Saturday, August 29, 2015

Building with ICF's


For those of you who don't really know - ICF's are Insulated Concrete Forms. Basically they are foam legos (4 feet long, 16" high and thickness varies depending on how thick you need the concrete to be). The foam blocks have an open core between the inner and outer layer that is filled with rebar and concrete to give you your structure.

Until we started building I thought all brands were essentially the same. Maybe the foam was thicker on some, but never thought they truly were different.

We researched Logix ICF and AMVIC ICF's. Both had some pro's and cons.

Logix has a higher R-value (R-22.5) and a more defined "web" to screw into, but the blocks need proper orientation for proper assembly (ie. Right hand and Left Hand Corners). AMVIC's blocks are universal - they can be used upside down and backwards to reduce waste. The R-value is slightly lower with the AMVIC (R-20), but we have the flexibility to increase the R-value of the basement wall slightly more to ensure compliance with the Passive House Standard. AMVIC also has to be installed in 2" increments due to the distance between the connectors (so your wall has to be 35'2" or 35'4"). Given we had not yet ordered the trusses and floor joists we had the extra play to slightly grow the wall an inch or so.

We decided to go with AMVIC ICF's.

The good news is - since we don't have a complicated shape (a rectangle) we could really minimize the waste and didn't use 3 entire pallets of material, which can be returned. This is the waste from the entire basement (7 blocks tall). Remember we had to cut some out for the 2 window opening as well. Actually in terms of waste weight and volume - the cardboard the ICF's were shipped in weighed more than all of the waste of the job. I was impressed! .

It was just enough to build a small playhouse for the kids!

Block Close Ups

The ICF is a pretty simple design. It has 2.5" of EPS foam, 8" of webbing adn 2.5" more of foam.
The black webbing made of plastic is spaced every 6" and provides the stability between the layers of insulation to hold the concrete in place. It also serves as a mounting bracket for drywall, lumber, etc. The plastic webbing also holds any necessary rebar in place with its clips. The outside of the ICF is expanded polystyrene (EPS) foam which has roughly an R-Value of R-4 per inch. So the overall R-Value is R-20. Trying to achieve an R-20 on a concrete block or poured foundation wall can be costly and add extra time to the build. For us to meet the Passive House standard we need an R-60 basement wall so the ICF is not enough insulation, so we will be installing a wall on the interior side of the ICF and insulating that with cellulose.

The Process

After the Capillary Break cured over night we began assembling the 1st course - starting in the corners. The 1st course blocks are zip tied together to hold them while you assembly the 2nd course. 

You assemble 2 full courses before you shim, level and glue to the footer.

Once the ICF's are level and glued to the footer you can continue building.

We finished 4 rows when we had to start assembling the bracing / scaffolding. The bracing we are using is called Plumbwall. The bracing is a free rental when you purchase the ICF's. 

It is screwed to the plastic webbing on each course 

It has a triangular support for the scaffolding and an angular support that is staked into the ground.

You can then set up the scaffolding on the top of the triangular supports. After getting all of the scaffolding up we could begin to build again. The final courses went fairly fast, again with the rebar in each course. Once all of the courses were done, the "new" courses had to be screwed to the Plumbwall system.

The final course we installed is called a "Tapertop" course. I don't know if this is a common course or not for homes, but because our exterior wall is so thick we needed some support for our flooring system and our exterior wall - since they are not on top of each other.

The tapertop course gives us more bearing area of concrete to set our exterior wall on. (Note: this detail is for a Logix Block - the AMVIC block tapers in both directions)

Once all of the rebar is installed and all of the braces are attached to all of the courses of block we  need to rough in our basement windows:

By thermal separating the window from the concrete we ensure there is no thermal bridging around the window. 

The window will be installed on the inside of the ICF foundation - really supported by the interior framed wall. This will place the window in the middle of the R-Value of the wall assembly.

Once installed, the entire perimeter of each window will have 2" of rigid XPS foamboard insulation.  We made our rough openings an extra 4" tall to account for this added insulation. 

Sadly enough it is all too common to have concrete directly in contact with the window frame with typical construction. We have added a simple extra step to make sure the concrete is completely insulated and isolated from all of the other components of the house. This is important to maintain a continuous thermal boundary.

Once all of this was in, there were just a few minor things we did to ensure a smooth pour. We added furring strips to the exterior to make sure the courses stayed together during the pour. 

We installed sleeves for the foundation wall penetrations (water line, plumbing line, etc). 

We also installed a string line to make adjusting the straightness of the wall simple.  

The Plumbwall system has a great adjustment strategy, simply adjust the nut at the scaffold level and you can move the top of the wall in or out to keep it straight.

All that is left now is to fill the ICF's with concrete!
The concrete pumper truck makes easy work of that!

Oh wait one more detail:
As you can see, the sill plate is recessed into the ICF form, so we can't bring the concrete all the way up to the top of the ICF.
If you look at the trowel the mason is using - it is basically a 2x4 with a small piece of OSB screwed to it to keep the concrete 1.5" below the top of the ICF so the sill plate lumber will fit into the groove!

Great Job by Ken Block and his crew! I'd highly recommend him -- visit his site for you next project

Next week we will be installing the sub basement slab insulation before framing begins!


  1. Wow very interesting technology, had no clue that even existed. My husband is usually the one who shows me these things as he is in the construction industry. This will make his day, thank you for sharing it. The pictures really help me understand what it is you were talking about thank you for that, again keep it up, thanks.

    Kendra Tran @ Leco Concrete Forms

  2. This is really great post thanks to share this post its information is really great.

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