What Is Spray Foam Insulation, How Does It Work, and Is It Better Than Fiberglass?

Quick Answer:
Spray foam insulation expands to seal air leaks, delivers higher R-value per inch, and controls moisture better than fiberglass for a more efficient, durable home.

What Is Spray Foam Insulation?

Spray foam insulation, or spray polyurethane foam (SPF), is a two-component material applied as a liquid that expands into a solid foam. Once installed, it fills gaps, adheres to surfaces, and creates a continuous insulating layer.

According to the Spray Polyurethane Foam Alliance, spray foam functions as both insulation and an air barrier when properly installed, which is a key distinction from traditional materials.

How Spray Foam Insulation Works

Spray foam is installed on-site using specialized equipment that mixes two heated components at the point of application. These materials react immediately, expanding and curing into foam.

There are two primary types:

Open-Cell Spray Foam

• Lower density and softer texture

• Expands more to fill irregular spaces

• Typically used in interior applications

Closed-Cell Spray Foam

• Higher density and rigidity

• Higher R-value per inch

• Acts as an air barrier and helps resist moisture

According to Building Science Corporation, controlling air movement through building assemblies is one of the most critical factors in managing moisture and long-term durability. Closed-cell spray foam directly addresses this by limiting both air and moisture intrusion.

Spray Foam vs Fiberglass: Key Differences That Matter

1. Air Sealing Performance

Fiberglass insulation slows heat transfer, but it does not stop air movement.

Spray foam does both.

According to the U.S. Department of Energy, air leakage can account for a significant portion of a home’s energy loss, and sealing gaps and cracks is essential to improving efficiency.

Spray foam creates a continuous air barrier, reducing drafts and uncontrolled airflow throughout the home.

2. R-Value Per Inch

R-value measures how well insulation resists heat flow.

Closed-cell spray foam provides a higher R-value per inch than fiberglass, meaning it delivers more insulation in less space.

According to the Spray Polyurethane Foam Alliance, this makes spray foam particularly effective in tight assemblies where maximizing performance is critical.

3. Moisture and Humidity Control

In coastal climates like Bluffton, Charleston, and the greater Lowcountry, moisture control is just as important as temperature control.

Fiberglass:

• Can absorb moisture

• Loses effectiveness when wet

• Can contribute to mold conditions

Spray foam:

• Resists moisture intrusion

• Reduces condensation risk

• Helps stabilize indoor humidity

This matters because moisture does not just stay outside. In many homes, a large portion of indoor air originates from below.

More than 50% of the air you breathe inside your home can come from the crawl space, bringing humidity, mold spores, and odors with it .

Spray foam’s air-sealing properties help block that pathway.

4. Durability and Long-Term Performance

Fiberglass insulation can:

• Sag over time

• Shift out of place

• Lose effectiveness

Spray foam:

• Bonds directly to surfaces

• Maintains its shape and position

• Does not settle or compress

According to NAHB Research Center, insulation performance over time is directly tied to proper installation and material stability. Spray foam’s adhesive and rigid properties contribute to long-term consistency.

5. Energy Efficiency and Comfort

Spray foam helps create a tighter building envelope, which leads to:

• More consistent indoor temperatures

• Reduced HVAC workload

• Improved overall comfort

According to ENERGY STAR, properly sealing and insulating a home can significantly improve energy performance while enhancing comfort year-round.

In many cases, spray foam systems can contribute to substantial energy savings depending on the structure and installation quality .

Tradeoffs to Consider

Spray foam is a premium solution, and it comes with considerations:

Cost

Spray foam typically has a higher upfront cost than fiberglass insulation.

Installation Quality

According to the Spray Polyurethane Foam Alliance, proper installation is critical. Temperature, mixing ratios, and application thickness all impact performance.

Product Selection

Choosing between open-cell and closed-cell depends on:

• Climate

• Building design

• Budget

When Should You Choose Spray Foam?

Spray foam is often the better choice when:

• Air sealing is a priority

• You are building or renovating in a humid climate

• Long-term performance matters more than upfront cost

• You want improved indoor air quality and comfort

Fiberglass may still be appropriate when:

• Budget is the primary concern

• A removable or non-permanent solution is preferred

Final Thought: Why This Matters in the Lowcountry

In the South Carolina Lowcountry, insulation is not just about keeping heat in or out.

It is about:

• Controlling humidity

• Preventing moisture-related issues

• Maintaining long-term structural performance

Spray foam insulation addresses all three.

By sealing air leaks, reducing moisture movement, and improving energy efficiency, it provides a solution built for the realities of coastal living.

And when you look beyond upfront cost and focus on performance over time, the value becomes clear.