It is useful to compare two types of Insulated Structural Systems – namely, Insulated Concrete Forms (ICFs) and Wood Waste Masonry (WWM) – from a green building design perspective. While insulated concrete forms (ICFs) are clear front-runners with a vastly larger market share within the construction industry, ICFs have inherent limitations from a green building perspective. While Wood Waste Masonry systems avoid many of these limitations, they do have certain limitations, which will be discussed.
Insulated Concrete Forms 101:
ICFs essentially consist of two parallel panels of polystyrene held apart by rigid plastic spacers. Builders stack ICFs like Lego blocks, brace the resulting forms, and then fill them with concrete. The polystyrene shell of the ICF units is left to remain in place as a permanent part of the building. ICFs are becoming increasingly common. Green advantages advertised for most product lines over conventional wood-frame construction include zero reliance on lumber, low air infiltration, high strength and fire resistance, good sound-deadening qualities, and much higher insulating values when as against conventional masonry construction.
Wood Waste Masonry 101:
Wood waste masonry units are fundamentally the same as ICF’s except that instead of polystyrene as the shell material, recycled wood content mixed with cement as the binder is used instead. The resulting units are laid up in just the same way as their ICF counterparts, and once laid up and braced, filled with concrete. Again, similar to ICFs the outer shell remains in place, a permanent part of the wall.
The Challenges of ICFs:
From a green building design perspective, although they offer a number of advantages, ICFs pose certain difficulties.
The first of these has to do with sourcing of the insulation material – polystyrene – necessary to the building system. Polystyrene comes from petroleum, which, as a nonrenewable resource, represents a fatal flaw from a green building perspective. This flaw can be overcome by using certain ICF product lines containing only 100% recycled content polystyrene, thus guaranteeing that the source material was not mined from fossil fuel.
The next limitation has to do with ICFs allocation of thermal mass, the heat storage “battery” in the thermal performance of a building. ICF’s core is concrete, thus creating the potential for high thermal mass within the system. But this mass, by design, is thermally isolated from doing its work by the highly insulative polystyrene shell. Thus, its effectiveness as mass is inherently limited.
The third challenge is that polystyrene outgasses.
According to Alex Wilson at Environmental Building News, “concerns have been raised about the brominated flame retardant HBCD that is found in all polystyrene insulation, both extruded (XPS) and expanded (EPS). There is now enough evidence that HBCD is hazardous to both human health and the environment that European agencies are moving to restrict its use.”
Discussion of Wood Waste Masonry (WWM):
In lieu of polystyrene, wood waste masonry units utilize 100% recycled wood content as the shell material. From a green building perspective this is advantageous when compared to ICFs in three ways:
• It removes any possibility of the use of “raw”, non-recycled polystyrene.
• It completely avoids the flame retardant/ outgassing issues associated with HBCD.
• The wood waste sequesters carbon from the atmospheric pool of greenhouse gases.
Moreover, although not all product lines incorporate it, the best (e.g. DurisolTM) rely on insulative inserts set within the concrete core to achieve the insulative goals for the wall system, and place these inserts to the exterior side of the concrete core. In this way the thermal mass potential of the wall system is more available to function as part of the building’s thermal battery as compared an ICF system.
Wood waste masonry is not without it’s own challenges. Among them, the cement used as binder in the wood waste shell material does add marginally to the embodied energy of the WWM wall system as compared to ICFs. This should be considered a minor fraction of the wall’s overall embodied energy given the concrete core inherent to either system.
Finally, WWM’s are relatively untried. Although insulated structural systems in general and ICF’s in particular have been deployed as building systems since the 1940’s, wood waste masonry’s long-term stability characteristics carry with them no inherent guarantee. For example, DurisolTM, seemingly the most viable product on the market, does not apparently offer a warranty on their product.
This is serious shortcoming for any product system, let alone one intended to support a sustainable lifestyle over the course of many generations.
Comparing two types of Insulated Structural Systems from a green building design perspective, while insulated concrete forms (ICFs) are the clear front-runners with a vastly larger market share within the construction industry, ICFs have inherent limitations from a green building perspective. Wood Waste Masonry (WWM) systems, although they are relatively untried, seem to offer superior advantages from a green building perspective.