Spray Foam vs. Fiberglass Batts: An Honest Comparison

Spray foam costs more up front than fiberglass batts, but it seals air leaks, fills irregular spaces completely, and in its closed-cell form resists moisture, which fiberglass can't do. Fiberglass is inexpensive, easy to find, and works fine in the right places if it's installed carefully. The better choice depends on the part of the building, your moisture risks, your budget, and how long you plan to own the place.

We install spray foam, so we have a bias. That's exactly why it's worth laying out where fiberglass makes sense too.

How Each Material Works

Fiberglass Batts

Fiberglass batts are blankets of spun glass fibers that trap air. The trapped air is what slows heat flow. Standard batts are rated around R-3 to R-4 per inch depending on density. They come in pre-cut widths for standard stud and joist spacing and can be faced with kraft paper or left unfaced.

Spray Foam

Spray foam is made on site by mixing two liquid components that expand and harden in place. Open-cell foam is about R-3.5 to R-3.8 per inch and stays soft and flexible. Closed-cell foam is about R-6 to R-7 per inch, dense and rigid. Both fill the cavity and stick to the framing.

Side-by-Side Comparison

	Fiberglass Batts	Open-Cell Foam	Closed-Cell Foam
R-value per inch (approx.)	R-3 to R-4	R-3.5 to R-3.8	R-6 to R-7
Air sealing	No	Yes	Yes
Moisture resistance	Poor when wet	Absorbs water	Resists water
Vapor control	Facing only	Vapor open	Vapor retarder at enough thickness
Fills irregular spaces	Poorly	Well	Well
Up-front cost	Lowest	Higher	Highest
DIY friendly	Yes	No	No

Where Spray Foam Has the Edge

Air Sealing

This is the biggest difference. Air moves through fiberglass. In a wall, a batt does nothing about the gaps around outlets, top plates, and wire holes. Spray foam fills those gaps as it expands. In Iowa, where winter winds and the stack effect push air through every opening, that difference shows up as fewer drafts and steadier temperatures.

Real-World Installation

Fiberglass performance depends heavily on installation. Batts need to be cut precisely around wires, boxes, and pipes, without compressing or leaving gaps. In practice, that's hard to do perfectly, and small voids reduce performance noticeably. Spray foam conforms to whatever shape the cavity has.

Moisture

Fiberglass that gets wet from condensation or a leak loses much of its insulating value and can hold moisture against the framing. That's a real risk in basements, rim joists, crawl spaces, and steel buildings. Closed-cell spray foam doesn't soak up water and keeps humid air off cold surfaces.

Pests

Mice love nesting in fiberglass. Foam isn't pest-proof, but it doesn't make an attractive nest, and sealing gaps closes some entry points.

Steel Buildings

Fiberglass on steel buildings has a long history of sagging, tearing, and trapping condensation against the panels. Spray foam bonded directly to the steel stops condensation and handles the odd shapes of purlins and girts. For a barndominium or heated shop, foam is the more common choice. See our page on barndominium insulation.

Where Fiberglass Makes Sense

Tight Budgets on Simple Walls

In a standard wood-framed wall where the exterior is already air sealed well with a good housewrap and careful detailing, fiberglass batts can do a reasonable job at a lower cost. Builders who take air sealing seriously can get decent results with batts.

Interior Walls for Sound

Fiberglass or mineral wool batts in interior walls are a common, affordable way to reduce noise between rooms. Open-cell foam also works well for sound, but batts are often the budget pick.

DIY Projects

You can buy fiberglass at any home center and install it yourself with basic safety gear. Spray foam requires professional equipment and training. Small DIY spray kits exist but are a different product for small jobs.

Floors and Areas With Easy Access

In some floor assemblies over unheated space, batts held tight against the subfloor can work, though air sealing the edges still matters.

The Hybrid: Flash-and-Batt

Some projects combine both. A thin layer of closed-cell foam is sprayed into the cavity to air seal and control condensation, then fiberglass fills the rest. This can lower the cost compared with full-depth foam while keeping the air sealing benefit. Whether it makes sense depends on the wall, the climate zone, and local code. Your building department and installer can help sort that out.

Cost Over Time

Spray foam is more expensive per square foot to install. Whether it pays off depends on how leaky the building is to start, how you heat it, what energy costs, and how long you'll own it. A heated shop or a drafty older home usually sees a bigger difference than a tight new house. We don't publish prices because every job is different, but a free estimate will give you a real number to compare against fiberglass.

Safety and Code

Both materials have safety considerations. Fiberglass can irritate skin and lungs during installation. Spray foam needs proper protective equipment for installers, and occupants stay out during application and for the re-occupancy period the manufacturer specifies, commonly about 24 hours. Building codes generally require a thermal barrier like half-inch drywall over spray foam in occupied spaces. Check with your local building department.

Key Takeaways

• Fiberglass is the low-cost option and works when installed carefully in well-sealed walls.

• Spray foam seals air, fills irregular cavities, and closed-cell resists moisture.

• Basements, rim joists, crawl spaces, and steel buildings favor spray foam.

• Flash-and-batt is a middle-ground option for some walls.

• The right choice depends on the building, budget, and moisture risk.

Frequently Asked Questions

Is spray foam really better than fiberglass?

It performs better at air sealing and in damp or irregular spaces. In a well-built wall with good exterior air sealing, fiberglass can still be a reasonable value. "Better" depends on where it's going.

Can I spray foam over existing fiberglass?

Generally no. Foam needs to stick to the framing and sheathing to seal properly. Existing batts are usually removed first.

Does fiberglass lose R-value over time?

Fiberglass that stays dry and uncompressed holds its R-value well. It loses effectiveness when it gets wet, compressed, settled, or disturbed by pests.

Get an Honest Recommendation

Not every project needs full spray foam, and we'll tell you where it makes sense. Check out our insulation services, then request a free estimate or call 319-403-1836. We serve Waterloo, Cedar Rapids, Dubuque, Iowa City, Decorah and anywhere in between.