Cellular Glass Roof Insulation

by Alvin Nunnikhoven, RRC

In continuing our series on roof insulations, this article focuses on the use of cellular glass insulation.  Cellular glass insulation is composed of crushed glass combined with a cellulating agent. These components are mixed, placed in a mold, and then heated to a temperature of approximately 950 degrees Farenheit.

During the heating process, the crushed glass turns to a liquid and decomposition of the cellulating agent causes the mixture to expand and fill the mold. As the mixture expands, it creates millions of connected, uniform, closed-cells to form a rigid insulating material. Cellular glass roof insulation was first used in commercial roof systems in 1943.

Cellular glass roof insulation is available in flat and tapered boards as well as flat and tapered blocks. Cellular glass insulation boards are produced with asphalt laminated kraft paper facers, while the cellular glass blocks are unsurfaced. Flat boards are available in 24" x 48" thicknesses of 1 1/2", 2", 3" and 4". Tapered boards measure 24" x 48", are tapered over the 24" dimension at either 1/8" or 1/4" per foot, and have a minimum thickness of 1" and a maximum thickness of 2 1/2". Cellular glass flat blocks are smaller in size than flat boards, measuring 12" x 18" x 1 1/2" thick and 18" x 24" x 2"-5" at 1/2" increments.  Tapered blocks are also smaller in size than tapered boards, measuring 18" x 24". Tapered blocks are tapered over the 24" dimension at 1/8", 1/4" and 1/2" per foot with a minimum thickness of 1 1/2".

Cellular glass has many desirable insulating properties. Cellular glass insulation is impermeable, non-aging, dimensionally stable, noncombustible, strong, and chemically resistant to, name a few. Cellular glass is impermeable, meaning it does not readily absorb moisture. Subsequently, cellular glass insulation has better thermal efficiency retention than other roof insulation types.

Cellular glass insulation has a linear coefficient of thermal expansion (0.0000046 in/degrees Farenheit) similar to steel and concrete. This low coefficient means that for a 100 degrees Farenheit temperature change over 100 feet, cellular glass insulation will expand approximately 0.55 inches. This minimal expansion creates a stable base for the roof membrane system, provided the insulation is properly secured or attached to the deck.

While cellular glass insulation has many desirable properties, it also has some disadvantages.  The kraft paper facers on flat and tapered boards are moisture sensitive and must therefore be protected from exposure to prevent moisture absorption and degradation. Because of its high compressive strength, the insulation may not conform to irregularities in the roof deck surface. This is especially critical if the insulation is being attached to the roof deck with hot asphalt. While mechanical fasteners are an acceptable means of attachment, the abrasive nature of cellular glass may remove the corrosion-resistant coatings typically applied to these fasteners.