Product Review from Environmental Building News
June 1, 2009
Retrofitting Exterior Foam on Existing Walls
Achieving necessary reductions in greenhouse gas emissions will require almost every home in colder climates of the U.S. to undergo a
deep-energy retrofit, often increasing wall insulation to R-30 or more and attic insulation to R-50. Although these retrofits are not technically challenging, they are extraordinarily expensive, running $50,000–$100,000 per house (see
EBN July 2007). A new product from Quad-Lock Building Systems, a Canadian manufacturer of insulated concrete forms (ICFs), can simplify the installation of exterior foam on existing houses. The R-etro System includes four components: 4¼"-thick (110 mm) panels (called Plus Panels), plastic clips (called R-etro Ties), galvanized steel starting track, and fasteners.
Adding exterior foam sheathing to a wall is the easiest way to add significant insulation value to a building because it avoids disruptive interior modifications. The strategy works well as long as steps are taken to avoid condensation problems. Many existing walls experience wintertime condensation on the interior face of the wall sheathing. In most cases such condensation causes few problems because the walls dry quickly to the exterior when warm weather returns in the spring. When foam is added to the exterior of an existing wall, the wall’s ability to dry to the exterior is reduced. That’s not necessarily a problem, though, because rigid exterior foam raises the temperature of the wall sheathing. To work well, exterior foam has to be thick enough to keep the interior face of the wall sheathing above the dew point; as climates get colder, the minimum thickness of exterior foam increases (see table above).
The easy-to-install R-etro System foam provides enough added R-value (R-18) to avoid condensation problems in all climates. Installers begin by fastening a length of galvanized-steel starting track at the base of a wall. The first course of foam panels, each of which measures 4¼" x 12" x 48" (110 x 305 x 1200 mm), is placed in the starter track. Each panel has a series of slots at the top; these slots are designed to accept the plastic R-etro Ties that secure the panels to the wall at 12" or 24" on center (305 or 610 mm), depending on the fastening requirements of the intended siding. The plastic ties are attached to the wall with screws or Tapcons, and hold the bottom of the next course of foam panels.
R-etro Ties have a base plate that establishes a ¼" (6 mm) gap between the foam and the existing wall, providing a drainage space for any water that gains entry. Accumulated moisture drains down and exits through weep holes in the starter track at the bottom of the wall.
According to Douglas Bennion, a technical representative for Quad-Lock, the air gap separating the exterior foam from the rest of the wall does not degrade the wall’s thermal performance. “The air gap actually adds to the wall’s insulation value,” says Bennion. “The gap is sealed at the top. Even at the bottom it’s not really open, in spite of the small holes where water can drain out. So there’s no convection—it’s a static air layer.” Building scientist John Straube advises, “The top of the cavity must be sealed, as should corners. It is critical to install a really good air barrier—for example, Tyvek, taped, and sealed—under the R-etro system.”
The R-etro System is a quick method of installing exterior insulation. Kerry Petersen, an ICF installer in Tucson, Arizona, has used the system over wood-framed walls. “We installed the foam right over the existing stucco,” said Petersen. “We used deck screws, and it went extremely fast.”
Like ICF walls, R-etro System walls can be finished with almost any type of siding. For a stucco installation, wire mesh can be mechanically fastened to the plastic R-etro Ties with Windlock fasteners. Wood or fiber-cement lap siding requires the use of special foam panels (Plus FS Panels) with integral plastic furring strips embedded at 12" (305 mm) on center.
Windows and doors require weatherproof exterior trim. “The details depend on where you want the windows to sit,” said Bennion. “Windows can be pushed to the outside edge of the foam, or they can be recessed. For recessed windows, you start with peel-and-stick flashing that is properly shingled. From there you trim it out with whatever product you want—some sort of metal flashing that extends out over the new insulated portion of the window sill.”
Windows that are installed at the outside edge of the foam are usually secured to a metal track installed around the rough opening. “The track is like a small metal stud,” said Bennion. “It’s a metal furring strip, about 2¼" (57 mm) wide. You make a saw kerf to accommodate one leg of the metal stud, and you fill the U-channel with spray foam and press it in the kerf. Or you can screw the track to the ties, so you have a mechanical connection on the sides.”
According to Bennion, the system is forgiving. “It’s important to realize that once this system is on the outside of the building, you don’t have anything in the system that is susceptible to rot,” said Bennion. “The system has superior drying capacity. Of course, you should take proper considerations for sealing up joints, but if there is any bulk water that gets inside this system, it is going to drain out.” Although it’s possible to install rainscreen strapping on the outside of R-etro System foam, Bennion believes that the system is so forgiving of moisture intrusion that a rainscreen is unnecessary.
– Martin Holladay
Martin Holladay is a senior editor for BuildingGreen’s homebuilder website, GreenBuildingAdvisor.com
For more information:
Quad-Lock Building Systems
Surrey, British Columbia
888-711-5625; 604-590-3111
www.r-etro.com
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