Spray Foam vs Fiberglass Insulation: A Complete, Honest Comparison

Fiberglass is the most-installed insulation in America. Spray foam costs more but performs fundamentally differently, not just "better" in the same way, but better at different things entirely. This comparison covers what the R-value number on the label doesn't tell you, and when each material is the right choice.

The Head-to-Head Numbers

The Problem with Comparing R-Values Alone

When most people compare insulation, they look at R-value per inch and installed cost. Those numbers favor fiberglass. But R-value only measures resistance to conductive heat transfer: heat moving through solid material. It says nothing about convective heat transfer, which is what happens when air moves through gaps, cracks, and the insulation itself.

Here's the critical fact: fiberglass is air-permeable. A fiberglass-filled wall cavity that tests at R-19 in a lab can perform at R-14 or less in the field because air moves through the batt, carrying heat with it. The Department of Energy estimates that air infiltration through the building envelope (not conduction) accounts for 25–40% of a typical home's heating and cooling costs.

Spray foam air-seals as it insulates. There are no gaps, no settling, no pathways for air to bypass the insulation. The R-value you spec is the R-value you get in the field.

Where Fiberglass Still Makes Sense

We're a spray foam manufacturer, so we'll give you the honest answer: fiberglass wins in specific scenarios.

Fiberglass is the better choice when:

• Budget is the primary constraint and air sealing is handled separately: a well-detailed fiberglass installation with separate air sealing can approach spray foam performance at lower total cost in some applications.
• DIY is required: spray foam requires professional equipment and training. Fiberglass batts can be safely installed by a homeowner.
• The cavity will be accessed repeatedly: spray foam is permanent. If the wall or floor cavity needs to be opened for future plumbing or electrical work, fiberglass is removable. Spray foam is not.
• Sustainability certifications with specific material restrictions: some green building certifications have restrictions on specific blowing agents. Consult your certification requirements before specifying foam type.

Where Spray Foam Wins Decisively

Crawlspaces and below-grade applications

Fiberglass in a crawlspace is a maintenance liability. It absorbs moisture from the soil, loses R-value when wet, and becomes a habitat for mold and pests. Closed cell spray foam applied directly to the crawlspace walls creates a sealed, conditioned crawlspace that maintains its performance indefinitely. This is not a close comparison; spray foam is the only appropriate choice in most crawlspace applications.

Roofdecks and cathedral ceilings

Achieving required R-values in a roofdeck with fiberglass requires either a very deep rafter (to hold enough batt thickness) or a complex assembly with rigid foam above the sheathing. Closed cell spray foam applied to the underside of the roofdeck achieves R-49+ in 6–7 inches, eliminates the ventilation requirements of a vented attic, and creates a conditioned attic that dramatically reduces HVAC load.

Rim joists

Rim joists are one of the highest heat-loss areas in a home and one of the hardest to seal with fiberglass. Closed cell spray foam at 2–3 inches fills every gap, adheres to irregular surfaces, and delivers R-13 to R-20 in a space where fiberglass typically delivers R-10 at best and provides minimal air sealing.

Metal buildings

Steel framing conducts heat roughly 400 times better than wood, a phenomenon called thermal bridging that can reduce the effective R-value of a fiberglass-insulated metal wall to nearly zero at the framing members. Closed cell spray foam applied directly to the steel eliminates condensation on the framing and provides far better effective R-value. It's the standard specification for any serious metal building insulation.

New high-performance construction

Homes and buildings pursuing net-zero energy, LEED certification, Passive House standards, or ENERGY STAR ratings consistently use spray foam. The air tightness (measured in ACH50, air changes per hour at 50 pascals of pressure) achievable with spray foam simply cannot be replicated with fiberglass without heroic detailing efforts.

The Durability Argument

Fiberglass batts sag over time, especially in vertical applications where gravity works against them. After 20–30 years, fiberglass in walls has often settled enough to create uninsulated gaps at the top of the cavity. Spray foam is a thermoset polymer; once cured, it does not settle, sag, or degrade under normal conditions. ThermoSeal's lifetime warranty covers the material for the life of the original structure, because the product genuinely lasts that long.

Which Should You Choose?

For most new construction and serious retrofit projects, the decision isn't really spray foam vs fiberglass; it's which type of spray foam for which zone, and where fiberglass can play a supplemental role. A ThermoSeal certified installer can assess your project and recommend the optimal insulation strategy for your climate, building type, and budget.