Spray Foam vs. Batt: An Honest Comparison

Insulation marketing is some of the most aggressive in the construction industry. Spray foam companies will tell you batt is obsolete. Fiberglass associations will tell you spray foam off-gases forever. Neither is true. Both materials have specific applications where they're the right answer, and the best insulation packages on the homes we build use multiple materials in different locations.

The materials

Fiberglass batts (the volume default)

Pink or yellow fiberglass batts pre-cut to stud-bay width. R-value ranges from R-13 (2x4 wall) to R-38 (12" attic batt). Cheap, fast install, well understood. Available in standard density and high-density (more R per inch).

Mineral wool batts (Rockwool)

Looks like fiberglass but made from spun mineral fibers (stone or slag). Denser than fiberglass, higher R per inch, fire resistant, sound absorbing. More expensive than fiberglass but cheaper than spray foam.

Open-cell spray foam

Sprayed wet from a truck, expands to fill cavities. Open-cell has air pockets in the cured foam — lower R-value (~3.5 per inch) but vapor-permeable and excellent at sound damping.

Closed-cell spray foam

Denser, fully sealed cells. Higher R-value (~6.5 per inch), structural rigidity, vapor barrier built in. Most expensive.

Blown-in cellulose

Recycled paper treated with borate. Blown into attics and walls. Excellent R-value per dollar, good acoustic properties, settles over time (re-fluffing needed).

Where each belongs — our standard spec

Walls (2x6 framing)

Three legitimate approaches:

• Flash-and-batt: 2" of closed-cell spray foam against the sheathing, then fiberglass or mineral wool batt filling the remaining cavity. Air seal from the spray foam, R-value from the batt. Our default for premium custom.
• Open-cell spray foam: fills the cavity, air seals, vapor-permeable. Good for tight envelopes in our climate.
• Mineral wool batts (Rockwool): excellent if you're air-sealing rigorously with caulk and tape. Better acoustic performance than fiberglass.
• Fiberglass batt: acceptable budget option ONLY if every penetration is meticulously air-sealed

Attic

Two architectures: vented (insulation at ceiling level) or unvented (insulation at roof deck, conditioned attic).

• Vented attic: blown-in cellulose or fiberglass at the ceiling level, R-49 to R-60 (DFW). Soffit and ridge venting. Cheapest, traditional, works well.
• Unvented (conditioned) attic: closed-cell or open-cell spray foam at the underside of the roof deck. Attic becomes conditioned space. Higher cost, but if HVAC equipment is in the attic, the efficiency gain often pays back.

Crawlspace (where applicable)

Closed-cell spray foam on the underside of the floor (in vented crawlspaces) or on the foundation walls + sealed (in conditioned crawlspaces). Conditioned crawlspaces are much better in our humid climate.

Basement walls (where applicable)

Closed-cell spray foam directly to the concrete — air seal, vapor barrier, R-value all in one application. The standard.

The claims to ignore

• "Spray foam pays for itself in two years." Almost never true. Spray foam costs 2–4x more than batt and saves 10–20% on energy bills. Payback is more like 8–15 years.
• "Closed-cell spray foam is a vapor barrier so you don't need anything else." True but only if installed at the right thickness (2" minimum for vapor barrier function in our climate).
• "Fiberglass causes cancer." Modern fiberglass is bound and irritation-rated; the IARC removed it from cancer-causing lists in 2001. Wear a mask during install — that's it.
• "Spray foam off-gasses forever." Properly cured spray foam (mixed in correct ratios, applied within temperature spec) doesn't off-gas in occupied space. Botched applications can — this is an installer quality issue, not a material issue.
• "R-60 is way better than R-49." Diminishing returns are real. Going from R-19 to R-38 doubles the R-value but only halves the heat loss. Going from R-49 to R-60 adds 18% R but only ~3% performance gain.

Air sealing — the layer everyone skips

Before insulation, your house needs an air barrier — a continuous seal that prevents air movement through the wall assembly. This includes:

• Caulk at every plate-to-subfloor and plate-to-sheathing joint
• Tape at every sheet seam in the wall sheathing
• Sealed penetrations at every wire, pipe, duct, and conduit through the air barrier
• Sealed top plates at every interior wall (a major air leak path most builders ignore)
• Gasketed electrical boxes on exterior walls
• Sealed at the rim joist between floors

Air sealing day is one of the most valuable days in the entire build. We have crews who do nothing but air sealing for two days before insulation. The result: blower-door tests at 1.5–2.5 ACH50 instead of the code requirement of 5.0 (and the typical builder result of 7.0).

Blower door test — the proof

A blower-door test measures how leaky your house is by pressurizing or depressurizing the house and measuring airflow. Result is expressed in ACH50 (air changes per hour at 50 pascals of pressure).

• Code typical (Texas): ~5.0 ACH50
• Energy Star: 4.0 ACH50
• Pretty Good House: 2.0 ACH50
• Passive House: 0.6 ACH50

Aim for 2.0–3.0 ACH50 on a custom home. The difference between 5.0 and 2.5 is roughly 20–30% lower heating and cooling energy use — permanent, requires no maintenance, and improves comfort.

The honest takeaway

There is no single right answer to "what insulation should I use." The right answer is layered:

1. Meticulous air sealing first
2. Closed-cell flash plus high-density batt in walls (or open-cell foam, well-installed)
3. Blown-in cellulose or batt in vented attic, OR closed-cell foam at roof deck for unvented attic
4. Blower-door test target of 2.0–3.0 ACH50

Insulation is one of the lowest-cost, longest-lasting performance upgrades in a home. Get it right, get it tested, and never think about it again.

— Daniel Caro, Construction Manager. Twenty years running jobsites — foundation, framing, mechanicals, and the unglamorous details that decide a great home. Get the free Ultimate Home Building Checklist for the field-tested list we walk every Angel home through.