open cell spray foam creates a continuous, seamless air barrier the moment it cures. A properly installed open cell foam assembly stops both conductive heat transfer (what R-value measures) and convective heat transfer (air leakage), which is why energy auditors consistently report dramatic improvements after spray foam installations.
Source: U.S. Department of Energy & International Energy Conservation Code (IECC)
The 2021 International Energy Conservation Code (IECC) prescribes minimum R-values for attics, walls, and floors that vary by climate zone. When specifying open cell spray foam, use the following thickness guide:
Because open cell foam eliminates air leakage, its effective thermal performance exceeds its nominal R-value. A 2016 study by Oak Ridge National Laboratory found that air sealing via spray foam reduced energy loads by 20–40% above what R-value alone would predict. Factor this into your energy model when comparing open cell foam to batt or blown-in alternatives.
■ Head-to-Head
Open cell and closed cell are the two primary types of spray polyurethane foam insulation. They share the same polyurethane chemistry and installation process, but their physical structures — and therefore their performance profiles — are fundamentally different. Understanding when to specify each type is the most important decision in any spray foam project.
Open cell’s exceptional expansion rate (up to 100×) fills every void in standard 2×4 and 2×6 stud bays completely. Its sound dampening properties reduce noise transfer between rooms.
For unvented (hot roof) assemblies, open cell foam applied to the underside of the roof deck creates a continuous air and thermal barrier. Excellent for complex roof geometries and cathedral ceilings.
The open-cell matrix absorbs sound waves rather than reflecting them. Media rooms, home theaters, recording studios, and shared walls in multi-family buildings benefit most.
At $0.50–$1.50 per board foot, open cell foam delivers spray foam air-sealing performance at roughly half the cost of closed-cell. Ideal for larger-volume applications.
Closed cell spray foam is the better choice when vapor control, structural reinforcement, or high R-value per inch in a thin profile are the governing requirements. Specify closed cell for exterior walls in moisture-prone climates, crawlspace floors and walls where water infiltration is a concern, roof decks in high-humidity coastal environments, and any application requiring a vapor retarder classification per building code.
Many high-performance building assemblies use a hybrid approach: a 1–2 inch lift of closed cell foam as a vapor retarder against the exterior sheathing, followed by open cell foam to fill the remainder of the cavity. This captures the vapor control of closed cell and the sound performance and economy of open cell in a single assembly.
■ Why Open Cell Foam
Open cell spray foam delivers four distinct performance functions in a single installation: thermal insulation, air sealing, vapor management, and acoustic control. No other single insulation material matches this combination, which is why spray foam consistently delivers energy savings that exceed its R-value on paper.
Open cell foam expands to 100× its liquid volume and bonds directly to studs, joists, plates, and sheathing. Every penetration, gap, and crack is sealed at the source. The U.S. DOE identifies air sealing as the highest-impact energy conservation measure available.
By eliminating drafts and cold spots, open cell foam creates consistent temperatures throughout the building envelope. Rooms that were previously unlivable in summer or winter become comfortable year-round without upsizing HVAC equipment.
The open-cell sponge structure absorbs sound energy rather than transmitting it. Open cell foam achieves STC ratings of 37–40 in standard wall assemblies — significantly better than fiberglass batts at STC 30–32 — and dramatically better in attic applications.
Homes insulated with spray foam see 20–40% reductions in heating and cooling costs compared to code-minimum assemblies. Most installations reach payback within 3–5 years from energy savings alone.
Plumbing penetrations, electrical conduit pathways, irregular framing, and irregular bay depths are no obstacle. Open cell foam flows and expands into every void before curing in place, eliminating the thermal bridging that batts always leave behind at penetrations.
Unlike fiberglass batts that sag and compress over time — losing both R-value and air-sealing performance — cured spray foam does not settle, shift, or degrade. It is a permanent, maintenance-free component of the building assembly.
ArmorThane’s ArmorFoam open cell spray foam is engineered from the same polyurethane platform that has made ArmorThane a global leader in protective coatings for 35+ years. ArmorFoam delivers verified R-3.8 per inch, a 100× expansion rate, and STC sound ratings that consistently test above competing open cell products. It is manufactured in Springfield, Missouri — not outsourced — and is backed by 24/7 technical support at (417) 831-5090.
▲ Applications
Open cell spray foam is one of the most versatile insulation materials available, suited to a wide range of residential and commercial building applications. Its high expansion rate, conformable texture, and acoustic performance make it the preferred material wherever air sealing, sound control, and cavity filling are the priorities.
Applied to underside of roof sheathing for unvented hot-roof assemblies. Eliminates attic bypass air leakage.
Fills 2×4 and 2×6 stud cavities completely. Stops convective looping within walls — the primary failure mode of batts.
Dramatically reduces sound transmission between rooms. Ideal for bedrooms, bathrooms, home offices, and media rooms.
Insulates and air-seals the floor assembly over vented crawlspaces. Eliminates cold floors and moisture-driven air infiltration.
One of the highest-infiltration areas in any home. Open cell foam seals rim joist bays faster and more completely than any other material.
Applied to above-grade sections of basement walls to stop air infiltration. Pair with closed cell on below-grade sections for vapor control.
Open cell foam applied to the interior of metal walls and roofs eliminates condensation (the steel framing never drops below dew point) and dramatically improves thermal performance.
Seals critical air barriers in refrigerated and frozen food storage facilities where thermal bridging and air infiltration create condensation and frost problems.
Open cell foam is the material of choice for broadcast-quality sound isolation. Its STC performance rivals purpose-built acoustic panels at a fraction of the cost.
Partition walls in commercial tenant improvements benefit from open cell foam’s combination of air sealing and sound privacy — a building code requirement in many jurisdictions.
Barns, poultry houses, and equipment storage buildings benefit from open cell foam’s ability to stop air infiltration and moderate temperature swings that stress livestock and equipment.
Post-frame buildings with steel roofing and siding are especially prone to condensation. Open cell foam applied to interior surfaces eliminates the condensation problem completely.
Open cell spray foam should not be used as the sole insulation layer in below-grade basement walls, in direct contact with standing water, or in applications requiring a Class II vapor retarder per code without a separate vapor control layer. In those assemblies, use closed cell foam — or the hybrid closed cell/open cell system described in the comparison section above.
■ Investment & ROI
Open cell spray foam is the most affordable spray foam insulation option, typically costing $0.50 to $1.50 per board foot installed. A board foot is one square foot of foam at one inch of thickness. For a standard 2×6 wall cavity at 5.5 inches deep, one square foot of wall surface requires 5.5 board feet of foam.
For whole-home insulation projects, total installed costs typically fall between $3,000 and $6,000 for a 2,000 square foot home, depending on the scope of work (walls only, attic only, or full building envelope), local labor rates, and the complexity of the framing. This is significantly less than closed cell spray foam for the same volume, while still delivering all of the air-sealing benefits that make spray foam worth the premium over fiberglass or cellulose.
Open cell spray foam consistently delivers strong ROI through energy savings. A typical residential installation reduces heating and cooling costs by 20 to 40% annually. For a household spending $200 per month ($2,400/year) on energy, that translates to $480–$960 per year in savings. At a $4,500 installation cost, payback occurs in 4.7 to 9.4 years — well within the product’s 100-year lifespan.
Beyond energy savings, open cell spray foam adds measurable market value to homes. Real estate appraisers and energy raters recognize spray foam installations as a premium feature. Studies by the Spray Polyurethane Foam Alliance (SPFA) indicate that energy-efficient homes insulated with spray foam sell for 3–6% more than comparable homes with conventional insulation. On a $350,000 home, that’s $10,500–$21,000 in added market value — often exceeding the cost of the foam installation itself.
Under the Inflation Reduction Act (IRA), homeowners installing qualifying spray foam insulation may be eligible for a tax credit of up to $1,200 (or 30% of the installed cost) through the Energy Efficient Home Improvement Credit (Section 25C). ArmorThane applicators can provide documentation to support your tax credit claim. Contact ArmorThane at (417) 831-5090 for details on qualifying products and current incentive programs.
► Step-by-Step
Open cell spray foam is a professional-grade material that requires trained installers, commercial-grade proportioning equipment, and proper safety protocols. DIY kits are available for small gap-filling tasks, but whole-cavity insulation must be installed by a qualified contractor with plural-component spray equipment. The installation process on a typical residential project looks like this:
All surfaces that should not receive foam — windows, doors, electrical boxes, flooring — are masked or covered. HVAC systems are shut off to prevent foam particles from entering ductwork. The installer establishes ventilation to remove vapors during application.
Surfaces must be clean, dry, and within the allowable temperature range (typically 60°F–80°F substrate temperature). Moisture content of wood framing is checked — framing over 19% moisture content must dry before foam is applied or adhesion and curing will be compromised.
A plural-component proportioner heats and pressurizes the A-side and B-side materials to the temperatures specified by the foam manufacturer. Hoses and the spray gun are purged and set to the correct pressure ratio — typically 1:1 by volume. Material temperatures are verified before spraying begins.
The installer applies foam in passes, building up to the specified thickness. Open cell foam expands rapidly — the full expansion occurs in 60–90 seconds. For stud cavities, the foam is sprayed to slightly overfill the cavity so it can be trimmed flush with the framing face.
After cure (15–30 minutes), excess foam extending past the framing face is trimmed flush using a foam saw or serrated blade. Trimmed surfaces must be smooth and planar for drywall to lie flat without telegraphing voids.
Per the International Building Code (IBC) and International Residential Code (IRC), open cell spray foam in occupied spaces must be covered with a 15-minute thermal barrier — typically ½-inch drywall. This is a life-safety requirement: foam is combustible, and the thermal barrier slows ignition in case of fire.
The installer documents ambient conditions, substrate temperatures, batch numbers, film thickness (measured by probe into the foam), and any special conditions. This documentation supports warranty claims, building permit inspections, and energy code compliance verification.
Open cell spray foam contains isocyanate (MDI) on the A-side, which is a sensitizer requiring proper respiratory protection during application and until cure is complete. All personnel in the application area must wear appropriate PPE: supplied-air respirators or combination cartridge respirators, chemical splash goggles, and chemical-resistant gloves and coveralls. The building must be unoccupied by non-essential personnel during application and for a minimum re-occupancy period specified by the manufacturer — typically 24 hours after application is complete.
ArmorThane provides complete Safety Data Sheets (SDS) and PPE guidance for all ArmorFoam products. Call (417) 831-5090 for the complete SDS package.
■ Product Specs
ArmorThane has been manufacturing polyurethane and polyurea protective coatings in Springfield, Missouri since 1989. ArmorFoam is ArmorThane’s purpose-built open cell spray foam insulation product line, engineered from the same polyurethane platform that powers our high-performance protective coatings. We manufacture the foam chemistry, manufacture the proportioning equipment, and train the applicator network that installs it — all under one roof.
ArmorThane is not a franchise operation. Every product — the foam chemistry, the proportioning equipment, the spray guns, and the technical training — is developed, tested, and supported from our Springfield facility. When you call ArmorThane at (417) 831-5090, you reach the manufacturer, not a franchisee’s call center.
Our global applicator network spans North America and 30+ countries. Every ArmorThane applicator completes hands-on equipment operation and application technique training before spraying a single job. The result is consistent, documented installations that meet or exceed building code requirements in every climate zone.
ArmorThane manufactures both open cell and closed cell spray foam systems, giving your project team the flexibility to specify the right foam for each assembly: ArmorFoam open cell for interior walls, attic floors, and soundproofing; ArmorFoam closed cell for exterior walls, roof decks, and vapor-critical assemblies. Hybrid systems combining both types are available and frequently specified on high-performance building projects.
Also see: Spray Foam Insulation Overview | Polyurea Coatings Guide
? Common Questions
Open cell spray foam is a type of spray polyurethane foam (SPF) insulation with a soft, spongy texture. Its cells are intentionally left open, allowing air to fill the spaces. This gives it an R-value of approximately R-3.5 to R-3.8 per inch, exceptional sound-dampening properties, and a 100× expansion rate that fills every gap and cavity. It is the most affordable spray foam option and excels in interior walls, attics, and soundproofing applications.
Open cell spray foam has an R-value of approximately R-3.5 to R-3.8 per inch. ArmorFoam achieves R-3.8 per inch per ASTM C518 testing. For an attic application requiring R-38, you would need approximately 10 to 11 inches of open cell foam. For a 2×6 wall cavity at 5.5 inches deep, the cavity provides approximately R-19 to R-21 of insulation value — with air sealing performance far exceeding what R-value alone suggests.
Open cell spray foam typically costs $0.50 to $1.50 per board foot installed. A board foot equals one square foot of coverage at one inch of thickness. For a typical 2,000 square foot home insulation project, total costs range from $3,000 to $6,000 depending on the areas insulated and local labor rates. While the upfront cost exceeds fiberglass batts, the 20–40% annual energy savings typically delivers payback within 3–5 years.
Yes. Open cell spray foam is excellent for attic applications. When applied to the underside of the roof deck in an unvented (hot roof) assembly, it creates a continuous air and thermal barrier that eliminates attic bypass leakage — one of the largest single sources of energy loss in most homes. For vented attic assemblies, open cell foam is applied on the attic floor between and over the joists. The sound-dampening properties are an added bonus, reducing roof noise (rain, wind, aircraft) significantly.
Open cell and closed cell spray foam differ in density, R-value, vapor permeability, and cost. Open cell has a density of ~0.5 lb/cu ft, R-value of R-3.5 to R-3.8 per inch, is vapor permeable, and expands up to 100× its volume. Closed cell has a density of ~2 lb/cu ft, R-value of R-6.0 to R-7.0 per inch, acts as a Class II vapor retarder at 1.5 inches, and adds structural rigidity. Open cell is preferred for interior walls and soundproofing; closed cell for exterior walls, roofing, and vapor-critical applications.
Yes. The International Building Code (IBC) and International Residential Code (IRC) require that spray polyurethane foam insulation in occupied spaces be covered with a 15-minute thermal barrier — typically ½-inch gypsum wallboard (drywall). This is a fire safety requirement because foam is combustible. The thermal barrier slows flame spread and provides time for occupant egress in case of fire. Uncovered spray foam in accessible attic and crawlspace areas must be covered with an approved ignition barrier unless the foam is listed for use without a thermal barrier.
Open cell spray foam is vapor permeable, meaning moisture vapor can pass through it — which is actually a feature in most climate zones because it allows the assembly to dry. However, open cell foam is not waterproof and should not be used in applications where it will be in prolonged contact with standing water or bulk moisture. For below-grade basement walls, crawlspace floors, or exterior applications subject to weather exposure, closed cell spray foam is the appropriate choice.
Open cell spray foam, when properly installed and protected with a thermal barrier, can last the lifetime of the building — often cited as 80 to 100 years. Unlike fiberglass batts, which compress, sag, and lose both R-value and air-sealing performance over time, cured spray foam does not settle, shift, or degrade. Its R-value and air permeance ratings remain stable as long as the foam is protected from UV exposure, physical damage, and direct water contact.
Properly cured open cell spray foam is safe for building occupants. The primary safety concern is during installation, when uncured isocyanate (MDI) vapors are present and require proper respiratory protection and ventilation. Once cured — typically 24 hours after application — the foam is inert and non-toxic. Buildings should remain unoccupied during application and for the re-occupancy period specified by the manufacturer. ArmorThane provides complete Safety Data Sheets (SDS) and installation safety guidance for all ArmorFoam products.
Small-volume DIY kits are available for gap-sealing and small cavity applications. However, whole-cavity insulation of walls, attics, and floors requires commercial-grade plural-component proportioning equipment, trained operators, and proper safety protocols. Improper mixing ratios, off-ratio material, or incorrect temperatures during application can result in poorly cured foam with compromised thermal and mechanical performance. For any application beyond small gap-filling, ArmorThane strongly recommends using a trained, qualified spray foam contractor.
Open cell spray foam significantly reduces the risk of frozen pipes in exterior walls and unheated spaces by maintaining more consistent temperatures around plumbing. Its air-sealing properties prevent cold drafts from reaching pipe surfaces. However, open cell foam’s vapor permeability means it is less effective than closed cell foam as a thermal blanket in severely cold climates. For pipes in extremely exposed locations, pair open cell foam air sealing with a heated pipe cable for maximum freeze protection.
Professionally installed open cell spray foam generally increases home value and makes homes easier to sell, not harder. Concerns about spray foam and home sales primarily relate to improperly installed open cell foam in roof spaces — specifically, foam that traps moisture around timber framing. Professionally installed ArmorFoam with proper ventilation design and documentation rarely causes issues. Keep installation records, use certified installers, and ensure the work meets current building codes to protect your investment.
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Talk to ArmorThane directly. We’re the manufacturer — foam chemistry, equipment, and technical support under one roof, supporting a global applicator network. Not a franchise. No franchise fees.
