By Steve Andrews

Insulated forms fall into three basic categories: stackable blocks, large-core molds and sheet foam systems. Stackable blocks are preassembled, but sheet foam products need some preparation. Large-core molds are stackable blocks that form the concrete into a post-and-beam wall.

Sheets foam systems use rigid insulation, usually extruded polystyrene. The sheets are joined with spacers that hold them 6 to 10 in. apart. This work can be done by the contractor or the dealer.

Stacking

At the job site, stacking the blocks or sheet foam assemblies is a lightweight task. That saves time and energy on all jobs. When the site is isolated or you just can't get a truck very close, the lightweight units can be a real back saver.

While the labor is physically a breeze, the job still takes planning. For most of the products, follow these simple rules:

• Secure wood guide plates to the footing.
• Start laying block at a corner.
• Stagger block joints.
• Stack cavities directly above one another.
• Brace the walls well.

During layup, rebar is placed horizontally on the metal or plastic cross members of every course. Vertical rebar spacing depends on the height of the wall and block design.

Bracing

Bracing requirements vary dramatically between products. On one extreme is R-Form, which uses horizontal bracing on 16 in. centers, both inside and out. On the other is Agrisol which calls for one plumb brace every seven ft. on the interior of basement walls. Some builders find they need more bracing for above-grade walls than manufacturers recommend.

After a few years of experience, some builders have found that applying adhesive to the joints reduces the need for bracing. One builder uses about 48 tubes of foam-compatible adhesive for a 1500 sq. ft. house.

Wind can cause problems. If the site is windy, extra bracing may be needed. Check for plumb for several hours after the pour--until the concrete sets up. If the wall shifts a bit, you can use the braces to straighten it before it sets up.

Although corner bracing is usually made from 2x6s, one builder switched to angle irons and saved time. Agrisol offers their Green Blocks with 45- and 90-degree corner blocks that eliminate the need for corner bracing. Several of the block systems use knob-like attachments to help keep the walls plumb.

Sheet forms can be "stitched together" quickly at butt joints by wrapping polypropylene bailing twine around the spacer tie plates on the form's outside surface.

Pouring the Walls

Stackable block systems require 25 to 33 percent less concrete than a standard 8-in. wall. Yet a number of builders selected sheet systems precisely because they would end up with a standard 8-in. poured wall.

Since the forms stay in place, concrete in foam forms cures for a full 28 days. That gives it greater strength. Claims run between 25 percent and 50 percent more strength than a conventionally poured wall. Cold-climate builders can pour in weather down to zero and still get a solid cure.

Pumper trucks are required when pouring above-grade walls, and many builders like to use them when pouring basements, too. Most builders pour their walls in 3 ft. to 4 ft. lifts. One builder managed to pour walls up to 22 ft.

Blowouts

Blowouts happen. Builders generally experience more blowouts during the first few jobs.

Developing a good understanding with your concrete supplier is the first step toward avoiding blowouts. If the delivered mix is too soupy, the extra water creates higher pressure which increases the number of leaks and blowouts. Most manufacturers recommend a five-slump mix of 3000 psi concrete.

Just in case, keep these blowout repair supplies on hand: two 3x3 ft. pieces of plywood and threaded rod. There should be a hole in the center of the plywood that has the same diameter as the rod. If a piece of foam bulges or pops, cut through the foam, scoop out some concrete and replace the foam. Then press the plywood over the spot. Slide the rod through the hole in the plywood and keep pushing until the rod extends through the form on the far side. Slide the other piece of plywood over the rod. Thread washers and nuts onto both ends of the rod and tighten them up.

Avoiding Voids

Because the forms stay in place, you'll never see voids that remain in the wall. So, you should invest some extra effort to prevent voids. First ask the concrete supplier to add plasticizer to the mix. This makes it slippery, so it flows better around obstructions.

Some manufacturers suggest vibration to eliminate voids. Corners may need special attention, because extra spacers can sometimes create voids. A few taps (gentle taps) with a hammer can help the concrete flow around the obstacles.

One builder uses a special needle-shaped vibrator. Following the pumper, a worker sticks the vibrator into the concrete every 3 to 4 ft. It only needs to run for a couple of seconds. Too long will bring water to the top.

Another builder, who uses sheet foam forms, takes the blade off a reciprocating saw. During the pour, he hits the tie plates for 10 to 15 seconds. He does this every three or four ft. along the form.

Size

A few builders noted that the forms were a hair off the published dimensions. One builder's 40-in. blocks were slightly less than 40-in. long. Over a long wall the accumulated error added up to over one inch.

Electrical and Plumbing

Nearly every builder agreed that installing wiring is straightforward. Using a router or a shaped hot knife, they make a groove in the foam just the right size for electric cable. Space for shallow boxes is cut out and boxes are glued or screwed to the concrete. Using a hotknife on foam can be faster than drilling studs in standard wood framing. Think ahead, you may have to preset a few vents, drains and electrical conduit.

Finishing

According to manufacturers, drywall and siding can be attached directly to the walls. On sheet forms, screws can grab hold of the tie plates. Other systems rely on non-solvent-based adhesives and screws driven into the wood sill plate.

Try it

Like any new system, insulated concrete forms require some getting used to. The first two or three projects will be full of lessons. To reduce the cost of those lessons, call on the manufacturers for technical support. Most builders report strong support.

The only way to find out how a new system works, is to give it a try. If you would like to start small, make your first project a simple foundation. That's a low-risk way to see what all the hoopla is about.

--Steve Andrews is a residential energy consultant based in Denver, Colo. This article first appeared in the Journal of Light Construction.

 

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