Most ICF's discussed on this board have the thermal mass sandwiched between two layers of insulation. I have a feeling this is the prevalent design because its easy to manufacture and install. I believe someone on this forum mentioned a study that found the greatest energy performance when the mass was located near the interior(sorry for being vague...maybe someone can post this study?) If the two walls had the same amount of mass and insulation ( insulation-mass-insulation vs mass-2xinsulation) how would these two walls perform differently? What impact would placing the mass on the interior have on HVAC? Is having the mass on the interior superior for all climates? Is having the mass on the interior only effective when combined with passive solar design?

Read this thread: http://greenbuildingtalk.com/Forums/tabid/53/view/topic/forumid/4/postid/49134/Default.aspx

Thermal mass is completely dependent on your climate. It works best in places where there are big swings in daily temperature around an average that's close to comfortable. Mass that takes hours to warm or cool acts as a thermal flywheel. Phoenix's hot days and cool nights suits high mass structures nicely. Minneapolis' cold days and colder nights do not. In places in between, mass can work in spring and fall.

Passive solar is separate but complementary in the sense that it can make mass effective in sunny cold climates like Colorado. Conversely, you'd limit mass in cloudy cold climates where there isn't enough solar energy to heat massive structures.

Insulated mass in any form can allow hvac load calculations closer to average temperature rather design loss temperatures, but energy savings are modest. If there's a benefit of thermal mass in your part of the world, it happens in 24-hour periods, and you'd want the mass exposed to the inside of your house.

Todd is right in that mass effects only come into play when you have temperature fluctuations over a 24 hour period. The greater the fluctuation, the greater the mass effects. Having the concrete on the interior is typically better than in the middle (even if the mass effects are minimal). The one exception to this would be something like a vacation home or a weekend home that is occupied for limited durations. Then you are spending energy to heat up / cool the walls just in time for your departure.

Durisol has a very good system that positions the concrete to the interior and some added benefits of providing a moisture flywheel as well as a temperature flywheel. That is to say it has thermal mass and hygric mass.

H20 (water) is the best / most cost effective "thermal mass" you can add to the interior. Not only does it have the storage capacity, but one can add / subtract containers to fine tune the system.

The best thermal mass is in an insulated storage room or tank - so you can overheat or overcool it when these things are cheaper without impacting the occupants. In the living space mass isn't worth much unless you are willing to tolerate temperature swings. If a high mass wall is at 70F at night, then it gets to 70F outside, then it gets cold and I heat to 70F, the wall has done absolutely nothing for me.

Best thermal mass - Glauber's Salt.

I am not sure you understand the concept, jonr. If your high mass wall is 70 degrees at 8 pm and your air conditioner kicks in at 10 pm, then we have very different definitions of what constitutes high mass.
I am building with autoclaved aerated concrete, which claims a thermal lag of 8 hours (takes that long for heat to work its way through.) So, a peak heat in air temperature at 6 p.m. should be shifted to 2 a.m. in my house, at which point the outside air temperature is much different. (How different determines the effectiveness of thermal mass.)
We are not taliking about heat storage at this point, but rather about daily averages, which should rarely get out of whack. Which will have the more consistent temperature: Your swimming pool or a gallon jug of water sitting on the pool's apron?
Even so, we can press on. If you are building a slab on grade for economy reasons and you use it to claim a thermal mass advantage, good on you! If you can incorporate passive solar in the process, better on you!

I understand it very well.  Let's take your example.  Since you mention air conditioning, this must be a cooling case (although your example would make a lot more sense if you used am instead of pm).  If your high mass wall is 70F at 8pm and your set point is 70F +- 0 and the outside temperature is anything more than 70F, heat will flow from outside to inside and the AC will kick in just after 8pm.  This is true no matter how much thermal mass you have.  Of course +-0 is never used, but the concept is important.

Thermal mass starts to have value as you increase the +-0 range.    But increase it too far and you get uncomfortable.  Unless that mass is not in the living space.  Then it can be anything you want.

OK, here are ORNL's DBMS calculations (dynamic benefit of mass systems) http://www.ornl.gov/sci/roofs+walls/research/detailed_papers/dyn_perf/results.html

Using DBMS as a multiplier you will see that a high mass wall in Phoenix rated at R-13 can be considered an effective R-33 because the mass is averaging out the extremes of daytime heat and nighttime cool (i.e. the temperature inside the house varies only a few degrees over 24 hours.) http://www.ornl.gov/sci/roofs+walls/research/detailed_papers/dyn_perf/figures/figure8.pdf

Even if you insist on 70 degrees you still save money because the house will have many more hours in the course of a year when it is 70 and has no hvac load. If you are familiar with UCLA's HEED modeling software, you can see for yourself.

Happily you don't have to choose between mass and active storage, assuming mass works in your climate. You can have both.

What variation in temperature did they allow (it's not in fig 8)?  70F to 77F is probably reasonable.

While interior mass is more efficient then exterior, it also removes the ability to turn down the heat/AC when sleeping or when not home for short periods (say at work).   I suspect that this factor might completely remove the advantages of interior mass in most climates.  Then consider condensation issues (cold walls and hot humid air aren't a good combination).

They didn't model active storage (which should perform much better).

Good heavens, man. ORNL devised DBMS as a means of comparing the energy performance of high-mass structures to regular stud walls, their concern being that R-13 for the Phoenix house above was a misleading number. With this goal in mind, it would be pretty dumb to set different standards for thermostat settings and air infiltration. (ORNL says this about a field study in the early 1980s comparing high- and low-mass structures in four locations: "The buildings were of identical construction except for the walls and were operated at the same thermostat setting.") In fact, ICF buffs complain that ORNL understates its performance by not recognizing it as inherently tighter than stick built.
Your point on thermostat setbacks is debunked here: http://www.ornl.gov/sci/roofs+walls/research/detailed_papers/thermal/index.html
Your theorizing on condensation can be checked in UCLA's Climate Consultant 4 by comparing dew point to average daily temperatures in your locale. (Not a problem in Pa., which is plenty wet in the summer.)
As for which is better, you're asking do I want steak or ice cream? I want both, thank you. My 25 tons of concrete slab is going both ways, passive and active when needed.

Let's see. Setback is worth 8% (1) or maybe 20% (2). Thermal mass is worth around 11% (your reference). Of course it all depends on the exact circumstances. But looks like about a wash. And the numbers are small enough that I would focus on other things (GSHP, active storage, solar, better insulation, etc).

1 - http://www.energystar.gov/index.cfm?c=thermostats.pr_thermostats
2 - http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V2P-4NKB1MB-2&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=b0587e3223939226870a858f763d4bb1

ORNL on setback

"The net effect of thermal mass in buildings containing heavyweight components was believed to cause the average indoor temperature and difference across the building envelope to be maintained at a more elevated level. As a result, night temperature setback caused the envelope heat-losses rate to be higher in massive buildings. All of this supported a common belief that night temperature setbacks in massive buildings caused a reduction in the setback energy savings. D. Burch investigated this penalty in setback energy savings and his research confirmed the fact that such a reduction took place. However, the magnitude of this phenomenon was very insignificant. For example, for a typical residence the difference in setback energy savings in the massive house and traditional wood-framed was predicted as only 0.3%."