Spray foam attic insulation: open-cell vs closed-cell explained

Spray foam attic insulation does two jobs at once: it slows conductive heat flow with its R-value, and it seals the air leaks that loose-fill and batts struggle to close. That air-sealing ability is why so many homeowners choose foam over other materials in an attic. It is sprayed as a liquid that expands and cures into a solid, filling gaps and bonding to the sheathing, framing, and other surfaces it contacts.

This guide walks the real decision a homeowner faces, in order: how open-cell works, how closed-cell works, a head-to-head comparison, where each type belongs, what it costs, the downsides to weigh, and the one heat-transfer mode spray foam leaves for a separate layer.

What spray foam does in an attic, and where it goes

Spray foam controls two of the three ways heat moves: conduction (heat through a solid) and air convection (heat carried by leaking air). Radiant heat off a sun-heated roof deck requires a separate layer, covered later in this guide. First, know the two places foam goes in an attic, because that single choice drives everything else.

These contexts behave differently. An unvented roofline assembly brings the attic into the conditioned space, which trims duct losses in both heating and cooling seasons. The U.S. Department of Energy’s guidance on where to insulate notes that you should seal air leaks before adding bulk insulation, which is exactly the job foam handles well at the floor plane.

The roofline approach is governed by building code. The DOE Building America Solution Center guidance on unvented conditioned attics covers the moisture rules that make this assembly safe, including keeping wood sheathing below 19% moisture content before foam is applied. Choosing between open-cell and closed-cell foam is the next decision, so start with how each one behaves.

Open-cell spray foam in attics

Open-cell spray foam delivers about R-3.7 per inch (typical range R-3.6 to R-3.8). Carbon dioxide blows the cells open during curing, so the cured material stays soft, spongy, and low-density at roughly 0.5 lb per cubic foot. The DOE Types of Insulation page describes open-cell cells as filled with air, which gives a lower R-value than closed-cell. Use the manufacturer’s data sheet for any code calculation.

Vapor behavior is the part that decides where open-cell fits. A perm is a unit that measures how readily water vapor passes through a material; higher perms mean more vapor moves through. At roughly 16 to 18 perms, open-cell is nearly as open as air. In colder Climate Zones 5 through 8, IRC 2021 Section R806.5 requires a Class II vapor-retarder coating over open-cell in an unvented roof assembly.

Open-cell fits best in two spots: unvented rooflines in hot-humid Zones 1 through 3, where vapor control at the deck is less critical, and air-sealing passes at the attic floor. Its installed cost is also lower than closed-cell at comparable thicknesses. Installed cost runs roughly $0.60 to $1.60 per board foot, per HomeAdvisor 2025 cost data.

Closed-cell spray foam in attics

Closed-cell spray foam delivers about R-6.0 to R-6.5 per inch, and premium products reach R-7.0. Manufacturer Johns Manville reports R-7 per inch at 2.0 lb per cubic foot for its closed-cell product. The aged R-value can decline slightly over time as the blowing agent diffuses out, so verify the number with the product’s ICC-ES report or SPFA technical data.

Its sealed cells are filled with a low-conductivity blowing agent, which makes a rigid, dense foam at about 2.0 lb per cubic foot. That rigidity also stiffens the roof framing. Because closed-cell bonds tenaciously to roof sheathing, reroofing later can require mechanical tear-out, a long-term commitment worth weighing before you commit.

That vapor behavior is why closed-cell is the foam for unvented assemblies in Zones 5 through 8 without an added coating. Building Science Corporation guidance states that in Climate Zone 5 and above, only high-density closed-cell foam should be used for unvented conditioned attics, and closed-cell meets the Class II vapor-retarder threshold at 1.5 inches.

Closed-cell fits best at unvented rooflines in any climate, especially mixed-humid and cold zones. It also works in hybrid assemblies, where foam at the deck pairs with an air-permeable insulation below. The trade-off is price: installed cost runs roughly $1.30 to $3.10 per board foot.

Open-cell vs closed-cell: a side-by-side comparison

Here is the head-to-head on the metrics that drive the choice, so you can scan rather than reread the two sections above.

In hot Zones 1 through 3, open-cell meets code at the roofline and costs less per board foot than closed-cell. In mixed and cold Zones 5 through 8, closed-cell is the practical default because it is a Class II vapor retarder on its own, with no added coating, and it hits the condensation-control R-value in fewer inches. Hybrid foam-plus-batt assemblies must keep the foam at the required share of total R-value to control condensation.

Where each type fits: roofline vs attic floor

Unvented, conditioned roofline

Foam at the roofline goes on the underside of the roof sheathing between the rafters. The attic becomes semi-conditioned, so HVAC equipment and ducts move inside the envelope and duct losses drop in both seasons. IRC 2021 Section R806.5 governs this assembly and requires air-impermeable insulation directly against the sheathing.

Closed-cell is required in Zones 5 through 8. Open-cell can be used in those zones only with an added Class II vapor-retarder coating, and no coating is needed in Zones 1 through 3. The IRC sets a condensation-control minimum R-value at the deck, and the foam must reach it before any supplemental permeable insulation goes below.

Vented attic with a flat ceiling

In a conventional vented attic, foam is most valuable only as the air-seal at the floor plane, sealing top plates, penetrations, and recessed-light boxes before blown-in cellulose or fiberglass goes on top. For the bulk fill itself, loose-fill beats foam on cost per R.

Either foam type works at the attic floor between joists, but it rarely earns its premium there. This is where layering with a reflective barrier comes in, since a foil layer at the roofline can cut the radiant load that floor insulation then has to absorb. For context on how conductive-only insulation performs, R-13 fiberglass batts top out around R-9.4 whole-wall once framing losses are counted and do nothing for radiant heat, which is why attics increasingly pair bulk insulation with foam air-sealing and a radiant barrier.

What spray foam attic insulation costs

A typical spray foam attic project runs roughly $1,500 to $8,000, per HomeAdvisor 2025 aggregate data. That average spans jobs of every size, from a targeted floor air-seal to a partial roofline. Full roof-deck coverage at code thickness sits at the high end and beyond, since it uses far more material. Spray foam is priced by the board foot: one board foot covers one square foot at one inch thick.

Work one example through. A 1,500 sq ft roof deck at 3 inches of closed-cell equals 4,500 board feet, which lands at roughly $5,900 to $14,000 installed, with a wide regional spread. The cost drivers that move price most are foam type, required thickness (colder zones need more inches), attic accessibility, prep work like removing old insulation, and the local labor market. Labor alone is roughly 40 to 60 percent of the total.

The practical tip: get at least three bids and confirm the installer follows ICC-ES compliance, so the assembly is installed to the correct thickness for your climate zone. Whether the investment pays back is covered in the FAQ below.

The downsides of spray foam in attics

Spray foam carries five real trade-offs worth weighing before you commit. Measure each one against your situation.

1. Highest upfront cost of any attic insulation type. Payback is longer than loose-fill on material alone. In a conditioned-attic build, the savings on duct leakage can shorten that payback, especially in mixed climates.

2. Off-gassing during the cure window. Per EPA guidance on spray polyurethane foam exposures, isocyanate vapors and VOCs are concentrated in the cure window, and manufacturers typically recommend at least 24 hours before occupants re-enter, though curing time varies with product formulation, thickness, temperature, and humidity. Fully cured foam is relatively inert.

3. Removal is difficult. Closed-cell bonds tenaciously to the sheathing, so reroofing or remediation can mean mechanical tear-out that damages decking and framing. Treat it as a long-term commitment.

4. Contractor quality varies. Bad mix ratios in two-component foam produce soft, under-cured foam with degraded R-value and a lingering odor. Verify certification, insist on ICC-ES compliance, and ask for a post-install check.

5. Moisture risk over wet wood. Per the DOE Building America Solution Center, sheathing must be below 19% moisture content before foam is applied. Foam over wet wood can trap moisture and hide decay.

The one thing spray foam cannot stop: radiant roof heat

Heat moves three ways: conduction, convection, and radiation. Spray foam handles the first two well. Reflecting radiant energy requires a different material entirely.

A roof deck in direct summer sun reaches 150 to 180 degrees F and re-radiates long-wave infrared into the attic, no matter how thick the foam beneath the sheathing is. Foam does not change the emittance of the deck facing the attic.

Emittance is how readily a surface gives off radiant heat; a dark, hot roof deck has high emittance and radiates strongly. That is a physics boundary, and no thickness of foam changes it. The hot roof deck acts like a radiator pointed down at the attic, regardless of the insulation beneath it.

The numbers back up why that radiant pathway is worth addressing. The DOE Energy Saver radiant-barrier guidance puts cooling-cost savings at 5 to 10 percent in warm, sunny climates. The DOE Building America Solution Center attic radiant barrier guide cites a reduction in attic-floor heat flow worth roughly 8 to 12 percent of annual cooling costs. And Oak Ridge National Laboratory large-scale climate simulator testing measured a 33 to 50 percent reduction in summer heat flow for foil-based radiant barriers on the roof deck or rafters versus an R-13 control, and 19 percent for a liquid-applied coating.

Once foam reaches code-minimum thickness, the radiant portion of the load remains. A radiant barrier addresses that portion directly, so the two layers are additive.

How Radiant Barrier RB+ complements spray foam

Radiant Barrier RB+ is a metalized foil sheet tested at 5% emittance (ASTM C1371), reflecting about 95% of the radiant heat from a hot roof deck instead of absorbing and re-radiating it into the attic. It complements spray foam in the same assembly, adding the radiant control that foam alone leaves open.

Installation is simple. You staple the perforated foil sheet to the underside of the rafters so it faces the attic air space, reflecting radiant heat before it loads the foam or the attic. The rafter depth gives the air gap that reflection needs.

The perforations measure 6.29 perms (ASTM E96), so vapor keeps moving and humidity does not build in a closed or semi-conditioned rafter bay. This matters in a spray-foam assembly where vapor is already managed. It does not absorb moisture or block vapor when installed correctly, so humidity moves through freely.

At the assembly level, RB+ contributes a system R-value of R-4.1 to R-14.5 for an attic or roof with heat flow down, calculated against RIMA, AIRAH, ASHRAE, and ISO 6946 standards. It ships in 500, 542, and 1,000 sq ft rolls and staples to rafter undersides or gable walls. The radiant barrier versus insulation comparison covers the system-versus-material math in detail.

For help sizing the two layers for your attic, contact our team.

Frequently asked questions

Spray foam attic insulation earns its place where air sealing and a high R-value per inch matter most: open-cell for hot-zone rooflines and floor air-sealing, closed-cell for mixed and cold-zone unvented assemblies. Match the foam type to your climate zone, confirm the thickness meets IRC R806.5, and budget for the labor that drives most of the price. Then add a perforated radiant barrier above the foam to reflect the radiant roof heat that no thickness of foam can stop, so the assembly handles all three heat-transfer pathways together.