Hi polycore.
The BTU loss or gain does not change because of increased mass inside a insulated structure. What does change is the amount of swing in temperature. (a big pot of water takes longer to heat up or cool down compared to a small pot.) There is nowhere near as dramatic of changes in temperature. Yes when the system is "fired up" the initial amount of energy needed to warm up or cool down will be more but beyond that there will be no difference.

As a personal example my passive solar/icf house has stained concrete floors throughout for the primary heat storage mass. It was without power/heat for four days during a ice>snow storm last winter. During that time the temperature of the house did not sink below the low 60's (63 or 64 I think). But it took most of the day to warm back up to the mid 70's when the sun came out on day 4.

Interior frost/condensation issues in winter occur if there are significant conductive pathways lacking substantial enough insulation in a structure from the inside to outside. One thing that could do this would be metal studs (a great conductor of heat) that are not insulated on exterior and/or interior surfaces with foam board etc.. (But for that matter every screw holding the foam onto the metal would be a small conductive pathway as well.) Or for that matter if the studs rest against a non-insulated exterior concrete wall.
Another example would be the older aluminum frame single pane storm windows. The metal gets the frost first.

David H.

One type of system you might consider would be the CIC shotcrete panels, SCIP, structured concrete insulated panels.  They have EPS in the middle with varying thickness/R-value and some type of welded wire mesh & trusses as the connections for two external shotcrete wythes.     Gives you good insulation value, concrete mass interior as well as shotcrete exterior, and a tight envelope.  You can do wiring and plumbing before applying shotcrete where there is room for wire/plumbing running under the wire mesh.  One version has a depth screed built in for consistent depth of shotcrete.

I've always been intrigued w/ that system but hesitant to be a guinea pig here in MN!

http://www.metrockscip.com/

http://www.envirolaststructures.com/wallsystem.html#

http://www.envirolaststructures.com/images/diagram.jpg

R10 in a poured wall isn't necessarily the deal killer you might imagine. Even with passive solar, you shouldn't ignore the direct benefit of thermal mass (i.e. in 24-hour periods when ambient air temperatures average 70ish, mass can provide consistency and comfort without HVAC.) Big R values aren't much help here because you want the mass to capture and hold daytime heat. The zero energy home in Tucson has R14 sheathing over CMU walls. http://www.toolbase.org/ToolbaseResources/level4CaseStudies.aspx?ContentDetailID=2473&BucketID=2&CategoryID=58
While Tucson's climate is ideal for thermal mass, you'd have stretches of similar weather in fall and spring in NW Ark. In fact, you'd look to the shoulder seasons for energy savings you aren't getting now in your passive solar/ICF house. Local climate means everything in this tradeoff between insulation and mass, but the figures are illustrative in this analysis of Minneapolis and Bakersfield by Oak Ridge labs. http://www.ornl.gov/sci/roofs+walls/research/detailed_papers/thermal/index.html In Bakersfield's cooling climate, R25 clearly is not 67 percent better than R15. CIC means concrete-insulation-concrete, while ICI is the opposite, or an ICF. Polycore, ICF is third best in these analyses because the interior foam is muting the mass effect.
Internet tools are another big change in the last five years. If you haven't found UCLA's design suite yet, you will be pleasantly surprised. http://www.energy-design-tools.aud.ucla.edu/ If Climate Consultant

to complete my last sentence (?!), if Climate Consultant says passive solar/high mass works well in your part of the world, interior mass is the way to go.

You might want to look at the new Quad-Lock system with up to 12 inches of insulation on the outside and only 2 inches on the inside the thermal mass properties are great. I prefer the R-53TMO option from them as far as economics.

Thanks toddm for that info. You are right I have not looked at software since my last build. This stuff looks great.
And Thanks John H. for the Quad-Lock info. I haven't looked at their site in quite some time either.
David H