While the use of plastics was spreading in the 1950s and 1960s, it is no surprise that someone came up with the idea of using rigid foam to both form concrete walls and then provide insulation, with the added bonus of not having to strip and clean the forms. Such insulating concrete form (ICF) systems now are offered by more than three dozen manufacturers in North America alone, and provide good performance and ease of construction. The surprising thing is that they are not more commonplace.
Most ICFs today are made of some form of foamed polystyrene. A German apothecary discovered polystyrene in 1839, but it wasn't until 1922 that someone figured out what might be done with it. Then in 1954 Dow Chemical Co. introduced Styrofoam, its trademarked name for foamed polystyrene.
Today foamed polystyrene, most often in the form of expanded polystyrene (EPS), is widely used for many things other than ICFs. It is electrically nonconductive, consists of about 95% air, and is 30 times lighter than solid polystyrene. Its light weight makes it easy to handle even the largest ICF components, while the air trapped in the foam contributes to its good insulation characteristics.
Structures built with ICFs often are touted for their energy efficiency. This comes from the cumulative effect of three factors: a continuous R-value, reduced air infiltration, and thermal mass. With this method of construction, the entire concrete perimeter wall is continuous. The result is a structure with highly uniform resistance to heat gain or loss.
The monolithic concrete core also forms an airtight barrier. Places where air infiltration may occur are easily identified—window and door openings, for example—and can be sealed easily as well. This same concrete core provides a thermal mass that helps moderate temperature swings in occupied spaces as the difference between the inside and outside temperatures increases.
The combination of these three factors on a structure's energy efficiency has been dubbed “the ICF effect” by the Insulating Concrete Form Association (ICFA). But there are additional performance issues where ICFs shine.
The temperature stability of the structure combined with the absence of cold spots and drafts offers increased comfort, particularly in residential applications. ICF structures provide a noticeable solidity, with reduced flexing of floors and little or no vibration from the wind or slamming doors. The reinforced concrete construction also makes ICF structures far more likely to survive storms and earthquakes, usually remaining intact and functional. Finally, ICF structures are quiet. ICFA reports only about 1/6 as much outside noise finds its way into an ICF structure compared to a standard frame structure.
Easy to use
The other big advantage ICF systems offer is their ease of use. Although the variations are endless, the fundamental concept is stacking hollow blocks and filling them with concrete.
Contractors who have worked with ICFs say crews adapt easily to this type of construction. So what makes working with ICFs so easy? First is their simplicity. Many systems offer highly flexible layouts with a minimal number of individual types of pieces. All are designed to fit together easily, both stacked and end to end. Special units are available for forming brick ledges, radius walls, 45- and 90-degree corners, and tees. Each system's component pieces easily fit together, even where trimming is required.
Building with ICFs is also quick and quiet. The lightweight pieces make it easy for crews to unload units and set them into place. Because it is not an equipment-intensive activity, jobsite noise is kept to a minimum.
Erecting ICF formwork has many similarities with setting up removable forms, but there are distinct differences as well. All ICF manufacturers provide instructions and reference materials detailing the proper use of their products; many are available for free on the Web. Some go beyond just the methods for using their systems and include basic concrete information as well, which turns out to be a good thing because many of those installing ICF walls today are coming from the ranks of builders and are newcomers to working with concrete. For concrete contractors already familiar with ready-mixed concrete, the substitution of ICFs for removable forms is simply an adjustment.
Lessons from the field
Remember as you consider trying your hand at using ICFs that your knowledge about how to work with concrete gives you a head start toward successful ICF construction. Here are some tips to help make the transition easier.
Think about penetrations before pouring the concrete. It's easy to insert a sleeve, such as a piece of PVC pipe, for wires, pipes, and other services after the forms are positioned and before placing the concrete. Seal gaps around the penetrations with foam adhesive and make sure they are installed angled down toward the outside so any water will drain rather than accumulate. Note that electrical wiring, conduit, and piping are installed after the concrete is in place by cutting channels in the EPS.
Door and window openings usually will be deeper with an ICF wall than a framed one. That means someone will have to coordinate with the window and door supplier so there's no surprise at installation time. Also, leave a little extra room in forming the opening—with a concrete wall it's easier to shim than to trim. Stabilizing cleats or scabs can prevent problems at window and door openings. Use them to bind the foam to the buck material on all sides. As an alternative, use prefabricated bucks.
Be careful attaching braces, because as ICFs are filled with concrete, they compress due to the friction as the concrete slides down the inside of the forms. This means bracing must be attached to permit that movement. One rule of thumb is to expect 0.5% vertical compression, or about ½ inch for an 8-foot wall (but remember that things such as door bucks will not settle). Braces designed for use with ICFs typically have slotted holes for this purpose, so attaching the fastener at the top of the slot will allow it to slide down as the concrete is placed.
Use additional bracing because ICFs are not as stiff as removable forms. Consider bracing at intervals even less than what the manufacturer recommends. Jake Vierzen, owner of R-Value Concrete Structures, Freeport, Mich., says, “Regular poured walls are straight vertically, but an ICF wall is straight for just one course at a time.” To make sure his walls stay flat and vertical, he puts braces 20% closer together than the industry standard. But that isn't necessarily as bad as it sounds—traditional forms must be braced on both sides. ICFs generally can be braced on only one side because their web structure holds the other side of the form in place.
Use good judgment when ordering the concrete. A maximum aggregate size of ¾ inch will work, but somewhat smaller—say 3/8-inch maximum—will make it easier to fill the forms. Use a boom pump for placement. Also, the sound of the concrete moving in the forms can give you an indication of whether the mix is right. “The best jobs are quiet,” says Will Oliver of Polysteel Southeast Distributors, Toccoa, Ga. “You can hear the concrete slide on the inside of the forms.”
Work with your concrete pumper, especially if there is some nervousness because it's your pumper's first time on an ICF project or he previously had a bad experience. He'll be pleasantly surprised how quickly a well-prepared job can go. Many contractors recommend placing a 3- or 4-foot lift all the way around the perimeter and then continuing back around with the next lift. The concrete should flow ahead of the placement, maintaining roughly a one-to-one slope. For a 4-foot lift, it should be flowing 4 feet ahead. Although the pumper's tendency might be to use a large hose and not move too fast, you'll probably be more productive with a 3-inch hose steadily supplying the concrete. Be sure to use two 90-degree elbows at the end of the hose to slow the concrete's velocity and limit the overall drop from the end of the hose into the forms.
Use a pencil vibrator, up to about 13/8-inch diameter, to ensure good consolidation. Always use it with caution, but especially near corners. External vibration systems are available, but for ICF work internal vibration generally is recommended.
Continue to check the wall straightness as the concrete is placed. Also keep an eye out for any bulges. It's handy to have several extra braces available in case you need to push an unexpected bulge back into alignment.
Have a blowout kit on hand that includes several 2-foot square pieces of plywood, sheetrock screws, a ladder, and a fully charged electric drill driver. If you have to use it, remember to stop the pump and the vibrator while repairs are made.
Although ICFs have yet to take the industry by storm, their use is growing and so is the variety of applications where they are being used as their familiarity grows among engineers and architects. The new PCA 100-07, “Prescriptive Method for Design and Construction of Residential Concrete Walls,” released just last fall, offers the first ANSI-accredited standard for building one- and two-family residences with ICFs. Its load tables and other design aides make it possible for builders to construct code-conforming concrete structures without resorting to an individually engineered design for each one.
The good news doesn't stop with the fact that ICF construction results in good structures. It's also a simple and straightforward approach to concrete construction, and the learning curve is very short. But like any concrete work, the success of the concrete placement depends upon good preparation and planning.