The Superior Wall System That You’re Not Using

By Matt Baker

Slice any single-family house built in the last hundred years in half and you’re likely to find the same thing again and again: dimensional lumber and fiberglass insulation. It has been the standard way to build for so long, it seems at times like it may be the only way.

But stick-built homes have many flaws. they don’t hold up to powerful storms, for example, and are susceptible to termites, mold and fire. And it’s not just safety issues. They are hard to effectively insulate and guard against air and moisture infiltrations. Even if you are able to put in stellar insulation, every stud is a thermal bridge that allows unwanted heat to seep in or out, depending on the season.

There is an alternative, however. Insulated concrete forms (ICFs) offer several wall components in one system: concrete forms, insulation, furring strips, air infiltration barrier and vapor barrier. “You pour this wall, you have a lot of work done already,” said Eric Barton of Biltmore Insulated Concrete, Inc.

ICFs are essentially two expanded polystyrene (EPS) panels separated by plastic or composite “webs.” Designed for quick installation, ICFs are stacked like masonry blocks, with the center void then filled with concrete.

The webs perform several functions. First, they keep the EPS panels separated during installation, but also hold them together during the concrete pour. They provide an incredible stability on their own, but they also have slots to accept horizontally-oriented rebar.

In lieu of 2x4s, the ends of the webs act as furring strips. Since the webs are all eight inches apart on center, subcontractors installing siding, brick ties or drywall will not only have the customary 16 inch stud width, they have a more versatile eight inch gap.

There are three types of ICF wall systems: post and beam, grid and the more common flat panel. The two former types can be stronger and use less concrete, but the challenge of vibrating concrete into all the nooks and crannies directs most contractors to the panel style. No matter the style, the concrete used with ICFs should use smaller aggregate—no bigger than ¾ inch—and have a high Portland cement content.

to the pouring of concrete, buttress the EPS panels and act as furring strips for the eventual installation of siding and drywall.

During the pour, the EPS panels act as concrete forms and as insulation after installation. It’s durable and cheap, but the star feature of EPS is definitely the insulation factor. “Think about it,” said Barton. “You have a 195 degree cup of coffee separated from your hand of 98 degrees by an eighth of an inch of EPS.”

A standard blown-cellulose wall can claim an R-value of around R-14. By contrast, a six inch ICF wall is more like R-24. The key to this is the foam-concrete-foam sandwich. Heat, whether outside in the summer or inside in the winter, is going to want to move through the wall. With ICF’s, it first has to fight through a minimum of 2¾ inches of EPS, though that could be thicker if the client desires. This alone takes a long time, but then there is the concrete core to heat up. On the other side, there is more EPS insulation.

This heat transmission is incredibly slow. “You can never stop that transfer of energy,” said Barton. “The goal is to slow it down and lose as little as possible.”

Approximately 25% of heating and cooling losses are due to air leaking in homes. The continuous, monolithic insulation also means that a properly installed ICF wall is air-tight. Because of this, there is no need to install air and vapor barriers above grade. With no cellulose to act as a food source, ICF walls are also mold-resistant.

One unintended benefit of these walls are the way that they cure compared to traditional, removable-form concrete walls. The secret to durable concrete is retaining moisture as long as possible. The longer it cures, the more substantial it will be. “The strongest way you can make concrete is to pour it and then put it in Lake Michigan,” said Barton.

Since the forms don’t come off of ICFs, the wall holds in moisture longer. The concrete will harden just as fast as in a traditionally-built wall, but it will cure for a longer time, leading to a virtually indestructible wall. The insulated panels mean that ICFs are also a good option for construction in the temperature swings of Chicago’s extreme summers and winters.

When the Greenbuild expo was in Chicago in 2010, the USGBC, Habitat for Humanity and some corporate sponsors teamed up to leave behind a legacy project: two LEED Platinum single family homes constructed side by side in Lake County. The two houses were identical in every way except that the shell of one was built with lumber and the other with ICFs.

The lumber-built house was made with 2×6 framing, with an air infiltration barrier, 1 inch of foam board sheathing on the outside and open cell, spray-foam insulation inside. By any account, it was a far superior wall system compared to traditionally-built homes.

And yet, the ICF house outperformed it with an R-value of R-24, 21% higher than the framed house. It was also much tighter, as determined by a blower door test performed on both houses. The framed house registered 2.76 air changes per hour, less than half of what was allowed by the International Energy Conservation Code at the time. The ICF house came in even better, however, with only 1.15 air changes per hour, outperforming its neighbor by nearly two and a half times.

ICFs are approved by building codes and have been available for nearly fifty years, but they still haven’t taken more than 10% of the residential market share. This is true even in commercial applications where concrete is more common. Masonry blocks, despite a longer installation time and without the built-in insulation, prevail by a wide margin.

But the time may be right for the technology to really get a foothold. It is, after all, five to ten times cheaper to conserve energy than to produce it. Those numbers mean a quick return on investment, something that no building owner—whether that’s commercial or residential—can ignore.

Photos:

Amanda DeVries
Kent Baxter/FEMA

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One Response to “The Superior Wall System That You’re Not Using”
  1. Rob Ross says:

    Great article guys. I was wondering if these were legal in Chicago, and I guess they are. It’s a great building material.

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