Spray Foam or Fiberglass: Choosing by Assembly, Not by Product

Decide on these five questions first

Most system arguments end quickly once the assembly is described properly. Before comparing products, answer the following about the specific surface being insulated.

• Is the surface continuous or irregular? Foam conforms to penetrations, odd geometry and unsealed transitions. Batt needs a defined cavity of the right dimension.
• Does the assembly need vapor control, or only thermal resistance? This is the question that most often decides between closed-cell and everything else.
• Can the assembly dry if it gets wet, and in which direction? An assembly that cannot dry is unforgiving of the wrong vapor strategy.
• Is the cavity accessible, and will it stay accessible? Future maintenance behind a foamed assembly is harder than behind removable batt.
• Does the space have a fire or acoustic requirement? Exposed foam plastic generally requires a thermal or ignition barrier; acoustic separation favours different materials than thermal performance does.

Closed-cell spray polyurethane foam

Closed-cell foam is the only one of the three that addresses heat, air movement and vapor in a single application. On an exposed steel roof deck or a metal building wall, where humid interior air reaching cold steel is the actual failure mode, that combination is what stops condensation rather than simply slowing heat flow. It also adds structural rigidity to light-gauge assemblies.

What it will not do is fix a wet building. Foam needs a clean, dry, sound substrate to bond to. Applied over a leaking roof or damp deck it conceals the problem and traps the moisture against the structure. It also requires ventilation and a re-entry period during application and cure, which an occupied facility has to plan around, and where it stays exposed it generally requires a thermal or ignition barrier.

Open-cell spray polyurethane foam

Open-cell fills and seals like closed-cell but stays vapor permeable and delivers less thermal resistance per inch. That permeability is a benefit in assemblies that need to dry inward and a liability in assemblies where vapor drive has to be stopped. In practice it does its best work on interior partitions, sound separation between tenancies, and irregular interior cavities where air-sealing matters more than vapor control.

Fiberglass, batt and blown

Fiberglass remains the sensible default in a framed cavity that is clean, dry, accessible and dimensionally correct. It reaches the specified thermal target at lower cost, it can be removed and reinstated for maintenance, and it does not require ventilation or a re-entry window during installation.

It is also the system most often installed badly. Batt compressed into an undersized cavity, cut short around bracing, or left with gaps at the edges loses a large share of its rated value, and fiberglass does nothing about air movement on its own. In an assembly where uncontrolled air leakage is the real problem, adding batt without an air barrier will not fix it.

Where each system is the wrong answer

The useful part of any comparison is knowing when to rule something out.

• Do not specify foam over a wet or unresolved substrate. Fix the leak first. The insulation is not the remedy for water entry.
• Do not specify closed-cell where the assembly must dry toward the interior unless the full assembly has been assessed for that behaviour.
• Do not rely on batt alone where air leakage is the failure mode. It is a thermal material, not an air barrier.
• Do not use open-cell where vapor drive has to be stopped, for example on the warm side of a refrigerated space.
• Do not leave foam plastic exposed where a thermal or ignition barrier is required by the applicable code.

Buildings often need more than one system

Treating this as a single choice for a whole building is usually the mistake. A distribution facility can reasonably take closed-cell at the roof deck for condensation control, fiberglass in the framed office cavities, and open-cell around interior partitions for sound. Specifying one product everywhere either overspends in the easy areas or underperforms in the hard ones.

What to ask a contractor proposing either system

• Which assemblies is each system going on, and why that one
• What the vapor strategy is for the assembly, and which direction it dries
• What thickness or performance target is being priced
• Whether a thermal or ignition barrier is required and included
• What the substrate has to be in before application, and who is responsible for getting it there
• For spray-applied work, what ventilation and re-entry period the building has to plan around

Sources and technical references

The code, standard and safety statements in this guide come from the publishers below. Adopted editions and local amendments vary, so confirm what applies to a specific project with the authority having jurisdiction.

• U.S. Department of Energy, Insulation materials and where each is applied. Reviewed August 2026.
• International Code Council, International Energy Conservation Code, current adopted editions. Reviewed August 2026.
• OSHA, Respiratory protection and personal protective equipment standards. Reviewed August 2026.

Code requirements, product approvals, fire-protection details, and required R-values vary by jurisdiction, building use, assembly, and project conditions. Confirm requirements with the applicable authority, design professional, and project specifications.