Why We Don't Use Spray Foam — and What We Do Instead

I want to talk about something that comes up on almost every project we discuss, and where I think a lot of builders and homeowners are making a decision they'll regret in ten years.

Spray foam insulation has become the default answer for high-performance insulation in custom home building. Walk through the framing of most new construction, and you'll see it everywhere: in the rim joists, in the roof deck, often in the wall cavities. Builders love it because it goes in fast and it seals and insulates in one step. Homeowners love it because it sounds like the premium choice.

I don't use it. Here's why.

The Day One Problem

Spray polyurethane foam comes in two main types: open-cell and closed-cell. Closed-cell foam, the kind used in walls and roofs for its higher R-value and vapor resistance, has a lot going for it on paper. High R-value per inch. Air sealing in the same application as insulation. Good compressive strength.

But closed-cell spray foam has a day one problem that the industry doesn't talk about enough.

The installation process is highly sensitive to temperature, humidity, substrate conditions, and the ratio of the two chemical components being mixed at the gun. When those conditions aren't exactly right, and in the field, on a real job site, they often aren't, the foam can cure incorrectly. It can shrink. It can pull away from the framing it's supposed to be adhered to. It can develop voids. And none of that is visible once the drywall goes up.

The Building Science Corporation has documented case studies of spray foam installations that looked perfect from the outside but were underperforming significantly because of curing issues that happened during installation. You paid for R-21. You might have gotten R-12 in certain sections. You'll never know, because there's no way to test it after the fact without tearing open your walls.

The Year Ten Problem

Even a perfectly installed closed-cell spray foam job has a longer-term issue that most people don't ask about because they're focused on move-in day.

Spray foam is a petrochemical product. It expands, it cures, and over time it can continue to move slightly as the building moves, as temperature cycles through seasons, and as the foam ages. In some cases, properly installed foam that performed well in year one develops small separations from the framing in year five or year ten. Those separations are invisible. They're small. But they're enough to compromise the air sealing that was the whole point of using the product in the first place.

There's also the off-gassing question. During and after installation, spray foam releases chemicals including isocyanates, which are among the most hazardous substances in the construction industry according to OSHA's occupational health guidance. Most of this dissipates within days to weeks after installation. But the timeline varies with product, application thickness, and ventilation conditions. The honest answer is that the long-term off-gassing profile of spray foam in an occupied home is not as well-studied as the industry would like you to believe.

What We Use Instead

At Parksdale, our wall assemblies use a combination of mineral wool insulation in the stud cavity and continuous rigid mineral wool or rigid foam board on the exterior of the framing.

Mineral wool is inert. It's made from rock or slag, it doesn't off-gas, it doesn't support mold growth, it doesn't move or shrink over time, and its performance characteristics are the same on day one as they are in year twenty. What you install is what you get, permanently.

Continuous exterior insulation solves the thermal bridging problem without relying on a chemically sensitive installation process. A layer of rigid insulation board on the outside of the framing breaks the thermal bridge through the studs and performs predictably. You can verify it visually. You can inspect it before it's covered.

The air barrier is a separate, dedicated layer. Rather than relying on spray foam to do double duty as both insulation and air barrier, we use a dedicated air barrier membrane or taped sheathing system that can be inspected, verified, and tested independently. This is a principle that the Building Science Corporation's Perfect Wall concept has advocated for years: control layers should be discrete and verifiable, not bundled into a single product that you can't inspect after installation.

The Blower Door Tells the Truth

At the end of every Parksdale build, we run a blower door test. That test measures the actual airtightness of the completed home and produces a number — ACH50 — that is an objective, third-party verified measurement of how well the building envelope was actually built. Our builds consistently achieve well below the 2021 IECC code requirement of 3.0 ACH50, and we target Passive House levels of airtightness on every project.

I'd rather build with a system that performs reliably under real conditions and verify it with a test than rely on a system that performs brilliantly under ideal conditions and hope for the best.

This Isn't the Popular Opinion

Plenty of good builders use spray foam and get good results. I'm not saying every spray foam job fails. I'm saying the performance of spray foam is more variable, more installation-dependent, and harder to verify than its reputation suggests. And when you're building a home you plan to live in for thirty years, variable and unverifiable isn't good enough.

My background is in building science. I spent years at Georgia Tech studying how buildings actually perform, not how products are marketed to perform. The gap between those two things is where most building failures live.

If you want to understand exactly what goes into our wall assemblies, roof systems, and air barrier details, reach out at info@parksdalebuilds.com or call us at 704-993-1030.