Open-Cell vs Closed-Cell Spray Foam: Which Do You Need?

Open-cell and closed-cell spray foam are not two versions of the same product — they're fundamentally different materials that solve different problems. Choosing the wrong one wastes money at best and causes moisture damage at worst. The decision comes down to three factors: where you're installing, whether you need moisture control, and how much R-value you need in a limited space.

Quick Answer: Use open-cell spray foam (R-3.5–3.8/inch, $1.00–$3.50/sq ft) for conditioned attics, cathedral ceilings, and interior walls where moisture drive isn't a concern. Use closed-cell (R-6.0–7.0/inch, $1.50–$5.00/sq ft) for basements, crawl spaces, rim joists, and any application needing vapor control or maximum R-value in limited depth. Both create air barriers. Closed-cell also adds structural rigidity and acts as a Class II vapor retarder at 1.5–2 inches.

Table of Contents

• Master Comparison Table
• R-Value Comparison
• Cost Comparison
• Moisture & Vapor Control — The Critical Difference
• Application-by-Application Recommendations
• Structural Properties
• Environmental Comparison
• Sound Performance
• Common Mistakes
• Key Takeaways
• FAQ

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Master Comparison Table

For detailed guides on each type, check open-cell spray foam and closed-cell spray foam. Our main spray foam insulation guide covers both in depth.

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R-Value Comparison

Closed-cell delivers roughly 2× the R-value per inch — and in fixed-depth cavities, that's a massive difference.

In a 2×4 wall, closed-cell achieves R-21+ — exceeding the 2021 IECC requirement for climate zones 5–8 (R-20+5ci per Table R402.1.3) from cavity fill alone. Open-cell in the same cavity delivers R-12 to R-13, which barely meets the zone 1–2 minimum (R-13).

For attic roof deck applications where rafter depth is 5.5 inches (2×6) or more, the R-value gap matters less in practical terms. Five and a half inches of open-cell gives R-19 to R-21 — adequate for most conditioned attic applications. The same thickness of closed-cell gives R-33+, which is excellent but expensive overkill for a conditioned attic where the goal is simply to keep the space above 55°F.

Winner: Closed-cell — nearly double the R-per-inch. But for many applications (attic roof deck, cathedral ceilings), open-cell provides sufficient R-value at significantly lower cost.

For the full R-value ranking across all materials, the R-value per inch chart has the complete data.

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Cost Comparison

Closed-cell costs 50–100% more than open-cell on a per-square-foot basis, and the gap widens at greater thicknesses (since closed-cell is priced per board foot and each board foot costs 2–4× more). The DOE's insulation guide recommends evaluating spray foam on a total-cost basis — material + labor + the value of air sealing — rather than comparing per-square-foot costs alone.

When the premium is justified: Any application where closed-cell's moisture control, structural reinforcement, or R-value density delivers a benefit you can't get from open-cell — specifically basements, crawl spaces, rim joists, and limited-depth cavities in cold climates.

When the premium is wasted: Conditioned attic roof decks (open-cell provides adequate R-value and air sealing at 40–50% lower cost) and interior wall applications where moisture drive is not a concern.

For full cost data by project type, our spray foam cost guide breaks down every scenario. The insulation cost calculator generates project-specific estimates.

Pro Tip: Both open-cell and closed-cell carry the same minimum job fee — typically $1,000–$2,000. For small projects like rim joists (150 linear feet = ~100–200 sq ft), closed-cell makes more sense than open-cell even on a cost basis because you need fewer inches (2" of closed-cell at R-12 to R-14 vs 4" of open-cell at R-14 to R-15) and the material difference is negligible against the fixed mobilization cost.

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Moisture & Vapor Control — The Critical Difference

This is the technical distinction that determines which type to use in most real-world situations — and getting it wrong causes the most expensive failures.

Open-cell spray foam is vapor-permeable: 12–20 perms at typical thickness. Moisture passes through it freely. It is NOT a vapor retarder. In below-grade applications, moisture drives continuously from soil through concrete into the building interior — open-cell does nothing to stop it. In cold-climate walls, interior moisture can pass through open-cell and condense on cold sheathing.

Closed-cell spray foam is a vapor retarder: At 1.5–2 inches, it drops below 1 perm — qualifying as a Class II vapor retarder per building code. At 2+ inches, it effectively functions as a vapor barrier. This built-in moisture control eliminates the need for a separate poly sheet or membrane in many assemblies.

What this means by application:

• Below-grade (basement, crawl space): Closed-cell always. The constant moisture drive from soil and concrete will pass directly through open-cell foam, potentially causing mold and deterioration behind the insulation.
• Rim joists: Closed-cell always. Rim joists are cold surfaces that condense moisture in winter — the vapor retarder property is essential.
• Conditioned attic (roof deck): Open-cell is typically fine. In cold climates (zones 5+), some codes require a vapor retarder on the interior side of open-cell foam — check your local code requirements. A coat of vapor-retarder paint over the open-cell satisfies this.
• Walls in cold climates: Closed-cell preferred. Its vapor retarder keeps interior moisture from reaching cold sheathing — a major cause of wall failures in northern climates.

For the full moisture management picture, our vapor barrier guide explains the complete system.

Pro Tip: The single most expensive spray foam mistake we see: open-cell on basement walls or crawl space walls. The foam goes on beautifully, passes inspection (air barrier requirement met), and then six months later the homeowner notices musty smells or visible mold behind the foam. Open-cell at 12–20 perms does nothing to stop the constant moisture vapor drive through concrete. Always closed-cell below grade.

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Application-by-Application Recommendations

Real Project Cost Comparison

To make this concrete, here's what these choices look like on common residential projects:

Project 1: Conditioned attic, 1,200 sq ft roof deck, Zone 4

• Open-cell (6", R-22): $3,600–$6,000 installed
• Closed-cell (3.5", R-24.5): $5,400–$8,400 installed
• Our recommendation: open-cell — saves $1,800–$2,400 with comparable R-value

Project 2: Crawl space walls, 400 sq ft, Zone 5

• Open-cell: not recommended (moisture risk)
• Closed-cell (2", R-14): $2,000–$3,600 installed
• Our recommendation: closed-cell only — the vapor control is essential below-grade

Project 3: Rim joists, typical 2-story home, ~160 linear ft

• Open-cell: not recommended (cold condensation surface)
• Closed-cell (2", R-14): $800–$2,000 installed
• Our recommendation: closed-cell always — highest-ROI spray foam application in any home

Can You Combine Open-Cell and Closed-Cell?

Yes — "flash and batt" or "flash and fill" uses a thin layer of closed-cell (1–2 inches) against the substrate for air sealing and vapor control, then fills the remaining cavity with a cheaper material (open-cell foam, fiberglass batts, or mineral wool). This hybrid approach is common in walls and cathedral ceilings where you want closed-cell's moisture properties but not its full-cavity cost. A 2×6 wall with 2" closed-cell (R-14) + R-11 fiberglass fill delivers R-25 at roughly 60% of the cost of filling the entire cavity with closed-cell.

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Structural Properties

Closed-cell spray foam at 2.0 lb/ft³ density adds meaningful structural rigidity to framing assemblies. Manufacturer testing data shows wall racking strength increases of up to 250–300% and roof wind-uplift resistance improvements of approximately 250% when closed-cell fills the cavity.

Open-cell foam at 0.5 lb/ft³ is soft and flexible — squeeze it between your fingers and it compresses easily. It adds zero structural benefit.

Where structural reinforcement matters: Hurricane- and tornado-prone regions, older homes with weakened framing, and any assembly where wind uplift is a design concern. FEMA and ICC have published guidance recommending closed-cell spray foam as part of wind-resistant building assemblies in high-wind zones.

Weight considerations: Closed-cell at 2.0 lb/ft³ is heavy compared to other insulation types. A 2×6 wall cavity (5.5" deep, 14.5" wide, 8 ft tall) filled with closed-cell weighs approximately 9–10 lbs per bay — noticeable in aggregate over an entire building but well within typical framing load capacity. Open-cell at 0.5 lb/ft³ is negligible at roughly 2.5 lbs per bay.

Closed-cell for rim joists and headers: Even when open-cell is the primary insulation choice for walls or roof assemblies, we often recommend switching to closed-cell for rim joists and headers specifically. These areas are the coldest spots in the building envelope, most vulnerable to condensation, and benefit most from closed-cell's combined R-value, air sealing, and vapor control. The added cost for these small, targeted areas is modest, and the moisture protection is absolutely worth every dollar. This approach lets you get the best of both worlds — affordable open-cell coverage where it's appropriate and targeted closed-cell protection at the most vulnerable junctions.

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Environmental Comparison

Open-cell is water/CO₂ blown with near-zero Global Warming Potential from its blowing agent. From an environmental perspective, it's comparable to cellulose and fiberglass.

Closed-cell with newer HFO blowing agents has a GWP of just 1–3 — essentially negligible. Older HFC-245fa formulations carry a GWP of 858, meaning each pound of blowing agent equates to 858 pounds of CO₂ in climate impact. The industry is actively transitioning to HFO, but not all products have switched.

Always ask your installer which blowing agent they use. If they're still using HFC for closed-cell, request HFO-blown product. Building Science Corporation and most building science organizations recommend HFO-blown closed-cell for environmental reasons.

Neither type can be recycled at end of life. Both go to landfill. If lifecycle environmental impact is a top priority, cellulose (80% recycled, compostable) or mineral wool (inert, recyclable) have better profiles.

Winner: Open-cell — lower environmental impact regardless of blowing agent generation. Closed-cell HFO formulations have closed the gap significantly.

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Sound Performance

Open-cell foam is slightly better for sound absorption due to its soft, flexible cell structure:

If soundproofing is a priority, neither spray foam type is optimal. Mineral wool outperforms both by a wide margin. We've had clients spend $5,000+ on closed-cell in shared walls for sound isolation and get worse results than a $1,500 mineral wool install.

Winner: Open-cell (marginally), but mineral wool beats both for sound.

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Common Mistakes

1. Using open-cell below grade. Open-cell foam at 12–20 perms allows moisture vapor from soil and concrete to pass through and accumulate in the assembly. Basements, crawl spaces, and rim joists require closed-cell for moisture control.

2. Over-specifying closed-cell in conditioned attics. A 1,000 sq ft attic roof deck with closed-cell costs $4,500–$7,000 versus $3,000–$5,000 for open-cell. In most conditioned attic applications, the extra R-value and vapor control of closed-cell aren't necessary — open-cell's air sealing and adequate R-value (R-19+ in a 2×6 rafter) do the job at 40–50% less.

3. Not accounting for thermal barrier requirements. Both open-cell and closed-cell are combustible (FSI ≤75, SDI ≤450) and must be covered by a 15-minute thermal barrier (typically ½" drywall) in occupied spaces per IBC/IRC. Exposed spray foam in a finished basement fails code inspection. See our fire safety guide.

4. Assuming open-cell and closed-cell are interchangeable. They are not. The vapor permeability difference (12–20 perms vs <1 perm) makes the wrong choice in the wrong location an active liability — not just a suboptimal insulation decision but a potential moisture damage problem.

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Key Takeaways

• 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) are not interchangeable. The application determines which to use.
• Both types create air barriers — spray foam's primary value is insulation + air sealing in one step.
• The critical difference is moisture: open-cell is vapor-permeable (12–20 perms); closed-cell at 2" is a Class II vapor retarder (<1 perm).
• Closed-cell always: basements, crawl spaces, rim joists, cold-climate walls (zones 5+).
• Open-cell usually: conditioned attic roof decks, cathedral ceilings, interior walls.
• Neither: unconditioned attic floors — blown-in cellulose/fiberglass is 2–5× cheaper for the same R-value.
• Closed-cell adds structural rigidity (racking strength up to 300%); open-cell adds none.
• Open-cell is water/CO₂ blown (low GWP). For closed-cell, insist on HFO-blown product (GWP 1–3) over older HFC (GWP 858).
• Both types require a thermal barrier (½" drywall) in occupied spaces — neither can be left exposed.

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FAQ

Which is better for attics — open-cell or closed-cell?

For conditioned attics (insulating the roof deck), open-cell is usually the better choice. Five to six inches of open-cell in a 2×6 rafter bay delivers R-19 to R-23 — adequate for most applications — and creates a complete air barrier. A 1,000 sq ft attic costs $3,000–$5,000 for open-cell versus $4,500–$7,000 for closed-cell. The higher R-value of closed-cell is overkill for most conditioned attics. Exception: in very cold climates (zones 6–8), closed-cell provides vapor control that open-cell doesn't. For unconditioned attics, skip spray foam entirely — blown-in insulation on the floor delivers R-49–R-60 at a fraction of the cost.

Can I use open-cell in a basement?

No — we strongly advise against it. Open-cell is vapor-permeable (12–20 perms), which means moisture vapor from soil and concrete passes through the foam into the basement assembly. Over months, this leads to moisture accumulation, potential mold, and damage to framing behind the foam. Always use closed-cell on basement walls and crawl spaces. Two to three inches of closed-cell (R-12 to R-21) provides insulation, an air barrier, and a vapor retarder in one step.

Is closed-cell spray foam worth the extra cost?

In applications that require moisture control (basements, crawl spaces, rim joists), absolutely — there's no adequate substitute. In conditioned attics and cathedral ceilings, usually not. The 50–100% cost premium buys you higher R-per-inch and vapor retarder properties that are essential below-grade but unnecessary in most above-grade applications. Run the numbers for your specific project using our insulation cost calculator.

How thick should open-cell and closed-cell be?

Open-cell: 3.5 inches minimum in wall cavities (R-12 to R-13), 5.5–7 inches on roof decks (R-19 to R-27). Closed-cell: 2 inches minimum to achieve vapor retarder status (R-12 to R-14); 3–4 inches for maximum R-value in limited cavities (R-18 to R-28). For rim joists, 2 inches of closed-cell is the standard. Check our insulation thickness chart for all R-value targets by material.

Can I combine open-cell and closed-cell in the same project?

Yes — and we do this regularly. The most common combo: closed-cell on rim joists and basement walls (where vapor control is critical), then open-cell on the attic roof deck (where it's not). Different areas of the same home have different moisture requirements. Using closed-cell everywhere would work but costs significantly more than targeting it where it's actually needed. Another combination: 2 inches of closed-cell on the cavity side of wall sheathing for air/vapor sealing, then filling the remaining cavity with fiberglass or cellulose — the "flash and batt" approach that delivers high performance at moderate cost.