Spray Foam vs Fiberglass Insulation: An Honest Comparison

If you are researching insulation for your home, you have probably come across two main options: fiberglass (either batts or blown-in) and spray foam. Both insulate your home, but they work in fundamentally different ways and deliver very different results. This comparison covers the real differences so you can make an informed decision.

The Core Difference

Fiberglass slows heat transfer by trapping air in tiny glass fibers. It works like a sweater: it helps retain heat, but wind blows right through it. Fiberglass does not stop air movement. It only resists conductive heat transfer.

Spray foam expands on contact and hardens into a solid barrier that stops both air movement and heat transfer. It works like a windbreaker and a sweater combined. This distinction matters because air leakage accounts for a large percentage of energy loss in most homes. Stopping air leaks is just as important as having a high R-value, and fiberglass only addresses one of those two problems.

Head-to-Head Comparison

Factor	Fiberglass	Spray Foam
R-Value Per Inch	2.2 - 3.8	3.5 - 7.0
Air Sealing	None	Complete
Moisture Barrier	None (absorbs water)	Yes (closed-cell)
Lifespan	10-15 years before degradation	Life of the building
Upfront Cost	$0.50 - $1.00 / sq ft	$1.00 - $3.00 / sq ft
Installation Time	1-2 days	1 day (most homes)
Settles Over Time?	Yes	No
Fire Resistance	Naturally non-combustible	Treated with fire retardant
Soundproofing	Moderate	Good (open-cell) to Moderate (closed-cell)

Where Fiberglass Falls Short

Fiberglass is the most common insulation in American homes because it is cheap and easy to install. But cheap and easy do not equal effective. The problems with fiberglass become obvious over time.

Fiberglass batts are cut to fit between wall studs and floor joists. In theory, they fill the cavity. In practice, they leave gaps around electrical boxes, wiring, plumbing, and any irregular framing. Every gap is an air leak. In an attic, those gaps allow conditioned air to escape and outside air to enter your home continuously.

Fiberglass also absorbs moisture. Wet fiberglass loses nearly all of its insulating value and creates conditions for mold growth. In crawl spaces and basements, fiberglass insulation regularly fails because of moisture exposure.

Over time, fiberglass batts sag under their own weight, compressing and leaving the top of the cavity uninsulated. Blown-in fiberglass and cellulose settle, reducing their effective R-value by 20% or more within a few years of installation.

Where Spray Foam Wins

Spray foam solves the problems that fiberglass cannot. Because it expands and adheres to surfaces, it fills every gap and creates a continuous air barrier. There are no seams, no gaps around wiring or plumbing, and no voids in irregular cavities.

Closed-cell spray foam also acts as a moisture barrier, making it the only insulation material that handles both thermal performance and moisture control in a single application. In humid climates like North Alabama, this is a significant advantage.

Spray foam does not settle, sag, or degrade over time. Once installed, it maintains its full R-value and air-sealing performance for the life of the building. There is no need to add more insulation in 10-15 years like there is with fiberglass.

The Cost Question

Spray foam costs more upfront. That is the main reason most builders use fiberglass in new construction: it reduces the build cost. But the homeowner pays the difference every month in higher energy bills for the life of the home.

A typical attic insulation job with fiberglass might cost $1,500. The same attic with open-cell spray foam might cost $3,500. That is a $2,000 difference. But if spray foam saves $80/month in energy costs (which is conservative for most homes), the payback period is about 25 months. After that, the savings continue for decades. For a detailed cost breakdown, see our spray foam cost guide.

When Fiberglass Makes Sense

Fiberglass is not always the wrong choice. For interior partition walls where energy performance is not a factor and soundproofing is the goal, fiberglass batts work fine. For rental properties or situations where the budget absolutely cannot accommodate spray foam, fiberglass provides some insulation value at a lower price point. And in some retrofit situations where access is extremely limited, blown-in fiberglass may be the only practical option.

The Bottom Line

If your goal is to make your home as comfortable and energy-efficient as possible, spray foam is the better insulation in almost every application. It costs more at the start and saves more over time. If your goal is to spend the least amount of money right now regardless of long-term performance, fiberglass is cheaper upfront.