Foamglas Insulation: A Great Option for Below Grade

For below-grade applications where moisture resistance and high compressive strength are needed, extruded polystyrene (XPS) and, to a lesser extent, expanded polystyrene (EPS) have long dominated the insulation market. But there are growing concerns both with the brominated flame retardant HBCD used in XPS and EPS and with the global warming potential of the blowing agent used in XPS (see “Polystyrene Insulation: Does It Belong in a Green Building,” EBN Aug. 2009, and “Avoiding the Global Warming Potential of Insulation,” EBN June 2010).

Foamglas building insulation has been made by Pittsburgh Corning since 1937 and is widely used in Europe. For over 60 years, however, it has only been actively marketed in North America for industrial applications. Now Pittsburgh Corning is actively marketing Foamglas for building applications.

What is Foamglas?

Foamglas is a rigid boardstock, cellular-glass insulation material that is impervious to moisture, inert, resistant to insects and vermin, strong, and fairly well-insulating. It can be used for insulating roofs, walls, and below-grade applications, including beneath slabs. The two most commonly used forms are an unfaced “T4+” product (also known as Foamglas One), and a faced form, Readyboard, with protective facings on both sides. The T4+ boards are available in 18" x 24" panels; Readyboard is sold in thicknesses from 1½" to 6", in ½" increments, both come in 2' x 4' panels.

Foamglas is made primarily from sand, limestone, and soda ash. Virgin raw materials are used in U.S. factories, while up to 66% recycled glass is used in European plants. These ingredients are melted into molten glass, which is cooled and crushed into a fine powder. The powdered glass is poured into molds and heated in a sintering process (below the melting point) that causes the particles to adhere to one another. Next, a small amount of finely ground carbon-black is added, and the material is heated in a cellulation process. The carbon reacts with oxygen, creating carbon dioxide, which forms the insulating bubbles in the Foamglas. This CO₂ accounts for more than 99% of the gas in the cellular spaces, and it is permanently trapped there.

If you scratch a piece of Foamglas with your fingernail, you will detect a rotten-egg smell from hydrogen sulfide, which is produced in small quantities in the manufacturing process. While hydrogen sulfide is hazardous at high concentrations, there is very little in Foamglas, and it’s locked tightly into the cellular glass. Even after 30 years in place, scratching Foamglas produces the same smell. “It’s proof that the cells are absolutely airtight,” says Axel Rebel, vice president and general manager of Pittsburgh Corning’s North American buildings division. Even during landfill disposal, the glass cells are unlikely to degrade as quickly as cells of foam plastics, and any release of hydrogen sulfide would be dwarfed by the production of this gas from anaerobic decomposition of organic matter.

Environmental attributes

Foamglas T4+ Technical Performance Properties

Foamglas has a number of important environmental and human health advantages over other insulation materials. It has no blowing agents that deplete ozone or contribute to global warming. Being noncombustible and inorganic, it has no flame retardants or other additives needed to improve fire resistance.

While the domestically produced Foamglas does not (currently) contain recycled content, the materials going into it are abundant and extracted with relatively low environmental impact. Fossil-fuel energy is used in manufacturing, but there is no hydrocarbon material in the finished product.

Foamglas is also highly durable. A West Virginia reader of a blog I recently wrote on the material said there was “absolutely no apparent deterioration” of Foamglas that his father had used under the floor slab and on exterior foundation walls in the 1950s—nearly 60 years ago.

Key performance attributes

High compressive strength.Foamglas T4+ will fully support almost any concrete slab—and may even reduce the necessary thickness of a concrete slab in some situations.

Waterproof and impervious. Foamglas is waterproof and impervious to water vapor. Foamglas panels are typically installed using an adhesive and asphalt sealer between the panels to ensure a continuous seal, making it airtight and thus a highly effective radon barrier. While neither moisture nor freezing in itself damages Foamglas, moisture exposure in areas with freeze-thaw cycles will gradually degrade Foamglas. In below-grade applications in those climates it should be protected to below frost depth, and Foamglas should not be used in an “inverted roof membrane” application in which the insulation is installed on top of the membrane.

Fireproof. Foamglas has exceptional heat and fire resistance, with a maximum service temperature of 900°F (500°C) and a melting point of over 1,800°F (1,000°C). There are no binders to burn, so virtually no smoke is produced in a fire.

Rot-proof and vermin-resistant. Being inorganic, Foamglas will not decompose and is not a food source. Termites, carpenter ants, mice, and rats will not tunnel through it; Foamglas is sometimes used as a termite shield when other below-grade insulation materials are being used.

Reasonable R-Value. Foamglas T4+ insulates to R-3.44 per inch with no degradation of thermal performance. This is a lower R-value than extruded polystyrene and most expanded polystyrene, which means that greater thickness will be required to provide comparable performance. A 6" layer will provide slightly over R-20. Foamglas is often used in Europe in buildings that achieve Passive House performance, with multiple layers used to achieve very high R-values.

Working with Foamglas

Foamglas is typically adhered directly to a substrate, though mechanical fasteners can also be used. In most applications, an asphalt-based sealer is used between the boards, and in roofing applications hot asphalt is often used directly on it. The integral bitumen (asphalt) facings on Foamglas Readyboard simplify roof installations by allowing the membrane to be melted in-situ with a torch.

For environmental builders otherwise attracted to Foamglas, use of asphalt sealant will likely be the greatest concern. According to Rebel, when there is no need to have the installation be vapor tight, sealant can be left out or a mineral adhesive (similar to mortar) can be used. Or a separate vapor retarder membrane can be used—though this option leaves a risk of penetrations. Pittsburgh Corning also offers a range of adhesive options, including low-VOC materials, but Rebel says that an organic layer is required with any insulation material if a continuous, truly impermeable layer is called for.

Cost and availability

Foamglas is significantly more expensive than the other commonly used rigid insulation materials. The typical cost of Foamglas T4+ is about $1.00 per board-foot, depending on quantities, according to Rebel—roughly two-and-a-half times the cost of XPS. On a cost-per-R-value basis, that difference is even greater. Rebel admits that if you’re comparing insulation materials simply on cost and insulation value, you’re not going to choose Foamglas. “We have to add another value,” he says. That value can come from replacing other layers in the construction system (vapor retarders, moisture barriers, radon-control layers, termite-proofing), from greater durability, and from environmental attributes. Rebel also notes, “We can reduce the thickness of the concrete slab because Foamglas is so rigid.”

Foamglas is manufactured at two U.S. factories (in Texas and Missouri) and can be shipped anywhere. Rebel told EBN that it’s no problem to supply it for individual houses—though shipping may increase the cost and result in some additional lead time.

User experience

Foamglas has a long history of use, especially in Europe—where there tends to be a willingness to spend more money for highly durable and top-performing construction materials. Building science expert John Straube, P.Eng., has used Foamglas on construction details that required high strength and decent insulation such as under footings and brick veneers, and he considers it a good insulation option. From a moisture management perspective and as a thermal break material, he says that it works very well.