Monthly Archives: June 2015

Fox Blocks, Phase 1


Posted on June 13, 2015 by

As part of my build, I wanted to experiment with a variety of different construction methods, including ICFs.

ICFs are “Insulated Concrete Forms” that you can use to build a very well insulated wall.  They stack like lego and include strong high density plastic (HDPE) inner supports that hold the sides together while the concrete is being poured.  This inner web structure is also used to position and support the rebar and the portion embedded in the polystyrene acts as furring strips for attaching things to the wall. After the concrete is poured, the forms are just left “in place” as insulation.

In addition to the 4 inches of polystyrene insulation, we will be adding 6 inches of concrete that will give the wall mass to retain heat, “dynamic R value“.

The front and back of my garage (Quonset hut) are flat walls that needed to extend past the Quonset hut and perform double duty as a parapet retaining wall, all without any complicated construction or difficulty attaching insulation.  ICFs seemed like the perfect choice for this application.

The final setup

The final setup

Installing the Fox Blocks ICFs was pretty easy, as you can see in the video.  Lessons learned are included in this post.

The Video:

Fox Blocks;

I looked at many different ICF companies and carefully considered their various advantages.  Some fold flat for easy shipping, others have longer or taller blocks or come in separate pieces that can be assembled in a variety of different ways.  Cost of materials and installation were also a concern.  In the end, Fox Blocks was my first choice.  For more on why, see this earlier post.

The blocks cost about 20$ each and I was able to buy them directly and install them myself without any fancy tools or equipment (or skills).  A regular concrete block wall (CMUs) would have been cheaper, but would have been much more work and required much more skill.  I would also have needed to insulate it, so maybe not even that much cheaper.

The Story (lessons learned in orange boxes):

The Setup;

ScreenShot_DimensionsWe started out by measuring and marking all the locations for doors and windows right on the concrete pad/footing. From this, we could easily mark all the locations for the rebar.  We drilled half inch holes 3 inches into the concrete at each rebar location.

The Fox Blocks stack very easily.  We also use the “Fox Clips” to clip the blocks together horizontally and vertically.  After each layer, we added the horizontal rebar.  My engineer specified one piece of #4 alternating near the front or back of the wall each 16 inches.  The internal web of the Fox Blocks had notches to hold the rebar so that we didn’t even need to tie it in place.

The lego bits (that Bonnie insists are called “nubbins”) are every 2 inches and the blocks can be stacked upside down or back to front, but you definitely want to line up the internal webs so that it is easy to clip things together vertically.  It also makes it easier to attach things to the outside of the wall if the webs are all lined up.


In the first section of wall, we had a T section for 3 levels on the back.  So when Bonnie got to the 4th level, it was a bit tricky to trim a block for that transition and you can see her trying a few different things (stacking and unstacking and then trimming and re-stacking) to get all the internal webs to line up.

After the wall was mostly up, we started dropping in the vertical rebar (half inch) and then using a hammer to tap the ends of the rebar into the half inch diameter holes in the footing.  These 3 inch deep holes were enough to hold the rebar vertical within the wall, however, in many cases, I still wire-tied the vertical rebar to the top horizontal rebar just to keep it all where it was supposed to be. This vertical rebar strengthens the wall and also helps the later layers hold across the cold joint at the top of this section of the wall.

When initially constructed, the blocks are securely attached to each other, so you have a monolithic form.  The vertical rebar keeps it from moving very far in any direction, but it is still very loosely positioned and not plumb.  You can see it moving around a lot in the video.  The wall will need to be plumbed and aligned as a final step.

We screwed 2x4s into the sides of the bucks.  It is very important to screw them in where the plastic reinforcement is.  In those locations (which act like internal furring strips), the screws bite in nicely and hold well.  Anywhere else, and they will just pull right out of the polystyrene.  these strips are hidden under the polystyrene. Fortunately, the blocks are clearly marked with the words “Fox Blocks” along the furring lines.

Unfortuneatly, I was not clear enough when working with my friends/family and assumed they all understood how the plastic strip locations were marked.  When their screws would not bite in, I would say something helpful like, “You need to make sure you screw it into the FOX BLOCKS.” and I would point to the line that said “FOX BLOCKS” vertically on the side of the block.  I would even screw one in for them to show them how it worked. They would nod and smile, but were actually thinking, “Yea, I am screwing it into the Fox Blocks, what did you think I was doing?  But its not working for me!”


It also didn’t help that we were (at least initially) using screws that were not threaded far enough up the shaft.  It was fine for the 2x4s, but when used with the thinner boards, the threads passed all the way thru the plastic and spun without really tightening up.  This caused quite a bit of frustration for Zack who was in charge of putting up the thinner boards.


Once the 2x4s were in place, we positioned and plumbed the ends of the walls using bracing and stakes to fix them in place.  Then we stretched string between the ends so we could align the rest of the wall.  Section by section, we used a level to plumb the wall and the string to align it.  The bracing at each section was individually staked.

This turned out pretty well, but I didn’t factor in potential movement at the bottom of the wall.  I had not fixed all the degrees of freedom and had relied on friction and the weight of the wall to keep the base where it was.  Of course, an ICF wall is relatively light and the strong winds shifted it in the 2 days between setting it up and the concrete pour.  I ended up needing to make some last minute adjustments.  Next time, I would also do something to secure the location of the back of the wall along the bottom edge.


Holes for the windows are cut out of the Fox blocks.  We used “Fox Bucks” to frame around the windows.  Fox Bucks are similar to the Fox Blocks (Polystyrene molded around an internal webing of tough HDPE plastic), but without the “snap together” feature. Instead, they must be taped into place and then the seams are held together with externally applied boards screwed in on both sides (the block and the buck).  Again, it is critical that the screws be in the plastic within the Fox Bucks, but the plastic fills pretty much the full sides, so it is hard to miss.

fox-buck1 fox-buck2

Not everyone helping understood that the concrete would be exerting hundreds of lbs of lateral force to push the bucks out and that tape and a couple screws would not hold them. I understood, but my big mistake was not fully inspecting (I was too distracted) that the boards were screwed into both the bucks and the adjacent Fox Blocks every 8 inches or so.  This lead to some blow outs and additional work down the line.  More images of this sort of mistake in the gallery at the end of this post.


The Fox Bucks are also used at the top and bottoms of the windows.  When used as window sills, we had to cut holes to allow the concrete to be poured in.  We also sloped the sills (by trimming the front of the underlying blocks) so water would drain off.

The full height of the wall is 12 blocks tall, but we only setup the first 4 levels because we wanted to be sure that the concrete would consolidate all the way down in the forms.  If we were more experienced, we may have tackled a deeper pour and got more done at once.  But as it was, I was glad we kept this first one simple.  Stopping at 4 levels also meant that I could let the first part harden before I added concrete across the tops of the windows and garage door, which probably saved me a disaster.  The cold joint that will occur is bridged by the vertical rebar.


The final steps were to place 6 mil plastic to separate the coming concrete from the Quonset hut steel and to spray foam some gaps and along the bottom of the wall. It not only fills gaps to keep the concrete from leaking, but works as a very effective glue.

The ICF wall was inspected and approved.  Keep in mind that the inspector checks basic things like if you have Rebar in place, etc.  He does not check for every screw.  As the general contractor, that was supposed to be my job.

Permit (Medium)

The Pour;

On the day of the pour, we also planned to take care of the basement floor and the concrete ribs while we had a pump truck on site.  I was actually running on fewer than 4 hours sleep and still frantically finishing some final details on the rib forms when the concrete trucks were rolling up.    I should have been inspecting the forms.  Is that enough foreshadowing for you?

Since I had experienced concrete guys on site to take care of the floor, I also asked them to help with the ICF walls also.    To make sure that the concrete was properly consolidated in the walls (without air pockets, etc.) I had bought a 5 ft long concrete vibrator for $99.00, which I would recommend to anyone doing similar work.

As soon as the concrete started filling the forms, the end started popping open.  It was immediately obvious that one of my helpers (who shall remain nameless) had not really understood that concrete would be trying to push its way out of these forms and had not secured things nearly enough.  For instance, on the first end, he had only fastened the top and bottom of the board.  Hundreds of lbs of lateral force were pushing out the middle and we had to scramble to brace it.  The windows bucks held up well, but then one side of the garage gave out and a few hundred lbs of concrete spilled out while we frantically grabbed scrap wood to brace it.  Seeing that not nearly enough screws had been used, I ran ahead of the concrete hose and frantically added more to the other bucks.


We made sure that the forms were not filled all the way to the top and we roughed up the top surface of the concrete so that the next layer would grip well across the cold joint.

It all happened very quickly… So quickly that, for the time-lapse, I had to slow it down by 50% so I could fit two sentences into the scene.

Once the day was done, I had time to think about my mistake and plan to do it better next time. There will actually be several more phases and I will need to wait until all the rib forms are done before I can complete the garage and put another ICF wall on the front of it.


Here are some pics from the day with descriptions.  Thanks to all those who helped me out.