Friday, October 16, 2015

Construction - Concrete Work - Insulated Concrete Forms

This is the fourth post on the concrete phase of construction.  It details the unconventional foundation under the stick-built walls.  The first post on concrete work covered the excavations for the footings.  The second dealt with pouring the wide footings and concrete walls.  A third detailed the narrow footings under the stick-built walls.  Here we dwell on the rather unique and unconventional foundation wall, poured in insulated concrete forms to support the stick-built walls.  (Click on any photo to enlarge it for closer inspection).

Shallow Frost Protected Foundation
Our foundation is based on a recent "green" innovation that has actually found its way into the International Building Codes.  It is called a "shallow frost-protected foundation" (see the earlier post on shallow frost-protected foundation).  Instead of the top of the footing for the foundation being below the frost line, it is protected by insulation so that it can be much closer to the surface.  Our plans call for a 20" foundation wall, i.e., the distance from the top of the footing to the top of the foundation is 20".  For termite reasons, the code calls for an 8" separation between the soil and the wood elements of the structure.  If this dimension is subtracted from the height of the foundation wall, the top of the footing is only 12" from the grade instead of the +/-30" needed to get below the frost line.  This represents a 46% dividend in terms of embodied energy in the concrete as well as a few dollars saved on the volume of concrete.  However, the forms do not come cheap (a little over $1,200 for the 164 linear feet  20" high).  It would have been almost twice as much if the forms were used to pour a conventional foundation that reached below the frost line.  However, it must be said that, for a first-timer, the installation was unexpectedly time-consuming and frustrating, due largely to the unevenness of our footing.

Amvic Insulated Concrete Forms
The brand of  insulated concrete forms (ICFs) that I selected was the Amvic System because it were available from a local supplier and was manufactured in Missouri, both of which help to minimize embodied energy which is an important sustainability issue.

Installing the Insulated Concrete Forms on the Narrow Footing
Let me say upfront that our installation had little in common with that detailed in the "ICF Technical & Installation Manual" for the Amvic System.  The need for improvisation was entirely my fault by not getting the footing perfectly level throughout.  In retrospect, it would have taken rented metal forms installed with huge precision to have met Amvic's spec of 1/4" variance for levelness.  But salvaged  2 x 8s, despite being a lot more work to install, not always straight-edged and impossible to install precisely, were free and produced a leveler footing than would have been possible by pouring directly against the soil in a trench.

The Amvic ICFs are available for wall thicknesses of 4" to 10" of concrete with 2 1/2" of expanded polystyrene ("styrofoam") on either side,  The styrofoam serves first as the outside and inside forms into which the concrete is poured.  The "Dixie Cup" walls are strong enough to handle the concrete by virtue of internal webs that hold the two sides together.  Secondly, the forms stay in place after the pour as R-22+ insulation.  Our ICFs were for 8" walls such that the overall thickness, styrofoam included, was 13".  The 20" height for the wall was achieved by using one course of blocks 16" high then adding a 4" so-called "height adjuster" course to the top.  The smooth top of the height adjuster also facilitated screeding the top of the wall when the concrete was poured.

Our house plans give the dimensions of the foundation wall in terms of the exterior planeof an 8" wall and the batter boards had been set to these exterior dimensions. The ICFs then had to be installed on the newly-poured footing so that the concrete poured into them matched the batter boards.  To this end, mason lines were strung one last time between batter boards so that the equivalent of corner posts could be established and marked with concrete screws on top of the footing.  A chalk line was then tensioned between screws and snapped to give the position of the inside edge of the exterior styrofoam, which is to say, the outside edge of the concrete when it is poured inside the styrofoam.
Chalk line wrapped around a corner screw and
 ready to snap for a layout line.

We used a technique for aligning the forms that the installation manual recommended, viz., 2 1/2" steel drywall tracks for precise positioning of the ICFs along the chalk line. The tracks were fastened with Tapcon concrete screws on the chalk line so that the outside styrofoam of the ICFs, when seated in the track, would give a perfectly straight wall.  But "straight" is not the complete answer -- the wall has to be level if the foundation is to be level.  Despite working so hard using wood forms to control the levelness of the footing, there was a variance of about an inch over 164 linear feet. Therefore, the variance had to be finessed as the ICFs were installed through a combination of shimming (raising) under the low spots and shaving the bottom of the ICFs (lowering) over the high spots (all of which was much more time-consuming than expected).

Leveling the ICFs Longitudinally 
Thank goodness for a rotating laser when working alone.  I used it first to survey the top of the entire footing at +/- ten foot intervals to see how much variance there was and where the highs and lows were located.  Based on that data, I picked a default height that was weighted towards shimming (raising up) rather than trimming the bottoms of the ICFs.  As a consequence, the highest shim was +/-3/4" and the most that the bottoms of the ICFs had to be trimmed was +/-1/4" and the latter occurred only over 10-15' linear feet distributed over three areas.  

Installing the Metal Track
The first task was to drill holes in the tracks so they could be screwed to the footing with Tapcon screws. The easiest way to do so was ahead of time with a drill press.The installation was started where the track could be fastened without shims at the default height.  As the installation proceeded around the footing, the following steps emerged as the best approach ("proximal end" means the end next to the last section installed and "distal end" means the end farthest from the last section):

-  Lay the track on the chalk line
-  Fasten the proximal end first guessing (based upon laser readings) as to the amount of
Friend, Glen, leveling
the track with laser
 guidance.  Click on
 the photo to see the
 shims under the track
 shimming necessary and placing the shim(s) adjacent to the screw rather than drilling through them 
-  Since part of the shims will remained in the concrete avoid using wood shims to avoid attracting termites; use plastic shims, either store-bought or made from styrofoam trimmings
-  Fasten the distal end of the track on the chalk line without regard to shimming
-  Check the proximal end with the laser; loosen the screw, adjust the shimming as necessary and retighten the screw
 -  Check the distal end with the laser, loosen the screw and shim as necessary
 -  Through each of the intervening pre-drilled holes in the track, drill holes in the
 -  Add the screws; shim beside each screw, guessing as to the thickness; tighten the screw and check with the laser; if necessary, loosen the screw and finesse the shim height
-  Helpful hint:  Before shimming, press down on the track above a screw hole with the laser measuring rod to get a read on the shim thickness required then add the screw and the shim; doing so eliminates some of the trial and error and tends to speed up the process.

Leveling the ICFs Transversely 
So the above efforts leveled the outside styrofoam but the inside styrofoam was still a mess. The ICF blocks are sufficiently rigid that the inside foam goes along for the ride longitudinally but they still need leveling transversely. And, of course, transverse leveling also plumbs the wall.  We tried two methods but the one that worked best was to wait on the inside shims until the forms were leveled longitudinally.  Then, in conjunction with plumbing and leveling the forms, shims were added as necessary.

Attaching the Forms to the Footing
The disadvantage of opting for more shimming and less shaving to achieve levelness was that there was a space between the metal track holding the outside styrofoam and even more space under the inside styrofoam that did not have a track taking up space.  The manufacturer recommends using a proprietary foam and sprayer to fill the space and "glue" the forms to the concrete. The customer service person at the local supplier suggested, since our project was a one-time and small project, not to invest in the sprayer but to use a spray adhesive instead. It wasn't until we were ready to use it did we realize that the styrofoam must be in intimate contact with the concrete for the adhesive to work.   So how do we DIY a way to fasten the forms to the footing and obliterate the gap under them so that the concrete does not raise the forms or leak out under them? 

(This then is the point of departure from the technical manual).  First we ripped enough 1 x
The 1 x 2s were fastened to the concrete.  To the right,
one layer sealed off the bottom but, to the left, two
layers were necessary due to unevenness of the footing.
2s from salvaged lumber to circumvent the entire ICF setup on both sides, butted them against the forms and fastened them to the footing with Tapcon screws.  This stabilized the bottom of the forms from the lateral pressures of the concrete during the pour and largely sealed off the space under the forms (there was the equivalent of 70 linear feet, randomly situated, that needed a second layer of 1 x 2s and 30 linear feet that sat directly on the concrete). However, the 1 x 2s did not keep the forms from lifting as would be expected from adhesive foam recommended by the technical manual.

While actually exceeding the manufacturer's specs for spacing of vertical bracing, we used 2 x 4s resting on top of the 1 x 2s and stopping just short of the top of the wall. The verticals were drywall-screwed into the plastic webs that tie the two sides of the forms together -- the same ones that serve as "studs" for fastening the exterior and interior finishes later. Then we used drywall screws and Tapcon screws to anchor to the footing by toenailing the verticals to the 1 x 2s or, where they did not exist, directly to the footing.  As recommended by the manufacturer, the basic 16" x 48" ICF straight blocks in the first course were tie-wired together then extra verticals were added where adjacent blocks were still a little unstable or unlevel.

Strongbacks and Lateral Bracing
In additional to vertical bracing, there needs to be lateral bracing.  We chose to do it on the interior of the forms so as to have it out of the way of pouring the concrete. The verticals on the interior of the forms provided solid fastening for the braces.The verticals on the exterior of the forms, though anchoring the forms and supporting the strongbacks, did not receive
View showing the verticals, strongbacks and braces
any braces since 
bracing was necessary only on one side of the forms.  The exception was a "T-wall" that intersected the exterior wall.  Here exterior bracing was necessary because, without the bracing, the weight of the concrete in the leg of the "T" might cause a blow-out.

Strongbacks would have been overkill if the wall had not been topped off with the 4" height adjusters that gripped the course below rather tenuously and looked suspiciously vulnerable to displacement during the pour.  So, as an insecure amateur, I needed the strongbacks as insurance..

Garage Door Cutouts
The garage floor will overlap the foundation below the two garage doors.  Therefore, the
Cutouts for the garage doors 
ICF forms needed to be 9" shorter under the doors, which is to say the forms had to be shortened by that amount.  Despite leaving the 4" extensions off, the blocks below needed to be shorten 5" but doing so presented a dilemma in that the all-important tops of the webs in the forms would have to be cut which would severely weaken the walls of the forms.  I elected to
Garage door area - final configuration
away only the unsupported foam between the webs, thinking that I could finesse the cutouts after the concrete cured.  And the vertical rebars that were fine for the rest of the foundation were too long at the cutouts.  It was a simple matter to shorten them after they were stabilized by the concrete.
Rebar extending into existing wall

In order to transition the concrete from the higher foundation walls to the lower cutouts, 2 x 8 pressure treated blocks-outs were cut at 8" to fit inside the ICFs. They were anchored in place with 6" TimberLok construction screws that pierced the vertical braces (existing or added specifically to support the block-outs) and the foam before screwing into the treated members.  The block-outs were held at least 1 1/2"  back from the eventual rough door opening so as to accomodate two-by framing for the door opening and still maintain a 9' opening.

Tying Walls Together
The joint between the existing concrete walls and the new foundation wall was a "cold joint", meaning they merely abut each other but are not physically joined. In order to keep the relationship between them from shifting, the rebar from the foundation wall was extended into the existing wall by drilling holes a couple of inches deep for the rebar in the foundation wall. 

Pouring the ICFs

It took 5.5 yards of concrete, a conveyor truck and five volunteer workers to pour the forms
Forms poured and anchor bolts in place
with relatively little effort -- definitely a DIY-capable endeavor.  And I am happy to report that the installation of the ICFs was sufficiently robust to eliminate any potential blow-outs.

Quality Control Issue with the ICFs
The ICFs are understandably fragile. Some components were damaged in transit and were unusable.  The height adjusters that were 1/8 to 1/4" shorter than the straight blocks and had to be split in half
Poured foundation after bracing removal
and, even then, somewhat forced to place. The design of the corner blocks have been upgraded for better retention of cladding materials but the height adjusters have not been ungraded.  Consequently, both the adjusters and the corner blocks had to be modified with a knife to maintain levelness. Cutting and fitting two halves of the adjuster separately left space that I filled with minimal expanding spray foam, which actually may have been an advantage in that the adjusters were glued together and perhaps more stable.

Would I use ICFs again?  In a heartbeat, but only after more research before buying.


  1. All the contents you mentioned in post is too good and can be very useful. I will keep it in mind, thanks for sharing the information keep updating, looking forward for more posts.Thanks concrete contractors south florida

  2. ICF construction has two significant benefits in terms of energy efficiency. Primarily it prevents air leakage through the walls because of its excellent seal. There are no thermal bridges between the two insulation layers since they are put on both the interior and exterior of a structure. There is a considerable decrease in heat loss due to the tightness of the building envelope. In the case of a Passive House, an off-the-shelf ICF system may not be thick enough to insulate the structure adequately. Insulation may not be enough if you reside in a cold environment like Canada or the northern United States. There is good news in that ICF manufacturers like WestPeak ICF have developed systems that can deliver an R-value of up to R-48 in colder locations.


As a do-it-selfer-in-training, I welcome your comments.