Concrete slab-on-grade application is increasing in our sustainable practice, partly because its effectiveness as solar thermal mass, and partly because it offers a straightforward medium for hydronic heating.
Subgrade insulation types sort into two broad categories, organic and inorganic materials. Organic (carbon-based) options are primarily polystyrene-based materials (either extruded or expanded polystyrene products) derived from petrochemical (fossil fuel) precursors. Inorganic (non-carbon-based) product options include modified natural mineral products (e.g. vermiculite and perlite), expanded-glass, and expanded-cementitious products.
From a sustainable perspective, the responsible designer needs to be ecologically informed when considering the decision fork between these two options, chemical vs. mineral. Polystyrene is the petrochemical-based (“plastic geofoam”) insulation option which now predominates in this application. Among the many viable mineral alternatives available, one is perlite.
Polystyrene probably enjoys 95-99% market share of under-slab insulation products in this country. Both XPS (extruded) polystyrene and its alternative expanded polystyrene (EPS) have their antecedents in the chemical processing of ethylene (from natural gas or crude oil) with another petrochemical derivative, benzene. The result of their reaction is ultimately polymerized into polystyrene.
Benzene is an undisputed carcinogen. Its reactant, styrene, is identified by EPA (Environmental Protection Agency) as “a possible carcinogen, mutagen, chronic toxin, and environmental toxin”. The processing intermediary, styrene monomer, is listed by the International Agency for Research on Cancer (IARC) of the World Health Organization as having high carcinogenic potential; some MSDS sheets list styrene monomer as a residual constituent in the product at up to 0.2 percent levels, according to Environmental Design+Construction.
The blowing agent utilized to expand EPS into a foamed insulation product was historically CFC-12, an ozone-depleting chlorofluorocarbon now banned internationally. The intermediary blowing agent now used is hydrochlorofluorocarbon HCFC-142b. Though less problematic, HCFC-142b is still destructive to earth’s ozone, and is anticipated to be discontinued as a blowing agent by EPA mandate in accordance with the 2002 Montreal Protocol. Successors to HCFC-142b in XPS production are likely to include HTC-134a and carbon dioxide (CO2), according to The Alliance for Responsible Atmospheric Policy.
One alternative blowing agent now in use, pentane, is not considered to be ozone depleting. Yet it and all alternatives proposed by the polystyrene industry – CFC-12, HCFC-142b, HTC-134a, and CO2 – are recognized greenhouse gases, contributors to anthropogenic global climate change.
These light-gas blowing agents, the “pneumatic supports” to the cellular structure of polystyrene, can be expected to escape over time – over how much time is disputed between industry and academic sources. But most certainly they will escape over time. As they outgas they will easily permeate the building and then escape into the troposphere.
Thus the ozone damaging and climate change-inducing gases used by the polystyrene industry – past, present, and proposed – present and unwholesome and persistent overburden to the global ecology for a great many years to come.
In terms of the building, this displacement of lighter gases by denser in-situ replacements (e.g. water) has been observed to result in insulation failure (critically diminished insulative value).
One Sustainable Option: Perlite
In light of these concerns in our practice we have sought to avoid polystyrene, and have variously considered a range of alternative products including such exotics as foamed glass (aerogel). Our alternative material of choice as of this writing is perlite. A naturally occurring expanded volcanic mineral, perlite is relatively available, affordable, easily installed, and inert, which is to say, non-degradable.
Properly detailed, underslab insulation utilizing this material will perform well over the lifespan of the building, which is to say it will not suffer the shortcomings of polystyrene: it will not degrade over time, nor expand/contract due to moisture, nor lose insulative value over time. Nor does it rely upon the fossil fuel socio-political industrial complex to assure a steady supply of the product.
The Perlite Institute describes the material as “a naturally occurring siliceous rock… that is inert, lightweight, sterile, permanent, incombustible, non-toxic, rot and vermin proof…” The extracted rock is heated resulting in expansion ratios of 4x to 20x the material’s original volume. The product can be specified to a unit weight as light as 2lb/ ft3, and its thermal conductivity is rated at 0.27-0.33 Btu-in/hr-ft2 (source: http://www.perlite.com). Its expansion by heat is of concern, since this raises its embodied energy quotient. Yet even so, this author deems the material as far and away a more responsible choice than that its ubiquitous rival, polystyrene.
Figure #1 details an in situ application for use as underslab insulation.
The author thanks David Yarborough P.E., technical advisor to Supreme Perlite, for his excellent technical suggestions contributing to this article.