Oak Ridge to the Rescue!!
So, last time we spoke, I was trying to work through the actual performance (equivalent R-value) of an insulated mass wall, within a mild coastal climate. In this post, I will review the one tool that I found which might give some insight, but less than ideal.
As it turns out, most energy modeling software that isn’t TRANSYS or EnergyPlus just doesn’t really like thermal capacitance. They model heat as ‘BTU’s in—BTU’s out’ which for the purposes of most energy modeling, is quite sufficient. In other words, it doesn’t really respect the 2nd law of thermodynamics. However with massive wall systems, all the heat slushing in and out results in some fairly tricky dynamics. Keeping track of the temperature of the mass can be important in some designs, and is definitely important when it comes to comfort.
The traditional energy modeling software don’t really mess with this level of detail because it’s tricky and is essentially a finite element model.
Oak Ridge Thermal Mass Calculator:
Google that and you’ll find a nice software tool circa 2000. There aren’t too many options within the tool, so I chose a 3000 square foot 2-story ranch house with R-19 continuous steady state insulation (the EPS on the outside/inside of the wall). The tool calculates an assortment of equivalent R-values based on the location of the insulation with respect to the mass—inside, outside, or in the middle. It then gives us charts like the following, which shows:
ICF 2-story residence in Phoenix (R-19 steady state R-value): R-31 equivalent
The ICF performance is on the right side, labeled as “ICI,” or ‘insulation-concrete-insulation’
ICF 2-story residence in Seattle (R-19 steady state R-value): R-26 equivalent
You can see that the equivalent R-value of the wall in a temperate, cloudy environment, such as Seattles, is substantially less than the performance in Phoenix. R-26 ain’t too shabby, however, as far as overall wall performances are concerned.
The heavy thermal mass wall system works well, and the ICF is a good product, assuming one can contend with its adaptations in building style. The walls tend to have superior air-tightness relative to light frame assemblies due to the requisite attention to details and the monolithic nature of the wall assembly.
Achieving R-26 in a light frame is not hard. Just put some rigid foam on the interior or exterior, dude.
Also, if you noticed on those charts that the interior mass wall universally worked the best. Take note of that! Interior mass is where the mass should be, exposed, naked for all the interior world to see. And, of course, if you want some passive solar benefit, you could also put your mass where the sun does shine! ;)