Is spray foam insulation worth the cost for multifamily?

It depends on the application. For standard wall assemblies, spray foam often has diminishing returns compared to high-quality batt installation with proper air sealing. For rim joists, irregular geometries, and below-grade walls, spray foam is often the most cost-effective option.

What insulation gives the best HERS score per dollar?

On most multifamily projects, the best return comes from R-21 batt insulation combined with robust air sealing details, rather than upgrading to higher R-value spray foam. Air sealing typically delivers 2 to 3 times more HERS improvement per dollar than wall insulation upgrades.

Will spray foam face restrictions in Massachusetts?

Future embodied carbon rules are expected to penalize materials with high carbon footprints. Closed-cell spray foam has one of the highest embodied carbon values among common insulation materials. Projects designed today should consider materials that perform well under both current energy code and future embodied carbon requirements.

Spray Foam Insulation Is Not a Magic Bullet for Energy Codes

On a 276-unit MA project, upgrading from R-21 batts to R-38 spray foam had diminishing returns. Here is when spray foam makes sense and when it does not.

April 13, 2026

On a 276-unit multifamily project in Massachusetts, we modeled the building with two insulation approaches. One used R-38 closed-cell spray foam in the exterior walls. The other used R-21 fiberglass batt insulation.

The spray foam path cost significantly more. The energy performance improvement was minimal. The developer switched to batts and redirected the savings to upgrades that actually moved the needle on their HERS score.

The Diminishing Returns Problem

Insulation has diminishing returns. Going from R-0 (no insulation) to R-13 is a massive performance improvement. Going from R-13 to R-21 is meaningful. Going from R-21 to R-38 is marginal.

The physics are straightforward. R-value measures resistance to heat flow. Doubling the R-value does not halve the heat loss. Going from R-21 to R-38 reduces heat flow through the wall by about 45%, but the wall is already performing well at R-21. The absolute amount of heat saved is small relative to the cost of achieving it.

On this 276-unit project, the R-38 spray foam walls improved the HERS score by less than 2 points per unit compared to R-21 batts. The cost difference was over $3 per square foot of wall area.

Where the Money Actually Goes

We redirected the insulation savings to two upgrades that had much better cost-to-performance ratios.

Improved air sealing. Air leakage is responsible for more energy loss than conduction through walls in most multifamily buildings. Spending $1 per square foot on better air sealing details delivered a 3 to 4 point HERS improvement, nearly double what the spray foam upgrade provided at triple the cost.

HVAC efficiency upgrade. A modest improvement in heat pump efficiency (moving from a standard unit to a high-efficiency unit) delivered 2 to 3 HERS points at a fraction of the spray foam cost premium.

The total result: better HERS scores, lower construction cost, and a building that performs better in practice because air sealing addresses the real source of energy loss.

When Spray Foam Does Make Sense

Spray foam is not bad insulation. It is excellent in specific applications.

Rim joists and band boards. These are notoriously difficult to air seal and insulate with batts. Spray foam in these locations is often the most cost-effective solution.

Irregular geometries. Cathedral ceilings, cantilevered floors, and complex roof lines where batts cannot achieve full contact with the substrate. Spray foam conforms to irregular shapes and provides both insulation and air sealing in one application.

Below-grade walls. Foundation walls where moisture management is critical and batt insulation would be at risk of condensation damage.

Eliminating continuous insulation. On some wall assemblies, R-38 closed-cell spray foam in the cavity can eliminate the need for exterior continuous insulation entirely. When the spray foam replaces both cavity insulation and CI, the net cost can actually be lower than batt-plus-CI assemblies. This is a case where spray foam is a net win, not because of the R-value, but because it simplifies the wall assembly.

The mistake is assuming spray foam is always better or always worse. The right answer depends on the specific wall assembly, whether CI is required, and the cost of alternatives. On projects where spray foam eliminates CI, it can be the cheapest option. On projects where CI is already absent, upgrading from batts to spray foam is rarely worth the premium.

The Embodied Carbon Factor

Massachusetts is moving toward embodied carbon requirements in future code cycles. Spray foam has a significantly higher embodied carbon footprint than fiberglass batts or cellulose. Dense-pack cellulose and wood fiber insulation board actually store carbon rather than emitting it.

Projects that over-specify spray foam today may face penalties under future embodied carbon rules. The regulatory direction suggests that materials with lower carbon footprints will be increasingly favored, and many of those materials are already cost-competitive for standard wall assemblies.