cellular glass insulation

Cellular glass insulation is a rigid board made by heating crushed glass with a foaming agent, typically hydrogen sulfide or another gas that creates bubbles within the molten material. As it cools, the gas is trapped inside, forming a honeycomb structure with closed cells that cannot absorb moisture. The result is a lightweight panel with thermal resistance typically between R-2.5 and R-3.5 per inch, and compressive strength ranging from 15 to 100 psi depending on density and formulation.

The material's defining advantage is its impermeability: water cannot penetrate the sealed cell structure, making it ideal for below-grade applications, roofing, and mechanical systems where vapor drive or liquid water contact is a real risk. Unlike foam plastics, cellular glass contains no organic binder that can degrade or off-gas. It is inorganic, non-combustible, and has a high melting point, which makes it valuable in high-temperature pipe insulation and around steam lines. Its rigidity also means it resists creep under sustained load better than fiberglass batts.

Handling and installation concerns

Cellular glass boards are brittle and can shatter if dropped or flexed sharply; they must be handled and stacked with care. The material is heavy relative to its insulating value, around 8 to 10 pounds per cubic foot, so it requires solid support and adequate fastening. It is easily cut with a wet saw or scored with a utility knife, but cutting generates fine glass dust that requires respiratory protection. The boards are often adhered with specialized mastics or mechanically fastened with sleeves and fasteners rated for below-grade use.

Cost is higher than foam board or fiberglass, and availability is limited compared to commodity insulation products. Installation labor is typically more exacting because the material cannot flex or conform to irregular surfaces. In crawlspaces and foundation applications, cellular glass performs well around basement walls and under slabs where capillary moisture would destroy other insulations. It is also used in cryogenic piping, where its low thermal conductivity and rigid structure support the extremely cold lines without degradation.

The name reflects its literal structure: a glass material containing cells. It is sometimes called cellular glass foam or glass foam insulation. Its immunity to moisture and biological attack, combined with its inorganic composition, gives it a service life essentially equal to the building envelope itself, making it a durable choice where replacement would be difficult or expensive.