Closed cell foam is made by reacting specific polymers with blowing agents that create tiny, sealed gas pockets throughout the material. The exact ingredients depend on the type of foam, but the most common varieties are polyurethane, polyethylene, and ethylene-vinyl acetate (EVA). Each starts with a different base polymer, yet they all share the same defining feature: fully intact cell walls that trap gas inside, making the foam dense, moisture-resistant, and structurally rigid.
How Closed Cell Structure Differs From Open Cell
All foam starts as a liquid or soft polymer filled with expanding gas bubbles. In open cell foam, those bubbles rupture during manufacturing, leaving behind only the thin struts where two bubbles touched. The result is a soft, spongy material full of interconnected air channels. In closed cell foam, the bubbles never rupture. They form sealed, individual pockets that stay intact, creating a denser and more rigid structure.
This distinction matters because those sealed pockets are what give closed cell foam its practical advantages. The trapped gas resists heat transfer, which is why closed cell spray foam delivers an R-value of 5.6 to 6.0 per inch in typical residential applications, roughly double what open cell foam provides. The intact cell walls also block moisture and water vapor. Closed cell spray foam acts as both an air barrier and a vapor barrier at just 1.5 inches of thickness, while open cell foam needs nearly four inches just to serve as an air barrier and never fully blocks vapor.
Polyurethane: The Most Common Type
Polyurethane closed cell foam is what most people encounter as spray foam insulation or rigid board insulation. It forms through a chemical reaction between two main ingredients: an isocyanate and a polyol. The isocyanate component is typically methylene diphenyl diisocyanate, while the polyol can be either ester-based or ether-based, with each type influencing the foam’s final flexibility and durability. A catalyst, usually an amine compound, speeds up the reaction.
The third critical ingredient is a blowing agent, which is what generates the gas that fills each cell. As the polyol and isocyanate react and the polymer matrix forms, the blowing agent expands into gas, inflating millions of tiny bubbles that become the foam’s closed cells. The type of gas trapped inside those cells directly affects the foam’s insulating ability. Gases that conduct heat poorly make better insulators than plain air, which is why blowing agent selection is so important to performance.
Historically, manufacturers used hydrofluorocarbon (HFC) blowing agents, but these are potent greenhouse gases. The industry has been shifting to hydrofluoroolefin (HFO) blowing agents, which have a dramatically lower global warming potential. As of January 1, 2025, the EPA requires that HFC blowing agents used in appliance insulation foam have a global warming potential below 150, and manufacturers must label products and report usage data. This regulatory push means most new closed cell spray foam on the market now uses HFO-based formulations.
Polyethylene Foam
Closed cell polyethylene foam starts with polyethylene resin, one of the most widely produced plastics in the world. To turn it into foam, manufacturers melt the resin, mix in a blowing agent, and then cross-link the polymer chains so the material holds its shape. Cross-linking is the step that transforms soft, flexible polyethylene into a foam with structural integrity and consistent cell size.
There are three main ways to cross-link polyethylene foam. Chemical cross-linking uses a peroxide compound mixed into the resin that creates bonds between polymer chains when heated. Radiation cross-linking bombards the material with an electron beam to forge those same carbon-to-carbon bonds, producing a smoother and more uniform surface. Silane cross-linking uses moisture-reactive silane compounds grafted onto the polymer. The chemical and radiation methods create structurally identical bonds between the polymer chains, but radiation-cross-linked foam tends to have a more homogeneous texture.
You’ll find closed cell polyethylene foam in packaging, athletic padding, flotation devices, and gaskets. It typically ranges from about 1.5 to 6 pounds per cubic foot in density, depending on the application.
EVA Foam
Ethylene-vinyl acetate foam is a copolymer, meaning it combines two different building blocks: ethylene and vinyl acetate. The ratio between these two components determines the foam’s properties. Higher vinyl acetate content makes the material softer and more flexible, while lower amounts produce a stiffer foam. EVA is a thermoplastic, so it can be heated, shaped, and milled into precise forms without losing strength.
EVA closed cell foam is prized for its combination of light weight and energy absorption. It shows up in shoe midsoles, yoga mats, sports equipment padding, and cosplay armor. Its closed cell structure makes it moisture-proof and gives it good cushioning and vibration isolation, all while remaining easy to cut and shape with basic tools.
Neoprene Foam
Neoprene closed cell foam is made from polychloroprene, a synthetic rubber. Nitrogen gas is typically used as the blowing agent, filling the cells and giving the foam buoyancy and thermal insulation. Neoprene foam is the material in wetsuits, laptop sleeves, and orthopedic braces. Its rubber base gives it excellent flexibility and resilience, stretching and compressing repeatedly without breaking down. It also resists oils, chemicals, and weathering better than most other foam types.
Additives That Shape Performance
Regardless of the base polymer, most closed cell foams contain additional ingredients that fine-tune their properties. Surfactants, often silicone-based, stabilize the foam cells during manufacturing and control their size and uniformity. Without surfactants, the cells would form unevenly, creating weak spots and inconsistent performance. Fire retardants are common in construction foams to meet building codes. Colorants, UV stabilizers, and antimicrobial agents round out the additive package depending on the intended use.
The density of the finished foam also varies widely based on formulation. Closed cell spray foam used in buildings typically falls between 1.5 and 3.0 pounds per cubic foot. Packaging foams tend toward the lighter end of that range, while structural or marine applications push heavier. Higher density generally means greater strength and better moisture resistance, but also higher material cost and weight.