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My First ICF Foundation - Continued

When it comes time to fill ICF blocks with concrete, the tops of the walls are more likely to want to tip out than in, since they are braced only on the interior. Because of this, the manufacturer recommended that we tip the tops of the walls slightly inward. Regardless of how hard we tried to plumb and line, as I sighted down a course of blocks, it was wavy; variations of as much as 1/2 to 3/4 inch were not uncommon (Figure 7).


Figure 7. A vertical 2-by braced against the bank helps keep a wall intersection plumb. The wall braces, which can be seen behind the ICF wall, support staging brackets, which provide access for stacking the upper blocks and pumping the concrete.

We laid rough planks on the brackets and continued laying blocks. The plans called for a step down into the living room, so we cut the last course of blocks to height to accommodate the jog. The foam blocks stacked imperfectly, so the last course multiplied the foam's imperfections, leaving an up-and-down top edge. To correct this, we shot elevations, snapped lines, and trimmed the top course with a handsaw and Surform. The results were imperfect at best. Next, we dropped precut 5/8-inch rebar down each core into the drilled holes in the footing and wired them to the top horizontal rebar.

Finally, since the top of the floor trusses would be flush with the top of the sill plate, we created pockets in the foam to match the truss layout. To form the pockets, I cut slots in the foam blocks. I doubled up rough-sawn 2x6 blocks, scabbed them together, placed them in the slots, and held them in place with 1x3 strapping screwed into the plastic tees. After checking the diagonals at the top of the block walls, we were ready for the concrete pour.


Figure 8. A pumper truck is essential for placing concrete in ICFs.

Don't attempt to fill ICF forms without a concrete pumper (Figure 8). The concrete recipe for ICFs is a cement-rich 3,500-psi mix with extra sand. The proportion of large aggregate — in this case, the maximum size of the aggregate was 3/8 inch — was less than usual, and the slump was between 5 and 6 inches. This oozy mixture is perfect for the concrete pumper and doesn't get hung up on rebar and web ties. The end of the pumper hose was reduced from 4 inches to 3 inches, lessening the force of the concrete against the easily damaged blocks (Figure 9).


Figure 9. Moving the hose evenly around the forms ensures that the concrete is placed in shallow lifts. Note the steel wall braces against the rear wall, which support staging planks.

It's critical to fill the forms gradually and evenly, in several lifts. Our guy circled the foundation three times, allowing the concrete to set up somewhat before adding more. Around windows, he carefully built up concrete on each side. Triangular peep holes cut in the foam under each window buck, in the center of the opening, allowed us to detect voids. At corners, he switched from side to side to keep the pressure equal.

I held my breath during the pour. Although there were no blowouts, we were prepared for this possibility with 3/4-inch plywood patches that could be screwed to the plastic studs to cover a rupture. At the end of the pour, we inserted anchor bolts 4 feet on-center.

Sticky Stuff Won't Stick

Insulated concrete blocks are not waterproof, so they have to be protected from groundwater by attaching a waterproofing membrane to the exterior (see "Waterproofing ICF Foundations," 2/00). I chose Grace's Bituthene 3000, a peel-and-stick elastomeric membrane.

Applying the membrane is a two-person job. We installed it vertically, rough-cutting the membrane in sections long enough to cover the wall from the sill to the bottom of the footing. We nailed one end of the membrane to the pressure-treated sill with roofing nails. Then, while one of us peeled away the paper, the other smoothed the Bituthene and patted it against the blocks. We ran our hands across the membrane, ensuring even adhesion. Although the seven rolls of Bituthene came from the same lot, some of the material would not stick to the ICFs (even though we had hosed the surface dirt off the blocks). The manufacturer was unsuccessful in identifying the cause of the problem. The only cure they came up with was to buy additional Bituthene in the hope that it would stick. I did, and it did. (A year later, the basement remains leak-free.)

With the membrane in place, we backfilled carefully, dumping a 2-foot perimeter band of sand against the foundation to avoid membrane punctures caused by sharp rocks, and then proceeding to backfill with excavated material. To hide the Bituthene and protect the foam above grade, I cut 2-foot-wide strips of a fiber-cement panel siding, HardiPanel, and nailed the strips to the sill.

Do They Stack Up?

If you've been mentally building this foundation as I've been describing it, by now you realize that there are many steps, and many opportunities for procedures to go awry.

Although the ICFs I used are a good product, the documentation and instructions could have been stronger. As I puzzled together the best way to assemble the forms, many of my questions could be answered only by phoning the manufacturer.

What little guidance I received was strong on concrete technology but weak on a builder's perspective of how a house goes together. For example, the recommendation that I form pockets into which to drop floor trusses — rather than supporting the trusses on a ledger — added unnecessary expense, wasted time, and was unforgiving of layout errors. Details are available for attaching a ledger to ICFs (see "Step By Step With Foam Forms," 12/95). I wish I had had this information before I started.

And then there were the problems that arose as I went about building a house on top of the foundation. For example, to disperse a point load, I had to place 3/4-inch plate steel under a post holding up a corner of the second floor because there was more polystyrene than concrete at the point of contact. When I went to attach corrugated steel window wells around the basement windows, I had nothing to screw into but foam. And when I went to screw plywood to the interior walls to support the electrical panel box and mechanical system components, I discovered that the fastening strips didn't hold the screws that well. Attaching electrical boxes to the form was also challenging.

The summer I built this foundation was one of the wettest on record. Laying up ICFs is time-consuming, and after each rain, the cellar hole sides collapsed, forcing us to bring back the excavator to clean out the hole. I lost two weeks dealing with mudslides, and another two weeks with the Bituthene problem. Had I used conventional plywood forms, I could have snuck a foundation in between storms and mopped on asphalt dampproofing, all in a matter of three or four days.

I'm planning to build a deck, but since I can't attach it to the foundation, it will have to be a free-standing structure supported by additional concrete piers — an added expense. If I decide to add a family room, there will be similar problems, as I work out the best way to attach a poured concrete frost wall to an ICF wall with a waterproof connection.

Sometimes, Low Tech Beats High Tech

ICFs entered the market as an innovative solution challenging a time-tested, low-tech building system for foundations — plywood forms filled with concrete. While the insulating benefit of ICFs is hard to ignore (my ICF manufacturer claims an R-value of 22, while others boast values as high as 40), I wonder if this feature is worth the time and trouble of learning a new set of skills, dealing with new materials, the potential for blowouts, the extra cost of using skilled labor, the extended construction time, and the need for long-distance technical support.

Could some of my problems have been avoided? Sure. I take full responsibility for plunging in before fully investigating ICF building technology and not putting in the extra thought needed to plan out an ICF foundation. On the other hand, some problems and system limitations would not have gone away. These include the extended building time, waterproofing problems, the cost of skilled labor, and structural restrictions.

My suggestion: If you decide to use ICFs, pull together information from several manufacturers, watch their videos, and then choose an approach that fits your building style. Also, buddy up with someone who builds with ICFs before attempting one on your own.

Lee McGinleyis a builder in Addison, Vt.