Spray Foam Insulation: Complete Guide to Cost, Types & When to Use

Quick Answer: Spray foam insulation comes in two types: open-cell (R-3.5–3.8 per inch, $1.00–$3.50/sq ft installed) and closed-cell (R-6.0–7.0 per inch, $1.50–$5.00/sq ft installed). Closed-cell also acts as a vapor barrier and adds structural rigidity. Both types create air barriers — combining insulation and air sealing in one step. Spray foam costs 2–5× more than fiberglass or cellulose, but that premium buys you air sealing that those materials can't provide on their own.

Table of Contents

• How Spray Foam Works
• Open-Cell vs Closed-Cell: Complete Comparison
• R-Value Performance
• Cost Breakdown
• When to Use Spray Foam (And When NOT To)
• Air Sealing: The Real Value Proposition
• Spray Foam by Application
• Health, Safety & Off-Gassing
• Environmental Impact
• DIY Spray Foam Kits
• Spray Foam Myths Debunked
• Common Mistakes
• Key Takeaways
• FAQ

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How Spray Foam Works

Spray foam is a two-component liquid polymer that expands on contact and cures into solid foam within seconds. The installer uses a heated hose and spray gun to combine the two components — the A-side (isocyanate) and the B-side (polyol resin with blowing agent and catalysts) — at the nozzle. The chemical reaction generates heat, creates gas bubbles, and the mixture expands to fill the cavity.

Open-cell foam expands approximately 100× its liquid volume. The cell structure is open (think of a sponge) — soft, flexible, and vapor-permeable. It's blown with water and CO₂, making it the lower-cost, lower-impact option.

Closed-cell foam expands roughly 35× its liquid volume. Each cell is completely sealed, trapping the blowing agent gas inside. This creates a denser, rigid material with higher R-value and built-in moisture resistance. The blowing agent is either HFC (older formulations, high environmental impact) or HFO (newer formulations, dramatically lower impact — more on that in the environmental section).

The expansion is what makes spray foam unique among insulation types. It fills gaps, conforms to irregular surfaces, and adheres directly to substrates — creating both an insulating layer and a continuous air barrier in a single application. No other insulation does both.

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Open-Cell vs Closed-Cell: Complete Comparison

These are fundamentally different products. Choosing the wrong one for your application is one of the most expensive mistakes in insulation.

For a detailed head-to-head, see our open-cell vs closed-cell spray foam comparison.

Pro Tip: The single most common spray foam mistake we see: using open-cell in basements and crawl spaces. Open-cell foam is vapor-permeable (12–20 perms) — moisture from the soil and concrete passes right through it, potentially causing problems in the wall assembly. Below grade, always use closed-cell. Its vapor permeability drops below 1 perm at just 1 inch of thickness, giving you a built-in moisture barrier.

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R-Value Performance

Closed-cell spray foam delivers the highest R-value per inch of any common insulation material at R-6.0 to R-7.0. Open-cell sits at R-3.5 to R-3.8 per inch — similar to dense-pack cellulose and standard fiberglass batts.

Here's what that translates to in real wall cavities:

The closed-cell numbers in a 2×6 cavity are remarkable — R-33 to R-38.5 from cavity fill alone, exceeding what most climate zones require for the entire wall assembly. A 2×4 wall with closed-cell spray foam delivers R-21 to R-24.5, which matches or beats the 2021 IECC requirement for zones 5–8 (R-20+5ci per Table R402.1.3) without any continuous exterior insulation.

That said, R-value per inch is only part of the equation. Open-cell foam at R-3.7/inch looks modest on paper, but it provides an air barrier that fiberglass batts at R-3.7/inch cannot. The air sealing component is where spray foam earns its premium. See What Is R-Value? for why the label number isn't the whole story.

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Cost Breakdown

Spray foam is the most expensive common insulation. We don't sugarcoat that — but we also don't think price alone tells the full story. Here's what you'll actually pay in 2025–2026.

Cost by Project Type (2025–2026 Installed)

Board Foot Pricing

Spray foam is priced per "board foot" — one square foot at one inch thick. Open-cell runs $0.35–$0.55 per board foot; closed-cell runs $1.00–$2.50 per board foot. To calculate your cost: multiply square footage × desired thickness in inches × board foot price.

Example: 1,000 sq ft of closed-cell at 3 inches thick = 3,000 board feet × $1.50/bf = $4,500.

That minimum job fee of $1,000–$2,000 is real and non-negotiable. Spray foam rigs cost $80,000–$150,000, and the setup/cleanup time is the same whether you're spraying 100 sq ft or 1,000. For small projects under $1,500 in material, you're paying mostly for the installer to show up.

For a detailed cost breakdown with regional pricing, see our spray foam insulation cost guide and insulation cost calculator.

Pro Tip: Always get at least three quotes for spray foam work. We've seen pricing vary by 40–60% between contractors in the same metro area. Ask each installer: what blowing agent do you use (HFO vs HFC)? What's your per-board-foot price? What thickness are you proposing? Are you SPFA-certified? The cheapest bid isn't always the best — spray foam installation quality varies dramatically.

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When to Use Spray Foam (And When NOT To)

This is the section that matters most. Spray foam is an exceptional product in the right application and an expensive waste of money in the wrong one.

Use Spray Foam When:

Rim joists — The single best application for closed-cell spray foam. Two inches of closed-cell (R-12 to R-14) on the rim joist provides insulation, an air barrier, and a vapor retarder in one pass. Rim joists are notoriously hard to seal with batts, and they're a major source of air leakage. We recommend closed-cell spray foam on rim joists in virtually every project. The rim joist insulation guide.

Basements and crawl spaces — Closed-cell directly on concrete foundation walls is our preferred approach. Two to three inches gives you R-12 to R-21 plus a complete air and vapor barrier. No need for a separate poly vapor barrier, no worry about trapped moisture. Our basement insulation guide and crawl space guide.

Conditioned attics (underside of roof deck) — When you want to bring the attic inside the thermal envelope (common with HVAC equipment in the attic), open-cell spray foam on the underside of the roof deck is the standard approach. Five to six inches delivers R-17.5 to R-22.8 and creates a conditioned attic space. The attic insulation guide.

Cathedral ceilings — Fixed rafter depth with no accessible attic space above. Open-cell or closed-cell spray foam fills the rafter bay completely and air seals. Check the cathedral ceiling insulation guide.

Limited-depth cavities — Anytime you need maximum R-value in minimum space, closed-cell foam at R-6.0–R-7.0 per inch is unmatched. A 2×4 wall with closed-cell delivers R-21+ — try that with any other material.

Do NOT Use Spray Foam When:

Open attics with unlimited depth — This is where we see the most wasted money. If your attic has an accessible flat floor with unlimited headroom, blown-in cellulose at $0.60–$2.30/sq ft achieves R-49 to R-60 for a fraction of spray foam's cost. A 1,000 sq ft attic to R-49: cellulose costs $1,200–$3,500 installed; closed-cell spray foam costs $4,500–$7,000. Same R-value, 2–5× the price. Cellulose wins unless you specifically need a conditioned attic.

Budget-constrained projects — If the budget is tight, fiberglass batts at $0.30–$1.50/sq ft installed will get insulation in the walls. Pair it with caulk and canned foam for air sealing at penetrations, and you'll get 80% of the performance at 20% of the cost.

DIY projects — Spray foam requires professional equipment, training, and safety protocols. DIY kits exist for small jobs (more on that below), but anything over 200 sq ft should be professionally installed.

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Air Sealing: The Real Value Proposition

Here's the truth about spray foam that manufacturers don't emphasize enough: its real advantage isn't R-value — it's air sealing.

The DOE estimates that proper insulation combined with air sealing reduces heating and cooling costs by 15–25%. Of that, air sealing alone accounts for 10–20% of savings. Traditional insulation (fiberglass batts, blown-in cellulose) addresses conductive heat transfer but does little to stop air leakage. Spray foam addresses both simultaneously.

The conventional approach to air sealing + insulation is a two-step process: seal all penetrations, top plates, recessed lights, and gaps with caulk and canned foam first, then install insulation second. Spray foam collapses both steps into one — it expands into every crack and gap while also providing R-value.

On a typical 1,500 sq ft ranch in climate zone 5, we've measured blower door results of 2.5–3.5 ACH50 (air changes per hour at 50 pascals) with spray foam versus 4.5–6.0 ACH50 with well-installed fiberglass batts and separate air sealing. That difference translates to meaningful energy savings — typically $150–$300/year in heating and cooling.

For a deeper comparison, see our guide on air sealing vs. insulation.

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Spray Foam by Application

Attic

Conditioned attic (foam on roof deck): Open-cell spray foam on the underside of the roof sheathing, 5.5–7 inches thick (R-19 to R-27). This brings ductwork and HVAC equipment inside the conditioned space, improving system efficiency by 15–20%. A 1,000 sq ft attic roof deck costs $3,000–$5,000 for open-cell or $4,500–$7,000 for closed-cell.

Unconditioned attic (foam on attic floor): Generally not the best use of spray foam dollars. Blown-in cellulose on the attic floor achieves the same R-value for 2–5× less cost. See attic insulation guide.

Walls

Closed-cell in a 2×6 wall delivers R-33+ — well above code for any zone. A popular cost-saving approach: "flash and batt" — 2 inches of closed-cell spray foam on the sheathing side (R-12 to R-14 plus air/vapor barrier), then fill the remaining cavity with fiberglass batt. This delivers roughly R-25 in a 2×6 wall at 40–60% of full spray foam cost. See wall insulation guide.

Crawl Space

Closed-cell on crawl space walls: 2–3 inches gives R-12 to R-21 with a built-in vapor barrier. Always closed-cell below grade. See crawl space guide.

Basement

Same approach as crawl space — closed-cell directly on concrete. We insulated a 1960s ranch basement in climate zone 5 last year with 3 inches of closed-cell (R-19.5) on the foundation walls. The homeowner's heating bill dropped $45/month that first winter — a payback period under 4 years on a $3,800 job. See basement insulation guide.

Rim Joists

Two inches of closed-cell on the rim joist (R-12 to R-14) — fast, effective, and the best application for spray foam dollar-for-dollar. Rim joists are responsible for a disproportionate amount of air leakage in typical homes. See rim joist insulation guide.

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Health, Safety & Off-Gassing

We'll be straight with you: spray foam involves chemicals that require serious safety precautions during installation. After curing, the risks are minimal.

During Installation: The A-side component (MDI isocyanate) is a respiratory sensitizer — exposure can cause asthma, breathing difficulties, and skin irritation. Professional installers wear full-face respirators and protective suits. This is non-negotiable.

Occupants must vacate the building during spraying and for at least 24 hours after application (some products require 48–72 hours). Ventilate the space thoroughly before re-entry. Pets must also be removed.

After Curing: Properly mixed and applied spray foam is generally considered inert after curing (24–72 hours). The EPA has published guidance on spray polyurethane foam confirming that cured foam produces minimal off-gassing under normal conditions.

Thermal Barrier Requirement: Per building code (IBC/IRC), spray foam must be covered by a 15-minute thermal barrier — typically ½-inch drywall — in occupied spaces. Spray foam cannot be left exposed in living areas, basements, or garages. In attics and crawl spaces not used for storage, an ignition barrier (thinner covering) may suffice per local code.

Spray foam IS combustible. Both open-cell and closed-cell have a Flame Spread Index of ≤75 and Smoke Development Index of ≤450. They require thermal or ignition barriers per code. Compare this to mineral wool (FSI 0, SDI 0, non-combustible) and unfaced fiberglass (FSI ≤25, SDI ≤50, non-combustible). See our fire safety guide.

For a deeper look at the health debate, see Is Spray Foam Toxic?.

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Environmental Impact

Spray foam's environmental story is complicated — and the type you choose matters enormously.

Open-cell spray foam is water/CO₂-blown with a Global Warming Potential (GWP) near zero for the blowing agent. From an environmental standpoint, it's comparable to fiberglass and cellulose.

Closed-cell with older HFC-245fa blowing agent has a GWP of 858 — meaning one pound of the blowing agent has the same climate impact as 858 pounds of CO₂. This is significant. A typical whole-home closed-cell job uses enough blowing agent to produce thousands of pounds of CO₂-equivalent emissions.

Closed-cell with newer HFO blowing agents has a GWP of just 1–3. This is a massive improvement and largely eliminates the climate concern. Major manufacturers like Honeywell and Chemours have transitioned most product lines to HFO, and SPFA member contractors are following.

Always ask your installer which blowing agent they use. If they don't know or can't tell you, that's a red flag. Insist on HFO-blown product for closed-cell applications.

One more environmental consideration: spray foam cannot be recycled or composted at end of life. It goes to landfill. Cellulose (80–85% recycled content, compostable) and fiberglass (40–60% recycled content) have better end-of-life profiles.

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DIY Spray Foam Kits

Consumer spray foam kits are available at home centers and online, typically covering 200–600 sq ft at 1 inch of thickness for $300–$600. They're closed-cell only (no open-cell kits for consumers).

When DIY kits make sense: Rim joists in an accessible basement — 150–200 linear feet of rim joist is the sweet spot. The kit pays for itself versus hiring a contractor with a $1,000–$2,000 minimum job fee. Apply 2 inches for R-12 to R-14 plus an air barrier.

When DIY kits don't make sense: Everything else. Consumer kits produce inconsistent thickness, the foam chemistry is harder to control (temperature-sensitive mixing), and you'll waste 20–30% of the material on learning curve and overspray. For any project over 200 sq ft, the per-square-foot cost of a DIY kit approaches professional pricing while delivering inferior results.

Our honest recommendation: DIY kits are fine for rim joists and small gap-filling. Hire a certified professional for walls, attics, crawl spaces, and any project over 200 sq ft.

If you do use a DIY kit: Store the tanks at room temperature (60–80°F) for 24 hours before use — cold tanks produce off-ratio foam that won't cure properly. Practice on a scrap piece of plywood first to get comfortable with the spray pattern and trigger control. Apply in multiple thin, even passes (½ inch each) rather than one thick application, and wear a full-face respirator with organic vapor cartridges. Keep the space ventilated and unoccupied for at least 24 hours after application. For more detailed cost comparisons between DIY kits and professional installation, check our spray foam cost guide.

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Spray Foam Myths Debunked

"Spray Foam Will Cut Your Energy Bills in Half"

Manufacturer marketing often claims 30–40% energy savings. Independent data from field studies puts the number closer to 20–30% compared to an uninsulated space — and much of that benefit comes from the air sealing, not the R-value alone. If you're comparing spray foam to an existing insulated wall with separate air sealing, the marginal improvement is smaller. The DOE's estimate of 15–25% savings for proper insulation + air sealing is more realistic for a whole-home upgrade.

"Spray Foam Causes Long-Term Health Problems"

The health concerns are real but limited to installation. MDI isocyanate exposure during spraying requires full PPE and building evacuation. After the foam cures (24–72 hours with proper ventilation), it's generally inert. Millions of homes have spray foam installed with no long-term health issues reported in the cured state. The key: hire a qualified installer who follows proper mixing ratios and application protocols.

"You Can DIY Spray Foam to Save Money"

Spray foam quality depends entirely on proper mixing ratios, substrate temperature, application thickness, and equipment calibration. Professional rigs maintain precise temperature and pressure control that consumer kits cannot match. A bad spray foam job — off-ratio foam that doesn't cure properly — can off-gas indefinitely and may need to be torn out entirely at enormous cost. This is not a DIY-friendly material for anything beyond small-scale applications.

"Open-Cell and Closed-Cell Are Interchangeable"

They are not. Using open-cell below grade allows moisture infiltration (12–20 perms vs <1 perm for closed-cell). Using closed-cell where open-cell would suffice wastes money — 2–3× more per square foot for applications where the moisture barrier and structural benefits aren't needed.

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Common Mistakes

1. Using open-cell spray foam below grade. Open-cell is vapor-permeable (12–20 perms). In basements and crawl spaces where moisture drives through concrete, this allows water vapor into the wall assembly. Always use closed-cell below grade. The basement guide.

2. Not hiring a certified installer. Spray foam chemistry is precise — the A-side and B-side must be mixed at correct ratios and temperatures. Off-ratio foam doesn't cure properly, can shrink, crack, or off-gas indefinitely. Verify your installer is trained and certified through SPFA (Spray Polyurethane Foam Alliance) or the manufacturer.

3. Spray-foaming an open attic floor instead of using blown-in. If your attic is unconditioned with a flat accessible floor, blown-in cellulose or fiberglass achieves R-49 to R-60 for $1,200–$3,500 versus $4,500–$7,000+ for spray foam. Spray foam makes sense on the roof deck for conditioned attics — not on the attic floor.

4. Skipping the thermal barrier. Spray foam is combustible and must be covered by ½-inch drywall (or equivalent 15-minute thermal barrier) in occupied spaces per IBC/IRC. We've seen unfinished basements with exposed spray foam that fails code inspection. Budget for drywall over the foam.

5. Not comparing total cost against alternatives. Spray foam's air sealing benefit is real, but a combination of caulk/canned foam air sealing ($200–$500 for a thorough job) plus blown-in cellulose ($0.60–$2.30/sq ft) often achieves 80–90% of spray foam's performance at 30–50% of the cost. Run both scenarios before committing. Use our insulation cost calculator.

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For head-to-head comparisons with other materials, check spray foam vs fiberglass and spray foam vs cellulose.

Key Takeaways

• Spray foam comes in two types: open-cell (R-3.5–3.8/inch, $1.00–$3.50/sq ft) and closed-cell (R-6.0–7.0/inch, $1.50–$5.00/sq ft). They are not interchangeable.
• Both types create air barriers — spray foam's primary value is combining insulation + air sealing in one step.
• Closed-cell acts as a Class II vapor retarder at 1.5–2 inches and adds structural rigidity (racking strength up to 250–300%).
• Best applications: rim joists (closed-cell), basements/crawl spaces (closed-cell), conditioned attics (open-cell on roof deck), cathedral ceilings, limited-depth cavities.
• Worst application: open attic floors where blown-in insulation achieves the same R-value for 2–5× less money.
• Always use closed-cell below grade. Open-cell is vapor-permeable and will allow moisture through.
• Spray foam is combustible — it requires a thermal barrier (½" drywall) in occupied spaces per building code.
• Occupants must vacate during installation and for 24+ hours after. Hire certified professionals only.
• For closed-cell, insist on HFO-blown product (GWP 1–3) instead of older HFC formulations (GWP 858).
• The DOE estimates insulation + air sealing saves 15–25% on heating and cooling — spray foam delivers both, but at a premium price.

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FAQ

How long does spray foam insulation last?

Spray foam material lasts 80–100+ years. Closed-cell foam may lose a small amount of R-value over decades as blowing agents slowly dissipate, but the reduction is minimal (a few percent over 20+ years). Open-cell foam maintains its R-value indefinitely as long as it stays dry and isn't physically damaged. Both types outlast fiberglass batts (which sag and gap over 15–30 years) and cellulose (which settles ~20%). The foam itself doesn't degrade — the assembly around it is more likely to need attention.

Is spray foam insulation worth the cost?

It depends entirely on the application. For rim joists, basements, crawl spaces, and conditioned attics — yes, spray foam is often the most cost-effective solution when you factor in the combined insulation + air sealing + vapor barrier benefits. For open attic floors with unlimited depth, no — blown-in cellulose or fiberglass achieves the same R-value for 2–5× less. For walls in new construction, consider the "flash and batt" approach (2" closed-cell + fiberglass fill) as a cost-effective middle ground. See our spray foam cost guide for a full analysis.

Can spray foam cause moisture problems?

Open-cell spray foam is vapor-permeable (12–20 perms) — it does NOT block moisture. Using it in below-grade applications or on the exterior side of a wall in cold climates can allow moisture accumulation. Closed-cell spray foam at 1.5–2 inches acts as a Class II vapor retarder (<1 perm), which is appropriate for most wall, basement, and crawl space assemblies. The key is matching the right type to the right application. See our vapor barrier guide.

Do you need a vapor barrier with spray foam?

With closed-cell spray foam at 2+ inches, no — the foam itself is a Class II vapor retarder. With open-cell spray foam, it depends on your climate zone and assembly. In cold climates (zones 5+), many codes require a vapor retarder on the warm side of open-cell foam assemblies — either a separate membrane or a vapor-retarder paint. Check your local code requirements.

How do I find a good spray foam installer?

Look for installers certified through the SPFA (Spray Polyurethane Foam Alliance) or directly through the foam manufacturer. Ask for references from jobs completed in the last 6 months. Verify they carry liability insurance — a bad spray foam job can require full removal at $10,000+. Get three quotes minimum. Ask each installer about their blowing agent (insist on HFO for closed-cell), proposed thickness, and whether they'll provide a thermal barrier. Avoid any installer who quotes by phone without seeing the job site.