If a single product can claim the status of a green building silver bullet, it might be foam insulation. Singlehandedly, these products can almost guarantee the highest practical levels of R-value per inch, effective air sealing, moisture management, and a higher performance as expanding foam usually fills even unseen gaps; plus, foam remains less susceptible to poor-quality installation, with most jobs done by trained applicators.

But with significant cost differences and subtleties in the performance characteristics and environmental impact of various foams, it’s important to understand the pros and cons of the basic foam products before choosing the best one for a specific project.

Foam Basics

If you’ve used a can of foam sealant, you already know the basics: The insulation comes as a sticky, thick liquid that reacts with air, expanding to flood any cavity you squirt it into. Chemical additives in the foam insulation harden the expanding spume into a resilient, solid substance.

Because of this expanding quality, properly applied foam will generally fill every nook in a wall cavity, band joist, floor joist, and attic rafter so that the entire surface has a tight air seal. Given equal R-values, foam delivers about 20% to 40% better overall performance than traditional fiberglass or cellulose insulation, says Rick Duncan, technical director of the Spray Foam Alliance. The reason for this significant difference, despite equal R-values, comes with trained installation and the built-in air seal.

Closed-cell foam insulation also manages moisture well enough to satisfy current ICC code requirements without an added interior vapor retarder in most applications. Perhaps most surprising, closed-cell insulation adds significant structural value, equivalent to about 75% of the ultimate shear strength of a ½-inch sheet of plywood, according to Duncan.

On the other hand, spray-foam installation costs between 100% to 300% more than professionally installed traditional insulation systems. And from a green building perspective, foam gets dismal marks in cradle-to-gate comparison, containing zero to very low renewable material or recycled content, compared to staple insulation products such as cellulose.

Most foam insulation is made with polyurethane, a two-part chemical composition that when blended creates gas bubbles, and then, like epoxy, sets up quickly, trapping the bubbles in a plastic matrix with excellent insulating properties.