Effective R Value of ICF Wall

Insulated Concrete Form (ICF) construction is becoming quite popular. The main reasons being the combination of structural integrity, high R value, relatively easy construction, and high thermal mass that ICF's provide.

The reasons for the good thermal efficiency of ICF's are: 1) minimal thermal bridging (that is no wall studs to short circuit the insulation), 2) the polystyrene forms provide a pretty good R value, 3) the high thermal mass of the wall results in an increase in effective R value.

There has been some debate about the actual effective R value of ICF walls. The R value of a typical ICF wall calculated in the traditional way (i.e. summing up the r value of each component) is not exceptional (typically about R16), but the claim made by ICF supporters is that the high thermal mass of the wall results in a higher effective R value. The ORNL (Oak Ridge National Labs) did a careful simulation analysis of various ICF configurations to try to determine the actual benefit of the thermal mass in the ICF wall.

To me, the figure below sums up the results of the study.

The figure was developed using a simulations of a typical 1500 sqft ranch style home with the usual stick construction with two types of insulated concrete form walls. The figure shows that a typical ICF wall has an R value about 1.5 times greater than what you would expect from summing up the R values of its components. That is, the thermal mass increases the effective R value of the wall by about 50%. The improvement due to thermal mass ranges from about 40% up to about 70% depending on the climate where the house is located. Warm climates appear to benefit somewhat more than cold climates. For example, if the nominal R value of a typical ICF wall is R16. The graph says that if you are building the home in Boulder, CO, it would perform comparably to a stick construction home with 1.4 times this R value -- i.e. the stick construction home wall would have to be R 22 to have the same heat loss. To me, this is particularly significant in that stick built walls tend to perform below their nominal R values.

Two types of walls are shown in the figure. The blue bars represent walls with typical ICF construction -- that is forms made of polystyrene with hollow cores filled with concrete. The yellow bars show a wall construction with insulation on the exterior, and the concrete mass on the interior -- for example a concrete block wall with exterior foam board insulation.

The bottom line (to me) is:

Conventional ICF walls have an effective R value of 1.4 to 1.7 times that of an equivalent insulation stick wall depending on climate.
(But, note update at bottom of this page)
Concrete or block walls with the insulation on the exterior perform better than the usual ICF construction -- the difference effective R value in some climates can be nearly twice that of ICF construction, but in most climates its more like a 20% gain.
The improvement for exposed thermal mass walls on passive solar homes might be even more?

I have heard and seen claims for ICF R values that go well beyond what the ORNL study supports, so be careful.

Overall, ICF's look like a pretty good choice to me, and might be well suited to owner built homes -- see the links in this section 1) "A Fast Foundation", Fine Homebuilding magazine, and 2) "One Year in A Foam House".

Figure from the ORNL report on ICFs

Update:

A recent study of and ICF building set out to measure actual R values, and see how much of a role the concrete thermal mass played in the thermal performance of the building. It appears that at least in this case, the thermal resistance of the wall can pretty much be explained by just the R value of the insulating form, and that the concrete did not add substantially to the performance of the building.

The full ICF R value paper...

Gary Nov 27, 2007, May 21, 2008