Passive Geothermal Heat in ICF Homes

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When considering building materials for houses, builders should be looking for materials that are going to be energy efficient for the homeowners. ICF is absolutely energy efficient but it tends to rank lower than it should because proper testing hasn’t been done on one of the biggest reasons for its energy efficiency: passive geothermal heat. While ICF wall materials should be considered as a passive geothermal heat source, further field research is needed to make it official. But we think it should definitely qualify! Let’s look at the science behind passive geothermal heat sources and why it’s important to ICF homes around the world. 

First off, we need to explore the meaning and importance of “R-value.” The definition of R-value is an “insulating material’s resistance to conductive heat flow is measured or rated in terms of its thermal resistance” (energy.gov) where higher numbers equate to a more effective insulation. R-value is important to builders and homeowners as it indicates the most effective material at maintaining a good internal temperature of a home and can change how a builder decides to implement heating and cooling measures. It also allows the homeowner to factor in a higher or lower monthly energy bill.

Foam has an R-value of 11. Each piece of ICF has two layers of foam and that puts ICF materials at 22, “which is moderately good.” (Larry Reid) But this R-value of ICF materials is rather skewed. And here’s why: testing the R-value in the lab vs testing in real life shows a wildly different result. ICF materials should actually be rated at about 50, much better than other building material options. Firstly, there are no cracks or drafts to worry about with the solid concrete and insular foam construction. Second, there is the heat conduction of concrete and the passive geothermal heat source to consider.

Concrete & steel, two main ingredients in the ICF building materials, conduct energy and heat – they can stabilize at 55°F. Passive geothermal heat sources creates a barrier between outside temps and inside temps by drawing up the temperature from the ground and allowing the HVAC unit to only have to swing the temperature from a more stable and even temp as opposed to bringing the temp up from 40 or down from 90. It keeps the heat inside in the winter and the heat out in summer. This creates a nice even temperature on a day-to-day basis, even when the hot sun fades to a cold evening.  

This barrier is not a perfect source of heat conduction. But it does allow for stability; the mass of larger objects takes longer to heat or cool than smaller objects. And when one side of the object is in constant contact with a fairly stable source of heat while also encased in a thick layer of foam, the geothermal heat is trapped within the concrete.

While we still need to study ICF as a passive geothermal heat source, we’re already asking the right questions to get us there. From the ICF Builder article on Heat Transfer:

“Research, for example, can be directed toward answering questions such as how much contact with earth and at what depth is optimal to achieve the functional equivalent of excellent insulation from temperature changes. Is it necessary for the top of the wall to be insulated to get considerable benefit from ground heat? Should there be different building practices (e.g., depth or kind of footings) for the deserts of Arizona compared to the plains of Canada to achieve less energy expenditure for indoor comfort? Perhaps the most interesting perspective is: How might the current practices for geothermal heating be incorporated into building practices using ICF walls?” (Larry Reid)

Whether ICF materials are a passive geothermal heat source or not, we still consider them to be the most energy efficient resource out there to create a comfortable home environment for the family inside while also saving energy and money on heating and cooling expenses.

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