Thoughts on ICFs (insulated concrete forms)

We continue to field discussions from various parts of the world on Insulated Concrete Forms (ICFs) so I thought I'd blog about them.  I wouldn't say there is a lot of misinformation out there - perhaps just a lot of misunderstanding.  As with any advanced building material, the foundation (no pun intended) of the concept lies in the science behind the product.  Sometimes the science is misdirected, or mis-applied, and expectations therefore often fall short of what yields from reality.

An ICF - or insulated concrete form - is pretty much what the name implies - two sidings of insulation sandwiching a core of concrete.  The idea is generally that you are building with "thermal mass" (concrete) and it is insulated (which is always good, right?) and therefore the combination of insulation and thermal mass must be a great thing.  Couple that with the inherent strength, fire resistance and longevity of most grades of concrete and it is often touted as a building product with no equal.

I believe the problem with this entire premise is that the way the ICF is constructed is actually contrary to the intended science behind it.  To get an idea of the way a homeowner would take advantage of thermal mass in a traditional sense, picture a house (northern hemisphere) with a southern exposure and glass windows that will allow certain levels of solar radiation to pass through the window panes unfiltered.  A massive stone floor or a large stone fireplace that is warmed by this solar radiation during the day would then re-radiate the sun's solar energy (hopefully) long into the cool evening.  This effect basically helps to level out the temperature swings associated with the diurnal cycle (night-day-night-day, etc.) and helps to reduce a homeowner's energy bills by supplementing a mechanical heat source with a passive heat source.  In order for a homeowner to take advantage of the thermal mass of the concrete in a structural wall that concrete must be allowed to do two things - 1) absorb thermal energy (like solar radiation) and 2) re-radiate that energy into the living environment of the homeowner.  Because an ICF typically has several inches of insulation on BOTH sides of the concrete (thermal mass), then by design an ICF is significantly retarding the concrete's ability to absorb and re-radiate thermal energy.  Ideally you would actually have lots of insulation on the exterior of the building envelope and zero insulation on the interior of your concrete wall - that way your concrete wall could absorb the thermal energy of your mechanical HVAC system - say 72degrees F - without influence from the hot or cold outside and then re-radiate that thermal energy at night when your thermostat is lowered to say 64degrees F or similar. 

Think about this - even if your concrete wall was exposed to the exterior warmth of the sun, how many walls are exposed for any extended period of time?  Odds are ONE to TWO.  That leaves at least TWO to THREE walls of your home in the shade most of the time - receiving virtually no benefit from solar radiation and only little impact from ambient air temperatures.

A local salesman for an EPS sip company out of Kentucky once told me that their R17 EPS panel actually performed like R26.  After some additional quizzing from my side he finally relented that it performed like an R26 fiberglass insulated stud wall (which will traditionally perform at an insulating value much less than the stated fiberglass insulation rating).  Unfortunately some ICF installers tout a similar claim of dubious origin - that ICF walls perform at better than R50.  Again, this is taking an old test and comparing ICFs against an "R50" fiberglass wall.  By definition concrete is a CONDUCTOR of thermal energy - with a specific heat capacity similar to glass or marble  - and the only insulator is the EPS foam that is on both sides of the concrete.  Technically the insulation value of the ICF is only equal to the insulating R value of the sandwiching EPS foam on either side of the concrete - if you have 2.5in of EPS foam on either side then you're probably looking at about R19.  All the concrete does in this thermal insulation equation is slow down the conductance of thermal energy flowing between the EPS insulation by virtue of it's ability to retain thermal energy for typically short periods of time.

Here is a great write-up from a nationwide ICF installer that helps to debunk the R-value myth of ICFs (so definitely an impartial source):

And here are some wonderful resources from the Australian government regarding thermal mass:

In summary I believe an ICF is a fine product that will provide a strong, durable foundation or building envelope.  But as a play on thermal mass, the effects will be greatly impacted by the insulation on both the interior and exterior of the building envelope.  In fact the primary source for thermal energy for an ICF wall will be the earth below where typically the concrete is often in direct contact.  In North Carolina the temperature of the soil in contact with most dug foundations is around 55-60deg F.  This will be the primary tune that wall will be singing - not one of re-radiating the sun's or your home's interior thermal energy because those are protected against by the EPS insulation skin.  Thermal mass is not typically a primary comfort strategy (except in the desert southwest, for example) - rather a secondary one.