Spray Foam Insulation — Evaluation

What Spray Foam Is

Spray polyurethane foam (SPF) is a two-component chemical insulation mixed and applied on-site. Component A (an isocyanate) and Component B (a polyol resin blend) are heated, pressurized, and combined at a spray gun. The two react on contact, expand, and adhere to whatever they touch, forming a continuous insulation layer with no gaps or seams. This is a neutral, factual description a manufacturer would agree with.

Closed-cell SPF (ccSPF) achieves roughly R-6 to R-7 per inch, adds rigidity, does not absorb water, and is vapor-impermeable at typical thickness. Open-cell SPF (ocSPF) achieves roughly R-3.5 to R-4 per inch, is vapor-permeable, softer and cheaper, and absorbs liquid water. Both are common in wall cavities, unvented attics, rim joists, and crawl spaces, and both have been heavily promoted over the past two decades for air-sealing and thermal performance.

The Two-Question Test Applied

Question 1 — Can it dry itself out?  Drying Ability: None (closed-cell) / Poor (open-cell)

Closed-cell foam applied to the interior of exterior sheathing creates a wall with no inward drying path. If moisture enters from outside — a flashing failure, a window leak, condensation at the sheathing, or rain during construction before the building was closed in — it has nowhere to go. Unlike a mineral wool cavity, where moisture can redistribute and dry through permeable layers, a closed-cell cavity holds moisture against the sheathing. In Good Air Homes' evaluation, that scores None. Open-cell foam is vapor-permeable in theory, but it absorbs liquid water and holds it against framing, extending the time a wall stays wet; that scores Poor.

Question 2 — Can you inspect it when something goes wrong?  Inspectability: Sealed

This is the failure that conflicts most directly with the standard. Once foam is in the cavity, it adheres to the framing and sheathing and hides everything behind it. A slow flashing leak, condensation at the sheathing plane, or a plumbing weep is invisible. In a mineral wool cavity you can open the drywall and see the problem; in a foam cavity, detection requires destructive removal. You cannot fix what you cannot see. That scores Sealed.

The Chemical Question Applied VOC Tier: Undisclosed

The chemical risk has two stages. During and shortly after application, spraying generates isocyanate vapors and aerosols along with amine catalysts and other compounds. Isocyanates — principally MDI in SPF — are recognized respiratory sensitizers; EPA notes there is no recognized safe exposure level for someone who has already become sensitized, and that isocyanates are a leading chemical cause of work-related asthma (EPA Safer Choice, Health Concerns about Spray Polyurethane Foam). The 2006 NIOSH MDI Alert documents asthma and a death from isocyanate exposure in related spray applications.

Long term, the honest answer is that the data is incomplete — and that uncertainty is itself the finding. EPA states plainly that the potential for off-gassing from SPF is not fully understood, that standardized methods to set safe re-entry and re-occupancy times do not yet exist, and that foam which appears "tack-free" within minutes is still curing and still contains unreacted chemicals (EPA Safer Choice, Potential Chemical Exposures from Spray Polyurethane Foam). Because the material is mixed and cured on-site with no way to verify the result after the fact, real-world emissions cannot be disclosed or confirmed for a given installation. In Good Air Homes' evaluation that is an Undisclosed VOC tier — not because the manufacturer hides a data sheet, but because the field-applied cure makes the actual in-place emissions unverifiable.

The Four Mold Inputs

Mold needs four things: spores, warmth, moisture, and a food source. You cannot remove spores or warmth from a home — they are always present. The standard works by controlling the other two. Here is how spray foam scores on each:

• Spores — not controllable. Present everywhere; no assembly changes this.
• Warmth — not controllable. Always present in a conditioned home.
• Moisture — worsened. Closed-cell foam removes the cavity's drying path; open-cell foam absorbs and holds liquid water. Either way, when water gets in, it stays longer.
• Food source — present and hidden. The foam itself is not food, but the wood framing and wood-based sheathing it bonds to are — and once they are wet behind sealed foam, they cannot be dried or inspected. Mold growth on the back of the sheathing is invisible and ongoing until damage reaches the interior.

Spray foam does not improve a single mold input, and it makes the two controllable ones worse.

The Field-Mixing Problem

This is the most important risk most sources understate. Spray foam is not a finished product delivered to the site — it is a chemical reaction performed at the site, in real time, by the installer. The A and B components must be combined at the correct ratio and temperature for the foam to cure completely. When they are not — equipment drift, cold substrate, depleted chemicals, an inexperienced applicator, or a too-thick pass that overheats and chars internally — the result is "off-ratio" foam that never fully cures and can off-gas for far longer than the manufacturer's re-entry window suggests.

The reason this matters more than a typical product defect: confirming whether cured foam is sound in place is difficult for a homeowner without specialized, and often destructive, testing. EPA's own position is that standardized methods to assess SPF's impact on indoor air quality and to set re-occupancy times are still needed (EPA Safer Choice). So the homeowner cannot easily confirm the ratio was right, cannot easily confirm the cure is complete, and cannot inspect the result without cutting into it — largely, they wait and see whether symptoms appear. In Good Air Homes' evaluation, an unverifiable on-site chemical cure is not an acceptable foundation for a home built to reduce chemical exposure.

Cost and Contractor Information

Installed cost: closed-cell runs roughly $1.50–$3.50 per board foot; open-cell roughly $0.50–$1.50. A 2,000 sq ft home with 2×6 walls and an unvented attic may run $15,000–$30,000 in closed-cell insulation alone.

Contractor availability: high. SPF is a mature trade with established certification. Finding an applicator is easy in most markets — but quality varies sharply, and the cost of poor installation stays hidden until symptoms appear. By comparison, mineral wool in a properly detailed assembly carries no off-gassing risk, full inspectability, and complete drying potential at a comparable or lower cost.