What Is Spray Foam Insulation? | Expands 30x for Sealing

A single can of spray foam does something no fiberglass batt can: it expands to fill every hidden nook, cranny, and drafty crack behind your walls. Formally called Spray Polyurethane Foam (SPF), it starts as a liquid blend of isocyanate (A-Side) and polyol resin (B-Side). When those two chemicals meet at the spray gun tip, they expand 30 to 60 times their liquid volume into a rigid, continuous seal. The payoff is a building envelope with dramatically lower air leakage, higher effective R-values, and better sound control compared to traditional insulation. But the two main types — open-cell and closed-cell — behave very differently, and picking the wrong one for your project is a costly mistake.

How Spray Foam Insulation Works

The chemical reaction between the A-Side and B-Side creates a thermoset cellular plastic that cures into a solid. During the reaction, trapped blowing agents cause the liquid to froth and expand, pushing the foam into every void. After curing — usually within minutes to a few hours — the material becomes a durable, moisture-resistant plastic that bonds directly to surrounding materials like wood, concrete, and drywall.

This bond means the foam acts as both insulation and a structural air barrier. Unlike fiberglass, which settles and leaves gaps, SPF conforms to irregular shapes and seals around wires, pipes, and joists on the first pass. The Spray Polyurethane Foam Alliance categorizes SPF by cell structure and density, which determines where each type performs best.

Open-Cell vs. Closed-Cell: The Two Types

The single most important decision you’ll make is whether to use open-cell or closed-cell foam. They share the same chemistry but produce very different materials suited to different parts of the house.

Open-cell spray foam (ocSPF), sometimes called low-density foam, creates a soft, flexible seal with a partially open structure that lets air and moisture vapor pass through. It has a density of about 0.5 pounds per cubic foot. It excels at soundproofing because the open cells absorb sound waves, but it offers poor moisture resistance, making it a bad choice for basements or exterior walls in wet climates.

Closed-cell spray foam (ccSPF) produces a rigid, dense material with fully closed cells. Its density is significantly higher, and trapped blowing agents inside each cell give it a much higher R-value per inch. Closed-cell foam also acts as a vapor barrier once its thickness exceeds about 1.5 inches, with a perm rating below 1.0. It’s the go-to choice for below-grade walls, roofs, and any area where moisture and high thermal resistance are critical. If you’re ready to buy, the best closed cell spray foam kit for your project can handle walls, crawl spaces, and rim joists with professional-grade results.

R-Value Per Inch: Thermal Performance

R-value measures how well the insulation resists heat flow — higher numbers mean better performance per inch of material. Here is how the two types compare based on data from industry sources and manufacturers.

Is Spray Foam Fireproof?

No. Spray foam is not fireproof, but it does have fire-resistant properties when treated. Many products achieve a Class A Fire Rated classification under the ASTM-E84 standard, meaning they resist flame spread and smoke development for a short period — sometimes up to nearly one hour. However, the raw chemicals (isocyanate and polyol) are flammable before they cure. Always keep spray foam away from open flames during application, and check the manufacturer’s fire rating for the specific product you buy.

Climate and Location: Which Type to Use Where

Your local climate and where the insulation sits in the building envelope matter more than personal preference. For above-grade walls in Climate Zone 5 and similar mixed-humid regions, open-cell foam is often the recommended choice because it allows walls to dry inward if moisture gets in. For below-grade walls in those same zones, closed-cell foam is the standard pick due to its vapor barrier properties.

In cold climates, closed-cell foam’s high R-value and vapor resistance make it the ideal insulator for the entire building shell — it keeps warm air inside and moisture out. In attics and crawl spaces, open-cell works well when you prioritize soundproofing over moisture resistance.

Cost Breakdown: Material and Installation

Spray foam costs more than fiberglass but pays back in energy savings over time. The price depends on the type of foam, the size of the project, and whether you hire a pro or use a DIY kit.

Open-cell material runs between $0.44 and $1.00 per board foot. Closed-cell material is $1.00 to $1.50 per board foot. Professional installation adds $1.00 to $3.00 per board foot on top of the material cost, covering equipment, labor, and cleanup. For a typical 2×4 wall cavity, one board foot equals one square foot at one inch thick.

How To Apply Canned Spray Foam (DIY Steps)

For small gaps around windows, doors, pipes, and electrical boxes, a do-it-yourself can of spray foam from Lowe’s or Home Depot is the most practical approach. Follow these steps from the official Lowe’s documentation for the best results.

1. Read the full label before you start — each brand cures at a slightly different rate.
2. Gear up: Wear safety goggles, work gloves, and long sleeves. Cover surrounding surfaces with a drop cloth or newspaper; the foam sticks to everything.
3. Mist the gap with water before and after applying the foam — water helps the foam cure faster.
4. Apply in a continuous bead with no breaks or voids. Don’t overfill; the foam will expand 30 to 60 times, so a small bead fills most gaps. The Lowe’s step-by-step guide recommends practicing on a piece of cardboard first.
5. Trim excess after curing using a serrated utility knife cut flush with the wall or framing.

Common Mistakes That Ruin The Job

Spray foam is forgiving if you follow the rules, but three mistakes cause most failures.

• Ignoring ventilation: An airtight house needs intentional mechanical ventilation (HRV or ERV) to prevent stale air and moisture buildup. The same tight seal that saves energy can trap pollutants.
• Foaming inside electrical boxes: Always turn off the main electrical switch before spraying near outlets, and keep the foam outside the box — it is a fire hazard and violates code.
• Applying in wet or cold weather: Most spray foams require a surface temperature above 40°F to cure properly. Cold surfaces cause delamination, where the foam pulls away from the wood and stops sealing.