Spray Foam vs. Fiberglass Insulation: An Honest Comparison

Fiberglass is the insulation most American homes already have. It's inexpensive, familiar and does its job when it's installed carefully. Spray foam costs more up front and does a broader job. Neither one is right for every house, and anyone who tells you otherwise is selling something.

We make spray foam, so we'll say that up front. To keep this comparison fair, we've stuck to published numbers: the U.S. Department of Energy's guidance for fiberglass, and the IAPMO evaluation reports for our two products, ComfortSeal 500 open cell and ComfortSeal HFO closed cell.

The short version

Fiberglass values: DOE Building America guide Energy Renovations: Insulation (PNNL and ORNL, 2012). ComfortSeal values: IAPMO ER-974 and ER-975, Table 1.

How each one works

Fiberglass is glass spun into fine fibers. The fibers trap pockets of air, and trapped air slows heat moving through a wall or ceiling. It comes as batts and rolls that are cut to fit between studs and joists, or as loose fill that's blown into attics and closed walls. The Department of Energy's insulation guide for contractors and homeowners lists standard-density batts at R-2.9 to R-3.8 per inch and high-density batts at R-3.7 to R-4.3 per inch, and it's candid about the trade-offs: batts are not an air barrier, gaps around their edges let air move around and through them, and they lose R-value when they're compressed.

Spray foam starts as two liquids that are mixed at the spray gun, expand on contact and cure in place. ComfortSeal 500 expands roughly 100 times its liquid volume and ComfortSeal HFO roughly 15 times. Because the foam expands against the framing and sheathing and fills the gaps around wiring, pipes and boxes, it seals air leaks at the same time it insulates. An earlier DOE Building America guide, Which Spray Foam Is Right for You? (2009), makes the same point: both types of spray foam are good at air sealing, while fiberglass and cellulose do much less to stop air flow.

R-value: compare what's installed, not the label per inch

R-value measures how well a material resists heat flow. Higher is better. Per-inch numbers are a useful starting point for comparing materials, but what matters is the R-value you actually end up with in each wall, ceiling or roofline.

For spray foam, that number comes from the product's evaluation report. The R-value of foam doesn't rise in a straight line with thickness, so you should never multiply the 1-inch value by the depth. Here's what IAPMO's Table 1 lists for the two cavity depths most homes have:

For fiberglass, the R-value is printed on the batt, and it assumes the batt is installed at its full thickness, with no gaps, folds or pinches. The DOE guide notes that careful installation is needed to limit leaks around the edges, because those gaps reduce the insulation's effectiveness. A well-installed batt performs close to its label; a stuffed or gappy one doesn't. To see thickness and R-value for our foams side by side, try the R-value calculator or read spray foam R-value by thickness.

Air leakage: the part R-value doesn't measure

R-value is measured in a lab under steady conditions. It doesn't capture air moving through gaps and cracks, and that moving air carries heat and humidity with it. That's the main difference between the two materials.

With fiberglass, air sealing is a separate job. Someone has to caulk and foam the top plates, the wiring and plumbing holes, the rim joists and the attic penetrations before the insulation goes in. The DOE guide treats air sealing and insulating as two halves of the same project, and a house with fiberglass and thorough air sealing can perform well.

With spray foam, the insulation and the air seal go in together wherever the foam is applied. ComfortSeal 500 has a published air leakage rate below 0.002 L/s·m² (ASTM E283). That doesn't make the rest of the house airtight. Leaks at windows, doors and areas that weren't foamed still need attention, and a foam job is only as good as its coverage.

How much either choice changes your energy bills depends on the house, the climate, how leaky it is today and what the new insulation replaces. Under the Federal Trade Commission's R-value Rule (16 CFR 460), anyone who claims insulation will cut your bills needs a reasonable basis for it. We'll put it the way the rule does: "Savings vary. Find out why in the seller's fact sheet on R-values. Higher R-values mean greater insulating power."

Moisture and vapor

Fiberglass lets water vapor pass through. When a vapor retarder is needed, it usually comes from a kraft, foil or vinyl facing on the batt or a separate layer. Put fiberglass in the wrong place and moisture becomes a problem. The DOE guide warns that batts between the floor joists of a vented crawl space in a humid climate can get damp, insulate poorly and let mold grow.

The two foams handle vapor differently:

• ComfortSeal HFO rates 0.45 perms at 2 inches, which qualifies it as a Class II vapor retarder under the IRC. It also showed no fungal growth when tested to ASTM G21. That's why closed cell is a common choice for exterior walls, crawl spaces and rim joists in humid climates.
• ComfortSeal 500 is vapor-open at 21 perms at 1 inch, so assemblies can dry toward the interior. Where the code for your area calls for a vapor retarder, ER-974 says one must be installed with it.

For more on this, see spray foam in humid and coastal climates.

Fire safety and code

Both materials have to be installed to code. The DOE guide notes that fiberglass batts have good fire resistance, though kraft paper facing is flammable. Spray foam is a foam plastic, so the building code requires it to be separated from living space by a thermal barrier, such as ½-inch gypsum wallboard, unless an approved alternative assembly is used. Both ComfortSeal products are Class I for surface burning (flame spread index 25 or less, smoke developed index 450 or less, ASTM E84, tested at up to 4 in.), and both evaluation reports spell out where coatings are needed in attics and crawl spaces. Our guide to ignition barriers vs. thermal barriers explains the difference.

What about cost?

Fiberglass costs less up front, and how much less depends on thickness, access and prep work. We don't publish installed prices for either material. The third-party price ranges we could verify for spray foam, and a checklist for comparing written quotes, are in what drives the cost of spray foam.

Where each one tends to make sense

Fiberglass is often the practical choice when:

• The attic is a simple, vented space with no ducts or equipment in it, and the ceiling below is easy to air seal.
• The walls are open, the budget is tight, and the crew will air seal carefully before the batts go in.
• You're topping up existing attic insulation that's in good condition.

Spray foam is often worth a serious look when:

• Ducts or HVAC equipment run through the attic. Foam at the roof deck can bring them inside the home's envelope. See unvented attics with spray foam.
• You're insulating a crawl space or rim joists, where air sealing and moisture control matter. See spray foam in crawl spaces.
• The cavities are shallow and you need more R-value in the depth you have.
• The house is drafty, with rooms that are hard to keep comfortable, or has framing that's hard to seal.

Some builders combine the two. A layer of closed cell foam goes against the sheathing to air seal and control vapor, and fiberglass fills the rest of the cavity. The 2009 DOE Building America guide describes this "flash and batt" approach and notes that the foam layer has to be thick enough for the climate to keep condensation from forming on its inner surface.

Questions to ask before you choose

• What R-value will be installed in each area, and at what thickness?
• For fiberglass: how will the air leaks be sealed before the insulation goes in?
• For spray foam: which product is going in each area, and what R-value does its evaluation report list for that thickness?
• Will any area need an ignition barrier or thermal barrier, and is it included in the price?
• What warranty comes with the work, and who stands behind the installation?

On that last point, here is our answer. Installs by certified ComfortSeal installers carry a limited lifetime warranty on the foam, for the life of the home; installation workmanship is the installing contractor's responsibility. See the warranty page for details.

Want a second opinion on your house? Find a certified ComfortSeal installer, or read our homeowner guide first.