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
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.
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).
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
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
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
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
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
Sometimes, Low Tech Beats High
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
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,