Insulated Concrete Form (ICF) Foundation

Re-building the foundation was not part of our original plan, but when we learned that the house’s existing foundation was deteriorating (check out the daylight coming in between those blocks!) we knew we had to take serious action.  The last thing we wanted was to invest a lot of time, energy, and money into creating a state of the art structure on top of a crumbling wet base. So, although it meant that our budget would take a big hit, we began to work a new foundation into our plans. Happily, this brought two big and unexpected advantages.

One was that it gave us the opportunity to excavate further to create a deeper basement—with a height of about 8 feet. This turned our once mostly unusable space into an expansive and comfortable room that will become a second classroom. This additional square footage for teaching and learning adds great value not only to the TREE House, but to the campus at large.  A new foundation also provided us with more and better options for achieving a tighter, well-insulated thermal envelope.

The design team settled on a foundation built of ICFs.


Different ICF products and brands vary in style, but the basic design of the ICF involves hollow foam blocks which stack easily and interlock side by side. Once assembled, the cavity in each column of blocks is filled with reinforced concrete. The result is an extremely durable, tight, an and energy efficient structure.  The ones we used are pictured above, from left to right: waiting and ready for action, a single block, and the ICF foundation in process.  

The advantages of using ICFs include a higher R-value and lower thermal bridging, helping ICFs deliver energy savings in heating, cooling, and ventilation for the life of the house.  But  ICFs are not without drawbacks.

One downside is that building with ICFs can cost more. According to the Portland Cement Association (2005), building a house with all ICF walls adds about 3% to overall construction costs, compared with a conventional wood-frame building.  Due to the many variables affecting a home's construction needs and performance, accurate cost comparisons are hard to find.  But one study comparing the construction of the foundation alone  finds that ICF foundations cost less per square foot of floor area than poured concrete and concrete masonry foundations. See their calculations below. 

Part of the savings likely comes from the reduced time and labor costs involved with assembly and finishing of the interior walls, which are already insulated and so only need a covering of drywall as a thermal barrier.

Another problem is that the materials in ICFs are not environmentally-friendly.  Concrete production comes with a significant carbon footprint, and the making of polystyrene foam involves various toxic chemicals.  See additional data from the Minnesota study to the right.  For these reasons, green building analyst Lloyd Alter is critical of those who call ICF construction "green."  He describes the use of ICFs above grade as architectural overkill, but admits that, “with rare exceptions all foundation technologies are pretty gross, primitive and invasive and few systems tread lightly on the landscape.  Alter concludes that, "ICFs are energy efficient for the occupant, solid and strong, and useful for foundations.”

On the bright side, an MIT study calculated that more than 90% of a house’s carbon emissions are due to the building’s operation phase, with construction and end-of-life disposal accounting for less than 10% of the total emissions. Because we’re working with an existing building andare interested in seeing it last another 100 years or more, the variables affecting our operating carbon emissions are even more important.  And an ICF foundation offers significant advantages in terms of energy efficiency and conservation.

Given all this, we determined that an ICF foundation was the best choice for the TREE House. 

Only after the foundation was long completed did we learn that there are alternative ICFs are made from recycled woodchips and mineral wool, rather than polystyrene.  Bummer!

Two things we have learned--are are reminded of again and again--is that in every situation there are  tradeoffs to be made (after being carefully calculated) and there are ALWAYS new things to learn.   Stay tuned for future updates about our ICF foundation and how it's working.

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