Open-Cell vs Closed-Cell Spray Foam Insulation: What’s the Real Difference?

Not all spray foam is created equal.

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Some of it’s soft and squishy, some of it’s dense and tough. One breathes. The other blocks. And if you pick the wrong one for your space, you could end up with insulation that’s doing half the job—or worse, holding onto moisture like a sponge in your walls.

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Let’s break down the difference between open-cell and closed-cell spray foam insulation so you can make the right call for your home—and your wallet.

What Is Spray Foam Insulation?

Let’s start with the basics. Spray foam insulation is like the Swiss Army knife of insulating materials—it seals, insulates, and strengthens in one go. Unlike traditional insulation that you roll out or blow in, spray foam starts as a liquid and expands into a solid foam, filling every nook, cranny, and gap along the way. It’s basically insulation with a superpower: air sealing.

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And not all spray foam is created equal. There are two main types—open-cell and closed-cell—and while they might sound like a minor technical distinction, the differences are pretty dramatic once you get into R-values, moisture resistance, and cost.

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Before we dig into those, here’s the punchline: spray foam doesn’t just sit in your walls—it actively blocks heat, air, and moisture from doing whatever they want. That’s a big deal in places like Connecticut, where the summers are sticky, the winters bite, and energy bills are always lurking.

Open-Cell Spray Foam — The Light and Airy Option

Open-cell spray foam is the easygoing, flexible type. It’s soft to the touch—kind of like the inside of a sneaker—and it expands like crazy when it’s sprayed in. We’re talking up to 100 times its original size. That makes it perfect for squeezing into tight, irregular spaces, which you’ll find plenty of in older Connecticut homes.

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So what makes it “open-cell”?
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It’s all in the structure. The tiny cells inside the foam aren’t completely closed—they’re left open, which gives the material that spongy texture. This also means air can pass through more easily than with its closed-cell cousin. That might sound like a bad thing, but it’s actually useful in the right context.

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The Pros

• Lower cost per board foot
• Excellent sound dampening
• Expands more, great for odd-shaped cavities

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The Cons

• Lower R-value (~R-3.5 to R-4 per inch)
• Not a moisture barrier
• No structural reinforcement

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Best Places to Use It

Attics, interior walls, vaulted ceilings—anywhere you want air sealing without needing a moisture barrier or high R-value. If you’re sealing up a drafty old Cape, this stuff can make a night-and-day difference.

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It won’t stop a flood, but it’ll stop a draft—and in a 1920s shoreline bungalow with paper-thin walls, that’s worth its weight in comfort.

Closed-Cell Spray Foam — The Dense, High-Performance Option

If open-cell is the laid-back cousin, closed-cell spray foam is the overachiever in the family. It’s dense. It’s rigid. It packs an R-value punch. When you need serious insulation horsepower—or you’re dealing with moisture, strength, or space constraints—closed-cell is your go-to.

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So what makes it “closed-cell”?
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Each microscopic bubble in the foam is sealed tight—no gaps, no air pockets. That structure makes it impermeable to water, incredibly strong, and insanely good at stopping heat flow. It also dries into a hard surface, adding structural integrity wherever it’s sprayed. It’s insulation and a mini fortification all in one.

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The Pros

• High R-value (~R-6 to R-7 per inch)
• Blocks moisture and vapor
• Adds structural support

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The Cons

• Higher cost
• Less expansion, requires more precision
• Not great for soundproofing

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Best Places to Use It

Basements, crawl spaces, exterior walls, under slabs—anywhere that needs to stay dry and tight. It’s the go-to for homes near the water table or for tight wall cavities that need big R-value in a small space.

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If open-cell is the cozy sweater, closed-cell is the weatherproof jacket—tough, tight, and built to take on the elements.

Open-Cell vs Closed-Cell: The Spray Foam Showdown

Let’s line them up side by side:

Quick Take:

• Need max insulation in tight space? Go closed-cell.
• Just want solid air sealing without breaking the bank? Open-cell might do the job.
• Dealing with moisture or structural needs? Closed-cell wins that round.
• On a budget and sealing an attic? Open-cell’s your friend.

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How to Choose the Right Spray Foam for Your Home

So, which one should you go with—open-cell or closed-cell? Like most things in home construction, the answer is: it depends. On what? Let’s break it down.

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1. What’s the job?

• Sealing up an attic or interior walls? Open-cell is usually plenty—and easier on the wallet.
• Trying to stop moisture in a basement or crawlspace? Closed-cell every time.
• Need to add strength to a roof deck or wall assembly? Closed-cell can actually stiffen the structure.

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2. How much space do you have to work with?

If you’re trying to hit a high R-value in a shallow cavity (like a 2x4 wall), closed-cell is the only way you’ll get there. Open-cell needs more thickness to reach the same thermal performance.

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3. What’s your budget?

No surprise here—closed-cell costs more. On average, 2–3 times more per board foot. But if you need a moisture barrier, structural support, or higher R-value per inch, it might be worth every penny.

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4. How’s the moisture situation?

If the space sees dampness, condensation, or risk of water intrusion, closed-cell is your shield. Open-cell can absorb and hold water—not what you want near wood framing or masonry.

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5. What kind of home are we talking about?

Here in Connecticut, we’ve insulated everything from 100-year-old shoreline colonials to modern new builds in the hills. Each one has different needs.

• Old homes with leaky attics? Open-cell’s great for chasing down air leaks.
• Basements near the water table? Closed-cell is non-negotiable.

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Final Thoughts: Don’t Just Spray and Pray

Open-cell vs closed-cell might sound like a niche debate, but when it comes to your comfort, energy bills, and moisture protection—it matters. A lot.

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If you’re insulating an attic in a drafty shoreline Cape, open-cell might be the hero you need. If you’re battling basement dampness or trying to squeeze max R-value into limited space, closed-cell is the heavyweight champ. Neither is “better”—they just solve different problems.

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Here’s the trick: spray foam is a powerful tool when it’s used correctly. But when it’s sprayed in the wrong place? You’ve just paid for high-end insulation that’s doing the wrong job.

Common FAQ's about Spray Foam

Can I mix open-cell and closed-cell spray foam in the same house?

You can mix open-cell and closed-cell spray foam in the same house. Use open-cell in attics for cost-effective air sealing and closed-cell in basements or crawlspaces for moisture protection. Choosing the right type depends on location-specific performance needs like air sealing, moisture resistance, and structural support.

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How much more does closed-cell spray foam cost compared to open-cell?

Closed-cell spray foam costs 2 to 3 times more per board foot than open-cell. The higher price reflects its density, superior R-value, and vapor barrier properties. Despite the cost, closed-cell is ideal for tight spaces or moisture-prone areas where performance justifies the investment.

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Is open-cell spray foam good enough for cold Connecticut winters?

Open-cell spray foam can be effective in cold Connecticut winters if applied thickly and with proper air sealing. Though it has a lower R-value per inch than closed-cell, it performs well in attics and walls when depth isn’t a constraint. Use closed-cell where space or moisture demands higher performance.

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Will spray foam insulation help with soundproofing between rooms?

Spray foam insulation can help with soundproofing between rooms if you use open-cell foam. Its soft structure absorbs airborne noise, making it ideal for interior walls and floors. Closed-cell foam is too dense for effective sound dampening and is better suited for moisture or structural applications.

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Does closed-cell spray foam really add structural support?

Closed-cell spray foam does add structural support by stiffening walls, roof decks, and other surfaces. While it doesn’t replace framing, it improves racking strength and wind resistance. This added support is especially beneficial in older homes or coastal areas with high wind exposure.

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