In my ICF structure design I have 2 options in regards to the 2 layers of foam (interior and exterior):
Option 1: 2 equal layers of 3 1/8" giving an R28 (R14 on each side) - see the below link for the R-28 configuration
Option 2: Interior layer would be 2 1/4" (R11) and the external layer would be 4 1/4" (R19) - a total of R30 - see the below link the configuration of R-30
See at the following link a graphical design of the configurations possible (this is a Quad-Lock system): http://www.quadlock.com/images/icf_components/R-Values/ICF_Walls_R-Values.png

My common sense, in regards to thermal mass of the concrete layer, tells me that I should go with Option 2 which favors the thermal mass of the concrete to better protect the interior space from the temperature swings. The idea would be that once the concrete gains some heat (in the winter) or some cooling (in the summer) due to the difference in the insulation that heat/cooling will take the minimal thermal resistance path which will be the inside of the building and not the outside.

However, in my geographical area (Vancouver, Canada) the day-night temperature swings are averaging about 8-10C degrees (about 20F degrees). The questions is if it actually makes any noticeable difference between Option 2 vs Option 1 (disregarding the R2 difference in the insulation - R28 vs R30)?

Many thanks in advance!

In my opinion, not science, just what I see

The EPS creates a well insulated shell, it reduces or eliminates any air transfer between the inside and out. I do not see any significant energy savings by having thicker EPS on the exterior, I am sure there is some, but not a significant dollar amount on your heating/cooling bill to justify the additional costs associated with the additional EPS.

The new code in Canada, not yet in effect, will require the higher R value. Every major manufacturer is making an update to their forms to meet this code, but behind the scenes they know this is only to satisfy code requirements and nothing more.

In my opinion, don't waste your money

While our climate in Calgary is a little cooler then what you would get, I would agree with Chris. The value is in the sandwich, not necessarily how thick the bread is.
The idea of thermal mass on the interior in our climates is over rated. Its a little like living with an elephant in the house.

Sailawayrb did a lot of energy modeling and adding the extra EPS really did nothing.

Posted By sailawayrb on 05 Jan 2015 11:45 AM

This was when all the ICF EPS insulation was nearly removed (i.e., I entered 0.01 for the interior/exterior EPS thicknesses). The conventional R-value was R0.69 (R0.60 from the concrete plus R0.09 from the remaining EPS) and the program predicted effective R-value was R0.77 or 1.12 times this conventional R0.69. I used R0.1 per inch for the concrete and R4.5 per inch for the EPS.  You can certainly use different R-values if you prefer too.

The observation here is that with or without the EPS insulation, the ICF effective R-value tends toward the conventional R-value as the concrete thickness is decreased. As the concrete thickness is increased, the ICF effective R-value tends toward the maximum possible effective R-value (which is about 3 times conventional R-value for our Summer outdoor temp profile)…even without ANY insulation.  No real surprise with this observation, but this does show the program is providing the results that would be expected.

When the interior EPS thickness is reduced to 0.1” and the exterior EPS thickness is increased to 4.9” (i.e., essentially put all the insulation on the exterior side, but keeping the total conventional R-value the same), the summer effective R-value went from R66.15 to R44.05. In this case the concrete temperature hovers very close to the indoor temp. The effective R-value remains the same (i.e., R44.05) when all the insulation is put on interior side. In this case the concrete temperature hovers very close to the outdoor temp. This would imply that both these non-symmetrical ICF designs result in an equal, but lower performance than the standard, symmetrical ICF design for this summer outdoor temperature profile. When this is repeated for the winter outdoor temperature profile, the winter effective R-value went from R21.34 to R21.96, i.e., a slight performance improvement.

2.5" EPS x 6" concrete x 2.5" EPS is about perfect but if code requires more, you are stuck adding more.