13 tips on working with insulating concrete forms
On the lookout for ever more energy-efficient building envelope solutions, a growing number of Building Teams are turning to insulating concrete forms as an alternative to light-gauge steel frame, wood frame, brick masonry, and precast concrete construction in the nonresidential market.
ICFs are 4- to 10-inch-thick concrete walls poured between expanded polystyrene (EPS) forms that stay in place permanently as an insulation barrier. The result is a highly insulated, virtually airtight wall system that offers a minimum insulation value of R-20.
ICF walls can reach heights of 100 feet or more, making them viable for all types of commercial buildings, including hotels, condos, K-12 schools, movie theaters, retail stores, churches, student residence halls, and office buildings. Commercial work makes up a quarter of all ICF jobs and is the fastest-growing segment for the building technology, according to the Insulating Concrete Form Association, Glenview, Ill.
Yet, despite their growing popularity, ICFs are still relatively new to much of the AEC community. To help educate industry professionals, BD+C asked several experts for tips and advice on designing and building with ICFs. Here's what they had to offer:
1. Reduce waste and speed construction by designing within the modulus. Architects can greatly reduce waste and speed construction on ICF projects by optimizing the dimensions of the building to work with the modulus of the ICF forms, says Vera Novak, the ICFA's director of marketing and technical services.
“Taking into consideration the block height when detailing the floor placement and the building height can reduce cut-off waste,” says Novak. She notes that a carefully placed change in concrete core width from one floor to the next may provide a ledger for either exterior stonework or interior beam support. “Designing to the modulus takes advantage of the design options and increases the constructability of the project,” she says.
Novak adds that structural engineers can optimize the placement of the rebar by adjusting the rebar placement schedule so that it corresponds with the height of each course of ICFs.
2. No need for air barriers or vapor retarders. When working with ICFs for the first time, some architects will want to incorporate air barriers, vapor retarders, and other weather-resistant barriers into the ICF walls system. “You don't need any extra layers with ICFs,” says Kelvin Doerr, VP of engineering and technical services for Reward Wall Systems, Omaha, Neb. “You just direct attach the veneer or apply the exterior finish to the ICF.”
However, Novak says that it is important for designers to remember that no concrete wall is considered to be waterproof; therefore, rain drainage and waterproofing details should be specified.
3. Be careful when vibrating concrete during pours. Strong foam-based forms have been known to blow out while vibrating the concrete during pours. Reward Wall Systems' Doerr recommends using a small-diameter mechanical vibrator, or “pencil” vibrator, to ensure that the concrete spreads evenly without harming the forms. Some installers use a reciprocating saw without a blade to provide the gentle vibration required to safely spread the concrete.
Rob Miller, PE, partner with M2 Design Group, Worthington, Ohio, suggests avoiding vibrators altogether. Instead, he recommends going with a higher-strength concrete mix—at least 3,500 psi.
“Increasing the strength of the cement mixture allows you to add more water without deteriorating the mix,” says Miller, whose firm specializes in commercial ICF design and engineering. “The result is a concrete that will flow better and avoid potential voids in the ICF walls.”
4. Try not to overcomplicate ICF construction. Doerr says structural engineers often overcomplicate ICF construction because they treat the technology as if it were a complex building system. “The ICF formwork is not a system or a structural component,” says Doerr. “In simple terms, ICFs are formwork that stays in place permanently for a cast-in-place concrete wall.”
As with any cast-in-place concrete wall construction, designers should reference ACI 318, Building Code Requirements for Structural Concrete and Commentary (www.concrete.org/bookstorenet).
5. Rightsize mechanical systems. A common issue in the commercial building industry is getting the local mechanical engineers to rightsize the mechanical systems to make the building as efficient as possible and reduce the heating and cooling costs, says Doerr. “If these systems are oversized, it can actually create humidity and moisture issues on the interior,” he says. “Mechanical engineers need to factor the tightness of the building envelope, the initial R-value, and the thermal mass that the ICF provides.”
Novak says that most commercial HVAC engineers have modeling software that can take into account the zero air infiltration through the walls and the continuous R-value of the insulation.
6. Corner windows can be a challenge. Designs that call for corner window units require extensive detailing, including “some sort of cantilever action for the rebar, because there's no corner post,” says M2 Design Group's Miller. “A corner window unit may be strong enough to support wood and metal framing, but it may not be able to support the weight of the concrete.”
7. Don't skimp on fasteners.When attaching anything to an ICF, use the fastener recommended by the interior and exterior finish supplier. Metal-webbed ICFs may require a self-tapping screw, while plastic-webbed ICFs work well with a sharp, course-threaded screw, according to Glen Klassen, regional manager with ICF manufacturer Arxx, Cobourg, Ont.
“I have seen people make their jobs difficult because they did not know that there are ideal fasteners to use,” says Klassen. “For instance, a smooth-finish pin nail will only work well temporarily. Also, certain screws and ring nails work better indoors than outdoors.”
8. Get it right at the roof line. In order to leverage the thermal characteristics of an ICF wall, the design team needs to pay close attention to the continuity of insulation and air barrier at the connection point to the roof, says Novak. “All too often, this detail is ignored, and the thermal envelope is greatly compromised,” she says. In addition, thermal breaks through the envelope, such as cantilevered concrete floors or exposed concrete slab edges, should be avoided.
9. Follow design guides. By utilizing the design guides that most ICF manufacturers provide, designers can make life easier for the engineering and construction teams. For instance, a proper design guide will show the recommended reinforcing requirements for a variety of building types, says Klassen. “This alone can save you money by not having the job over-designed,” he says.
10. Choose the right bracing and alignment system. “Simply put, if you have a wall that ends up not being level and straight when completed, you probably did not use the proper scaffolding and bracing systems,” says Klassen. Most ICF manufacturers recommend specific scaffolding and bracing systems that work best with their systems.
“Every so often, I will see a builder try to save money by creating their own bracing system,” says Klassen. “In the end, their jobs suffer by taking much longer to build.”
11. Use a qualified, licensed ICF contractor. In the past, Building Teams struggled to find three licensed ICF contractors to meet the requirements of the public project bidding process. That should not be an issue anymore, says ICFA's Novak, given the growing number of licensed ICF contractors across the U.S., including several firms that operate nationally.
“There are at least 25 ICF installers with commercial construction experience and who are bondable,” says Novak. “The best source of these contractors is the ICF block manufacturers themselves.”
12. Cantilevers can be costly. ICFs are very efficient and fast when building standard wall structures. However, when the designers add special structural features, like cantilevers, things can get tricky—and costly, says Miller. “It can be a challenge supporting the additional dead load of the heavier concrete walls when upper exterior walls are not stacked above foundation walls.”
13. No need for weather-resistant paper. With the adoption of the 2006 International Building Code on January 1, 2007, weather-resistant paper is no longer required over ICF walls.