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Using Foamglas instead of polystyrene to insulate beneath our basement slab and on the foundation walls.

Eli Gould cutting Foamglas for use under our basement slab. Click to enlarge.
Photo Credit: Alex Wilson

In my role with Environmental Building News and our GreenSpec Product Database, I get plenty of opportunity to research and write about innovative building products. That’s one of the really fun aspects of my job.

On occasion I also get an opportunity to try out new or little-known materials. In the construction of our new home in Dummerston, Vermont—actually the rebuilding of a 200-year Cape—I’ve had opportunity to get some real experience with lots of products. One of these is a cellular glass insulation material known as Foamglas (check out Foamglas in GreenSpec).

Why we need a product like Foamglas

I’ve written often about the problems with extruded polystyrene from an environmental and health perspective. Relative to performance, extruded polystyrene (XPS) is a great product. It is water-resistant so can be used below-grade; it has high compressive strength so can be used beneath a concrete slab floor; it insulates very well (R-5 per inch); and it’s inexpensive. These properties make XPS the nearly universal choice for sub-slab and exterior foundation insulation today.

We installed 4"-thick Foamglas as sub-slab insulation. Click to enlarge.
Photo Credit: Alex Wilson

But along with these benefits are some significant downsides. All XPS today (as well as expanded polystyrene, EPS) is made with the brominated flame retardant HBCD that has recently been added to the Stockholm list of Persistent Organic Pollutants (POPs) and is being banned in much of the world. HBCD provides some level of fire protection, though some studies suggest that its benefits are greatly exaggerated—and that that protection, if real, is irrelevant below grade.

In addition, XPS is currently made with the blowing agent HFC-134a, which is a potent greenhouse gas that contributes to global warming. And some of the petrochemical-derived raw materials, including benzene and styrene monomer, are carcinogenic—though once converted into polystyrene, that carcinogenicity is not present.

From a performance standpoint, XPS—like most other foam plastic insulation materials—is readily tunneled through by subterranean termites, carpenter ants, and other wood-boring insects.

In this photo you can sort-of see the cellular structure of the rigid material. Click to enlarge.
Photo Credit: Alex Wilson

Foamglas to the rescue

Foamglas is a cellular glass, rigid boardstock insulation material. It has high compressive strength, excellent moisture resistance, and tremendous fire resistance without the use of flame retardants. It is moderately well-insulating at R-3.4 per inch (32% lower than XPS), and it’s made without environmentally damaging blowing agents. It is also about the only insulation material that is totally impervious to wood-boring insects—a useful property for below-grade applications—particularly in a warming planet with termites extending their ranges north.

Foamglas has actually been around a long time—since Pittsburgh Corning introduced it in the 1930s—but it is used primarily for high-temperature industrial applications, such as insulating steam pipes and furnaces. It’s use as an insulation material for buildings remains very uncommon, though this use is increasing in Europe.

Even though Foamglas is significantly more expensive than XPS and its per-inch insulating value is lower, the environmental and health benefits made me want to try it out on our own home.

We installed 6" Foamglas blocks on the exterior foundation walls using a polymer-cement adhesive. Click to enlarge.
Photo Credit: Alex Wilson

Our use of Foamglas

We installed four inches of Foamglas under the basement floor slab and six inches on the exterior of the foundation walls. Our designer/builder, Eli Gould, and his six-person crew not only did admirably with this little-known material, but he came up with what I believe is a great option for adhering Foamglas to a foundation wall.

We were debating whether to use Pittsburgh Corning’s recommended solvent-based adhesive (“tar”) or their acrylic formula (a greener, water-based tar), which apparently doesn’t have quite as good performance properties as the solvent-based option. But the recommended solvent-based formulation sounded quite hazardous (it’s a two-component adhesive with one component consisting of three different types of diisocyanate and the other component consisting of petroleum asphalt, coal bitumen, naphthenic distillate, and hydrocarbon solvents). We wanted a well-performing adhesive, but the solvent-based option didn’t sound like something we wanted to expose workers to during installation or surround our home with. 

Eli tested different engineered cement products, as modern polymers have dramatically changed the adhesive capabilities of cement in the last couple decades. They are also free of VOCs and sounded far safer from a health and environmental standpoint.

Mixing the Ardicoat waterproofing material—an acrylic gets mixed with a proprietary mix of Type I and Type II portland cement.
Photo Credit: Alex Wilson

We settled on a polymer cement product  made by Ardex used for adhering stone veneer onto masonry walls, and it worked beautifully. The two companies (Ardex and Pittsburgh Corning) were so intrigued by our field-testing that they have begun conversations about testing and developing this alternative adhesive system.

Ardex also supplies a waterproofing coating that we applied over the Foamglas on the foundation walls: Ardicoat Plus. We used this in place of conventional asphalt-based (tar) coating, and I feel really good about not having hydrocarbons from the coating seeping into the groundwater or being released as VOCs.

Innovation and performance

Our foundation ends up with a respectable R-12 under the basement slab and R-22 on the exterior of the foundation walls. That’s not up to Passive House standards, but it should be good enough to enable us to achieve net-zero-energy performance with a PV system supplying power for an air-source heat pump. And it should last literally hundreds of years—a lifespan that I believe we should be aiming for in home building today.

We spent more for the Foamglas foundation insulation than we would have with XPS, but it feels good to have put my money where my mouth is relative to spurring product innovation and demonstrating greener building material options.

Eli and I also hope that by leading this sort of collaboration we may be able to help drive down the costs while broadening the market for Foamglas and other innovative products. With Foamglas and other inorganic products like this that may come along, we hope to see more durable, insect-resistant foundation systems that can help reduce energy consumption while minimizing health and environmental impacts.  

A layer of polypropylene mesh gets embedded into the Ardicoat for strength and flexibility.
Photo Credit: Alex Wilson

Foundations are not the only part of the building in which Eli and I plan to help companies “connect the dots” in developing better buildings. We’re working on innovative window solutions for existing homes, superinsulated roof systems, and modular components to speed construction—but those are topics for future columns.

Who knows, maybe we can even convince some leading manufacturers to move to the Brattleboro area and help to spur economic development in the region.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. He also recently created the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.

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