Insulated Concrete Form (ICF)

Conventional ICF

Traditional masonry foundations do a good job of supporting the structure above them, but they also have some serious limitations. For one thing, they’re a large source of heat loss in a home. A masonry foundation is a nearly infinite heat sink that will absorb any amount of energy you throw at it. Then, if the drainage around the foundation isn’t right, it may also transfer water into the home.

So, at Meadowlark Builders, we’re fans of insulated concrete form (ICF) foundations. ICFs usually consist of a molded expanded polystyrene (EPS) shell that stacks together like Legos to make forms that concrete is then poured into. When these forms are filled with ICF concrete they make a poured masonry wall that is comfortable and dry. This is a picture of an EPS ICF block:


Unlike a traditional uninsulated masonry wall, however, this wall has about an R-25 insulation value. By contrast, the traditional masonry wall has an R-1 value—the same as a single pane of glass. Here’s what R-1 looks like next to R-25 in a thermal image:


You can clearly see that a masonry wall transmits a lot of thermal energy while an ICF house keeps the heat much better.

But why stop at the foundation? ICFs make strong, tight, exterior walls as well. In fact, mid-rise hotels, particularly in hurricane-prone areas, are built of ICFs. For a residential home, ICF walls are energy efficient, quiet on the interior, and extremely durable. How durable? Look at these images of various disasters:



This is an image of a neighborhood where an F-4 tornado came through. The frame houses have become just litter on the landscape, while the ICF house, which was under construction, is ready to continue building.


Last, this is a picture of a car hitting an ICF home at 85 mph. The car is totaled, while the ICF house needs a bit of stucco repair. Don’t try this with a wood-frame structure!

ICFs aren’t without environmental issues, however. For one thing, concrete takes a lot of energy to produce, mostly because 15 percent of concrete is composed of Portland cement. This material is basically a mixture of rocks and minerals—limestone, clay, shale, or sand, for example—that are ground to a powder and heated in a kiln to about 1450 degrees Centigrade.

However, recently fly ash has become a good substitute for Portland cement in concrete. Fly ash is a residue of coal combustion, which used to be released into the atmosphere from inefficient coal-fired plants. Released into natural systems, fly ash is devastating to wildlife and waterways, but it also has cement-like properties. Now, due to more efficient coal combustion, what used to go up the chimney can replace up to 30 percent of the Portland cement used in concrete. In fact, about 40 percent of fly ash produced in the U.S. is recycled into concrete.

Finally, there’s the polystyrene shell of the ICF. Polystyrene is a petroleum by-product, and there are good environmental reasons to avoid these products. Straw bale houses are one alternative, as are Durisol ICFs.

So, while ICFs are expensive to produce and install, they create tight, energy-efficient houses that last a really long time. How long? These are some concrete walls the Romans built:


In short, a building that lasts for centuries—or millennia—probably saves much more energy over its lifespan than the energy that went into constructing it.

Durisol ICF

Cement fiber ICFs were developed in Europe after World War II. This type of ICF is basically a block of Portland cement impregnated with other materials—ground lumber after World War II or mineral wool, a non-compressible, rot-proof insulation.

Durisol is one brand of cement-fiber block. It’s a natural product and easy to work with. Durisol blocks are also hygroscopic, meaning they absorb excess moisture and release it slowly, providing a more even moisture content in the air. But the best part is that the blocks bias the concrete to the inside of the building. That’s important, because it allows the building to take advantage of the large thermal mass of the concrete.

Most ICFs have the concrete in the middle. They make a great structure, but the polystyrene shell then releases as much stored energy to the outside of the structure as to the inside. That creates a good buffer against interior temperature swings, but it doesn’t capture the inherent energy of the structure.

By biasing the concrete to the inside, Durisol blocks store more of the energy used to heat the house. When paired with south-facing, low-energy windows, the house is heated by the sun during the day, and the excess energy is stored in the concrete blocks, extending the solar effect by many hours. This is the concept of a passive house, which is a home heated nearly exclusively with solar energy.

Meadowlark Builders has developed a special technique to maximize the performance of Durisol Blocks. By wrapping the home with polystyrene board insulation, we provide a thermal break from the outside elements. Then we install siding with a venting system called a Rain Screen. This method creates a very tight exterior wall with an insulation value of R-34; it biases the concrete to the inside, and provides a great buffer for temperature and moisture content of the interior air. Even if the power goes out, the sun alone can keep the home reasonably comfortable in winter and naturally cool in summer. A drainage plane behind the siding and roof also helps cool the building.

We constructed the Phoenix House using this method, and even though we built it in the winter, the interior temperature would get up to 63 degrees during Michigan’s cold February days.

Although Durisol blocks have many advantages, the system is expensive to build with a payoff period of over 20 years. With a southern exposure, however, a house can be heated and cooled with very little energy or maintenance. A Durisol ICF foundation has to be viewed as a long-term investment both in the environment and in a durable, energy-efficient structure.

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