Differences in ICF forms

There are well over 50 different manufacturers of ICF forms in the U.S. Certainly this is due to the popularity of the forms, but also we see some manufacturers who have "jumped on the bandwagon" without investing the time to make a truly good form. There are even some who market directly to the homeowner, selling their product through "big box" stores and/or convincing people that they can build their own home with this product!  I believe this is a grave mistake.  Every horror story we hear about ICF's involves a homeowner doing it him/herself.  This article will attempt to classify the major differences in the blocks used in cold climates. Then you may well be able to see why certain blocks should not be sold to a general audience. It should also help you determine which block is best for your application.

Below we will cover only "true" ICF forms. Not like E-maxx or some other products which are intended to insulate only one side of the concrete wall. Such systems use standard poured wall forms, then insert the foam on one side in a variety of different ways. These systems add to the time it takes to form and pour a wall, and likely cost the same as a true ICF wall. The only advantage is that a poured wall contractor can install them with no previous experience. So caveat emptor! (Buyer beware). Just because someone advertises that they install insulated basements, does not mean they are an experienced ICF installer!

Flat wall vs. post and beam or waffle grid

A flat wall block is exactly as it sounds. It delives a concrete core of a consistent thickness. Most block manufacturers offer concrete core widths ranging from 4" to 12". There are a few blocks that can go as large as you want. R-Value uses only flat wall blocks.

Post and beam and waffle grid are both exactly as they sound. Post and beam delivers a vertical post and a horizontal beam of concrete, usually 6" in diameter, usually every 8" each way. The rest of the area is solid foam. Waffle grid blocks have the posts and beams, but they also have a solid concrete wall in between them, typically 2" thick.

Negatives of post and beams are significant; including a lack of ties/studs to attach finishes to, difficulty ensuring good concrete consolidation, difficulty in reinforcement, less thermal mass, shorter possible lintel spans, and lower ultimate strength. Positives include higher r-values, (possible) lower installed costs, and lower concrete bills.

Pre-assembled vs. job assembled

Most ICF's these days are pre-assembled. Both sides of the form are connected by a tie already from the factory, eliminating any time spent on the job assembling forms. Pre-assembled block come with either a fixed tie or a foldable tie. The foldable ties collapse to reduce the size of the form for shipping purposes. Since there is a significant labor savings in using pre-assembled blocks, and there doesn't appear to be any cost savings for the materials of a job assembled block, R-Value prefers pre-assembled blocks.

There are still a number of ICF's that require jobsite assembly. They come as the two sides, and the installer must connect them via a tie. These block have the advantage of lower shipping costs, and the ability to use the same form for different wall widths simply by using different ties. (The lower shipping costs really don't come into play until you are looking at homes large enough to normally require two truck loads of pre-assembled forms.) Another advantage is the ability to build a block around existing structures, such as columns and beams, pipes and wires. Different width ties can also be used to create architectual elements into the ICF wall, such as corbels and molding.

A sub-category of the above are the forms that require the installer to set each side, then install the ties, and then set the next two sides, ties, etc.; rather than assemble a bunch of forms, then install them in the same manner as the pre-assembled blocks. These type of block require even more labor to set.

Vertical vs. Horizontal

Horizontal: The vast majority of ICF's are a horizontal block, typically 16" tall and 48" long. These go together in a fashion similiar to masonry block.

Horizontal block are easy to use, and generate little waste. They also are often pre-assembled, saving precious time on the job.

This design is more subject to compression, in which the blocks compress as they are filled; and lift, in which two or more courses of block separate slightly during the pour. They also need vertical bracing to keep the wall from bowing at the joints, much like a mason uses his level to constantly check the blocks he sets. None of these items are a problem for an experienced contractor, who knows how to address each potential concern.

Vertical: There are a few ICF's that are vertical panels the full height of the wall, joined by fullheight ties/studs every 12"-16" depending upon the system. These are very similiar to a standard basement wall form, and are a natural for the poured wall contractor to begin using. Vertical ICF's are always jobsite assembled.

This design make it easy to change wall widths, as well as foam thicknesses. A thicker form can be used on the outside of the wall to shift the thermal mass closer to the inside of the wall. A vertical block is also very straight top to bottom, and one side of the wall can be set at a time, making utility installation easier in some cases.

Vertical ICF's tend to spread at the top, and care must be taked at the corners to eliminate this concern.

The interlock

The interlock is truly what makes each form unique. This applies only to the horizontal-type ICF. The interlock is what fixes two block to each other, and is especially important in determining how straight the corners will be, as well as how flat the wall is. There are several major kinds of interlock:

Tongue and groove. Fortunately there are not many of these blocks around. They use either a tongue and groove or a ship lap joint to join the blocks. This requires that each block be glued to the next, adding time and cost. Not only that, but care must be taken to insure that the ties line up well, and that the corners are braced very well against concrete pressure, since the form itself has a difficult time containing the pressure. It can just slide out of the joint (the glue isn't that strong). It can be done, but it is more difficult to get straight corners and a plumb, flat wall when using ICF's with a tongue and groove or ship-lap joint.

Interlocked. These block have a solid interlock which can't be flipped upside down, but most can be installed backward. The interlock automatically lines up all the ties with each other, as well as holds the corners from pushing out under concrete pressure. This is an extremely significant benefit over tongue and groove type block. The interlock also automatically plumbs the corners and levels the walls. The corners cannot "grow" as you go higher, since the interlock keeps them in place. With interlocked block, scrap pieces of form that are at least as wide as the tie spacing can be reused, not thrown away as in a tongue and groove type. Different blocks have different interlock spacing, interlocking on increments from 4" to 1". Smaller interlocks are more desireable, since they reduce the amount of splice joints in a wall. Small interlocks are most beneficial in biildings with lots of corners and/or windows. An interlock joint reduces labor costs!