The short answer
Open-cell spray foam is a low-density spray polyurethane foam, typically about 0.5 to 0.8 pounds per cubic foot, that expands to fill cavities, insulates, and at its tested thickness stops air leakage. The Spray Polyurethane Foam Alliance puts typical products at R-3.6 to R-4.5 per inch. It stays soft, lets water vapor through and can absorb liquid water, so it is neither waterproofing nor a vapor barrier. It suits dry, above-grade walls, interior partitions and properly designed attic rooflines.
The catch? Open-cell foam is not a substitute for a waterproofing system or a universally suitable vapor barrier. Whether it’s the right insulation depends on climate, roof or wall construction, moisture exposure and the amount of space available.
For most homeowners, these are the numbers that matter:
| Property | Typical open-cell spray foam |
|---|---|
| Density | About 0.5–0.8 lb/ft³ (SPFA typical range)[1] |
| R-value | About R-3.6–R-4.5 per inch (SPFA typical range)[1] |
| Air sealing | Can form an air barrier at tested thickness; SPFA cites about 3.5–5.5 inches for open-cell[1] |
| Vapor control | Generally vapor-permeable; requirements depend on assembly and code |
| Feel after curing | Soft and relatively flexible |
| Liquid water | Can absorb and retain water[1] |
| Main use cases | Appropriate attic rooflines, above-grade framing cavities, interior partitions |
| Installation | Professional installation is generally preferable for full-building areas |
Typical values are for orientation, not a product specification. Confirm the exact foam’s technical data sheet, approved thickness and evaluation report.
What Is Open Cell Spray Foam?
Open-cell spray foam is a type of spray polyurethane foam (SPF) installed by mixing chemical components that react and expand against the surface being insulated.
As the material expands, many of its tiny internal cell walls remain open or interconnected. That produces a lighter, softer material than closed-cell spray foam.
The result is an insulation product that can:
- Fill irregular gaps around framing and penetrations.
- Provide thermal resistance across a continuous sprayed area.
- Restrict uncontrolled airflow when installed at the manufacturer’s required thickness.
- Reduce sound transmission in some wall and ceiling assemblies.
- Fit projects where sufficient cavity depth is available.
Unlike fiberglass batts, spray foam adheres to the substrate and expands in place. But it still needs correct substrate preparation, uniform coverage and appropriate building-assembly design.
Don’t confuse open-cell building insulation with a can of expanding gap filler. Professional two-component SPF used across an attic or wall is a different system from the one-component sealants sold for small cracks.
How Does Open Cell Spray Foam Work?
Open-cell foam works in two related ways: it slows heat transfer through the foam and, when installed to the necessary thickness, helps stop air leaking through the insulated surface.
Thermal resistance. The foam’s cell structure slows heat movement. R-value measures thermal resistance: the higher the number, the greater the resistance under the test conditions.
Air sealing. The material expands around irregular shapes, reducing gaps that otherwise allow conditioned air to escape or outdoor air to enter. Not every foam meets the same air-barrier criteria at the same thickness; check the tested product.
Vapor movement. Air leakage and vapor diffusion are different processes. Open-cell spray foam can be an effective air barrier while still allowing significant water vapor to diffuse through it. That distinction is crucial in roof and wall design.
Foam alone also cannot fix a leaking roof, plumbing leak, damp foundation or badly detailed flashing.
Open Cell Spray Foam R-Value per Inch
The Spray Polyurethane Foam Alliance (SPFA) gives a typical range of R-3.6 to R-4.5 per inch for open-cell foam[1], and individual products vary. One manufacturer’s open-cell line, for example, is listed at R-3.7 to R-4.1 per inch[2].
Here is an illustrative R-value calculation using R-3.7 per inch:
| Foam thickness | Estimated nominal foam R-value |
|---|---|
| 2 inches | R-7.4 |
| 3 inches | R-11.1 |
| 3.5 inches | R-13.0 |
| 4 inches | R-14.8 |
| 5.5 inches | R-20.4 |
| 6 inches | R-22.2 |
| 8 inches | R-29.6 |
| 10 inches | R-37.0 |
These numbers are simple multiplication, not whole-wall or whole-roof R-values. Wood or steel framing, thermal bridging, coverings, installation quality and other assembly components affect overall performance.
How Thick Should Open Cell Spray Foam Be?
There is no correct thickness for every property.
The installer should first establish the assembly’s required insulation performance under the locally adopted energy code. They should then use the specific product’s tested R-value and installation instructions to determine thickness.
For example, achieving R-30 of nominal foam resistance with an R-3.7-per-inch product would take roughly 8.1 inches (30 ÷ 3.7). That is a mathematical example, not a code recommendation.
A contractor who proposes “three inches everywhere” without discussing climate zone, framing depth, thermal bridging or code requirements is not giving you a complete specification.
Where Is Open Cell Spray Foam Used?
Open-cell foam is most useful in dry, above-grade building assemblies where its air-sealing benefits and lower material density suit the design.
Attics and Roof Decks
Open-cell SPF is commonly considered for the underside of roof sheathing in an appropriately designed unvented, conditioned attic[3][4].
This can bring attic ductwork and equipment closer to the home’s conditioned environment. But it’s not as simple as spraying the underside of the roof and sealing every vent.
The roof assembly needs a suitable moisture-control and drying strategy. Existing roof leaks must be fixed first[5]. Depending on the climate and assembly, a vapor retarder or other design changes may be required: DOE’s Building America guidance says open-cell foam under a roof deck in climate zones 5 to 8 needs a Class II vapor retarder, installed or applied as a coating[4].
See our attic spray foam insulation guide and roof-deck insulation guide before choosing this application.
Exterior Wall Cavities
Open-cell SPF can fill irregular spaces in above-grade framed walls and provide both insulation and air sealing.
The question is whether the wall assembly will meet its full insulation and moisture-control requirements. The nominal foam R-value isn’t the same as whole-wall performance, particularly where framing creates thermal bridges.
For further considerations, see spray foam insulation for exterior walls.
Interior Walls and Sound Control
Open-cell foam’s softness and ability to fill cavities can make it useful in partitions where reducing sound transmission matters.
However, soundproofing depends on the entire wall system, including drywall mass, seals, mechanical connections and other materials. Foam by itself cannot guarantee a particular sound transmission class (STC).
Where Open-Cell Foam Is Often the Wrong Choice
Be cautious about open-cell SPF in:
- Below-grade basement assemblies with uncertain water intrusion.
- Crawl spaces or other damp environments.
- Areas exposed to bulk water.
- Assemblies needing high R-value in very limited depth.
- Roof systems without an adequate moisture and drying assessment.
That does not mean open-cell foam is prohibited in every one of these situations. It means the application requires a clear, product-specific assembly design rather than a blanket recommendation.
Open Cell vs Closed Cell Spray Foam: Key Differences
Closed-cell foam is denser (about 1.8–2.3 lb/ft³ against 0.5–0.8), gives more R-value per inch (about R-5.8 to R-6.9 against R-3.6 to R-4.5) and lets far less water vapor through[1]. Open-cell costs less per board foot and suits deep, dry cavities; closed-cell suits shallow cavities and moisture-sensitive spots. The side-by-side table and the part-of-the-house decision are on the open-cell vs closed-cell comparison.
Benefits of Open Cell Spray Foam Insulation
1. Air Sealing Alongside Insulation
Correctly installed open-cell foam can reduce unwanted air movement while providing thermal resistance. This combination can be valuable where existing framing or penetrations make conventional batt installation difficult.
2. Filling Irregular Cavities
Its substantial expansion helps fill uneven spaces around timber framing and other detailed areas. The installer still needs to inspect for voids and trim excess material where necessary.
3. Lower Cost per Board Foot
Open-cell SPF is generally cheaper per board foot than closed-cell SPF: HomeAdvisor’s national installed ranges are $0.60–$1.60 per board foot for open-cell and $1.30–$3.10 for closed-cell (updated June 2026)[6]. But it may need greater thickness to achieve the same nominal R-value, so compare completed assemblies, not just unit pricing.
4. Potential Acoustic Benefits
In appropriate interior partitions, open-cell foam can contribute to sound control. Ask for tested assembly data if noise reduction is a major project requirement.
5. Useful for Certain Roof and Wall Designs
Because open-cell foam remains comparatively vapor-permeable, it may support inward drying in some assemblies. That can be an advantage only when the building-science design calls for it.
Drawbacks and Risks of Open Cell Spray Foam
Moisture Sensitivity
Open-cell foam can absorb liquid water[1]. Persistent moisture can damage adjoining building materials and undermine the intended insulation system.
Before installation, investigate any history of roof leaks, condensation, stains, mold or wet wood. Never use foam to hide an unresolved leak.
Lower R-Value per Inch
Where available space is restricted, closed-cell foam or another insulation system may provide the required thermal resistance in less depth.
Application Quality Matters
Problems can arise from poor chemical mixing, incorrect temperatures, unsuitable substrates, inconsistent thickness and inadequate site controls.
A beautiful-looking foam surface is not proof of correct application. Request documentation and inspect the installation against the agreed scope.
Occupant Exposure During Installation
The uncured chemical components and application process can create health risks. Building occupants and unprotected workers should not be present in the application area.
The US Environmental Protection Agency advises following the specific manufacturer’s ventilation, cure and re-occupancy instructions[7]. EPA notes that some manufacturers recommend 24 hours before re-entry for professional two-component foam, but says the time varies[8], so 24 hours is not a universal safety rule.
Fire Protection and Coverings
Spray polyurethane foam is combustible. Applicable building codes and product approvals may require thermal barriers, ignition barriers or specific tested protective systems[3]. See spray foam building codes. Do not assume exposed foam is permitted just because it is inside an attic.
For a fuller overview, read spray foam insulation problems and is spray foam insulation safe?.
How Much Does Open Cell Spray Foam Cost?
HomeAdvisor’s national installed range for open-cell is $0.60–$1.60 per board foot (one square foot, one inch thick)[6], so 1,000 square feet at 3.5 inches (3,500 board feet) is about $2,100–$5,600 before minimum charges and extra work. Measure the surface being sprayed, not the home’s floor area. Prices by thickness and project size are on the open-cell spray foam cost page, and the cost calculator runs your own numbers.
Does Open Cell Spray Foam Need a Vapor Barrier?
Sometimes a separate vapor-control layer is needed, and sometimes it isn’t. The answer depends on the climate, assembly, indoor humidity and the foam’s tested vapor permeance at the installed thickness.
An air barrier controls moving air. A vapor retarder limits diffusion of water vapor through materials. These are not the same function.
Open-cell SPF can meet specified air-impermeability criteria while remaining relatively vapor-permeable. That’s why a roof or wall detail that performs in one climate may be risky in another.
Have the installer explain:
- Where the air-control layer is located.
- Where moisture could enter the assembly.
- How the assembly will dry if it becomes wet.
- Whether a separate vapor retarder is required.
- Which locally adopted code provisions and product approvals apply.
If those questions aren’t answered, the insulation recommendation is incomplete.
Is Open Cell Spray Foam Safe After It Cures?
Most of the documented risk is during spraying and curing, not after; but a safe re-entry time can’t be judged by appearance or smell, and it varies by product. Ask the installer for the product’s safety data sheet and written re-occupancy instructions, and report lingering odor or irritation. See is spray foam insulation safe? and spray foam off-gassing.
How to Choose an Open Cell Spray Foam Contractor
Compare the exact product and assembly, not just the price: ask for the product name and data sheet, the thickness for each area, the moisture-control plan, any leak repairs, the fire-protection plan, the ventilation and re-occupancy plan, and an itemized price. The full list is in spray foam contractor questions to ask; then find contractors near you.
The Bottom Line
Open-cell spray foam is a useful insulation and air-sealing material where the building has enough space for the required thickness and an appropriate moisture-control design. Its lower density and generally lower board-foot cost make it attractive for suitable above-grade applications.
The wrong decision is to choose it solely because it’s cheaper than closed-cell foam. Select the assembly first, confirm the insulation requirements, then compare the products and contractor specifications.
Next steps: Compare open cell vs closed cell, estimate insulation costs, or find a contractor.
Questions owners ask
Is Open Cell Spray Foam Good for Attics?
It can be, particularly in appropriately designed unvented roof assemblies. The roof must be assessed for leaks, local climate conditions, moisture control, code compliance and how the attic will be conditioned or ventilated after conversion.
What Is the R-Value of Open Cell Spray Foam?
Typical products provide around R-3.5 to R-4.0 per inch. Actual rated performance depends on the product, so use its manufacturer data sheet rather than a generic industry average.
Is Open Cell Spray Foam Waterproof?
No. Open-cell foam can absorb and hold liquid water, and it should not be treated as waterproofing.
Can Open Cell Spray Foam Cause Mold?
Foam itself does not eliminate mold risk. Water leaks, condensation and persistently wet adjacent materials can still support mold growth. Resolve moisture problems before insulating.
Can You Install Open Cell Spray Foam Yourself?
Large two-component SPF installations involve chemical exposure, specialized equipment and strict application controls. For whole attics and walls, a qualified professional is generally the safer choice. Small consumer gap-fillers are not substitutes for a whole-building SPF system.
Can Open Cell Foam Be Used in a Crawl Space?
It should not be a default recommendation for moisture-prone crawl spaces. The appropriate insulation depends on crawl-space moisture, drainage, conditioning, code requirements and assembly design.
How Long Does Open Cell Spray Foam Last?
Installed service life depends on the product, substrate conditions and whether the assembly remains dry and intact. Avoid relying on an unsupported blanket lifespan figure; review the manufacturer's warranty and inspect for water intrusion or other damage over time.
Sources
- Frequently Asked Questions, Spray Polyurethane Foam Alliance (SPFA). Accessed October 19, 2026. Industry trade association. Page metadata shows modified April 27, 2026. Typical density, R-value per inch, air-barrier thickness and vapor permeance ranges.
- ENERTITE Open-Cell Spray Foam, BASF. Accessed October 19, 2026. Manufacturer product page, cited only as an example of one product line's published values. No date shown.
- Residential Applications, Spray Polyurethane Foam Alliance (SPFA). Accessed October 19, 2026. Industry trade association. Modified September 24, 2026. Application table references the 2018 IRC/IECC.
- Unvented Conditioned Attic with Spray Foam Insulation Below Roof Deck, U.S. Department of Energy, Building America Solution Center. Accessed October 19, 2026. DOE guide for new homes. No date shown.
- Below Deck Spray Foam Insulation for Existing Roofs, U.S. Department of Energy, Building America Solution Center. Accessed October 19, 2026. DOE retrofit guide. No date shown.
- How Much Does Spray Foam Insulation Cost?, HomeAdvisor. Accessed October 19, 2026. National consumer cost guide, updated June 17, 2026. Installed cost per board foot; national ranges, not quotes.
- Ventilation Guidance for Spray Polyurethane Foam Application, U.S. Environmental Protection Agency (archived). Accessed October 19, 2026. Last updated November 5, 2020; archived EPA page.
- Vacate and Safe Re-Entry Time for Spray Polyurethane Foam Application, U.S. Environmental Protection Agency (archived). Accessed October 19, 2026. Last updated September 2, 2020; archived EPA page.