Thermal Mass can work for or against you. In general, when the temp goes above "comfortable" during the day, and "below comfortable during the night, is when you get the most benefit. The easiest way to think of it is that the wall mass will maintain the average temp between the two. In reality, the amount of time it's in a given temp, and whether the sun hits it will effect the avg, but you get the picture.
That mass will give off heat to the inside if said mass above comfortable, or absorb it if inside is below comfortable. Concrete is a good conductor of heat, so ground temp will give you added benefit.
If you are in an extrem temp, it won't be as high a benefit because it's trying to reach a constant temp somewhere between outside temp and your inside temp.
But ALWAYS keep this in mind. The most extreme temps you have around the most poorly constructed ICF, will far exceed that of the best built, stick frame, (outside laboratory construction, such as Oak Ridge).
Also, in southern climates, most places aren't required by code to have a vapor barrier. That lets LOTS of humidity into a stick framed house. Lots. And condensing humidity out of the air is the LARGEST loss of energy and stress on an A/C system.

Jelly,
I'd say JDCPE pretty much explains it. I lived in Louisiana for 15 years so I know what you're facing. In the climate there the daily temperature swings in the summer are 20 to 30F, but always pretty much above the indoor comfort level. What the thermal mass of the concrete will do is average out the high & low temps so the heat flow into the house will be max out a lower level than with a stick house. The same thing in reverse in the winter. The greatest benefit you would get by using ICF in LA is the ~R 25 insulation value and the air tightness of the house envelope. If you have a lot of sun striking your house the concrete will give you benefit because the sun heated siding will cool off before much of the heat makes it to the indoors.

Along with ICF be sure to study up on the benefits of proper attic insulation. A lot of the heat that came into the house I had in LA came in through the ceiling during the heat of the day. I wish I would have known 15 years ago what I know now. Would have done a lot more to improve the energy effciency of the house.

I would say if you're thinking concrete don't bother with anything other than ICF.

JDCPE,

You make a comment about vapor barrier not being used in the south. Take a surfing trip around the 'net, especially this site and building science sites, and you'll soon find that vapor barriers are being abandoned in all but the coldest of climates, like in Canada. Water barrier and vapor permeable are in, vapor barrier is out.

Thanks for the input - so far I'm getting that concrete's thermal mass does it's thing regardless of where the insulation is?

ICF is looking pretty good to me. But when I price it out block by block, the fully grouted and reinforced CMU with EIFS is still coming out significantly cheaper than ICF in terms of material costs. And ICF seems a lot less DIY friendly, so that means more labor costs.

Now if I put rigid foam board on the inside of the CMU as well as on the outside, then the material costs are getting closer to ICF but still the bottom line is cheaper. How important is the internal layer of insulation?

Less friendly than CMU? You must be a mason! :-) Compared to what little mortar work I've done I would have to say in my book stacking ICF blocks is a lot easier. Or are you looking at dry stacking with concrete poured in the voids? Compared to the "Lego" style ICF blocks with the lugs to align and hold them together, I would think even dry stacking would be more challanging, but what the hey, what do I know? I don't have my ICF house built yet and I've never built with CMUs.

Where are your temperature extremes? Outside. So your thermal mass must be protected from this, if it is to protect you. Insulate well on the outside. Insulation on the inside contributes to the extent that it's more insulation.

But no matter what you do, the laws of physics mean that your concrete will on average be hotter in Summer. It may not cool the house, but it will stabilize the swings.

And if you think CMUs are user-friendly, wait till you try them.

My humble opinions:
1. ICFs are much easier to use than CMUs for a DIY (lighter, quicker, easier to keep straight and plumb)
2. The thermal mass is great at leveling out the temp swings during the course of a single day or 2 but doesn't help much with longer term changes - it basically drives your temps toward the median temperature of the day. So when the daily highs and lows swing past your desired indoor temperature it helps out. When the daily high and low both fall above your desired indoor temp then it can still help a little because, while it will not reduce your cooling demand, it will still even the cooling demand swings so your AC will actually run a little less during the day but a little more at night. So you may get by with a slightly smaller AC that runs more and dehumidifies better.

Right, I'm talking about dry-stacking and then filling (look at Azar and Haener self-aligning systems), not mortaring (I won't even pretend to have the skill of a mason). Drystacking regular CMU's is probably harder to get plumb and level, but for almost the same cost per block you can get the self-aligning ones. And what seems to make CMU more DIY friendly is that you can take your time even when it comes to grouting because you can grout in lifts, and use a bucket instead of a boom (if you want to).

Compare it to ICF where you have to take care to brace the entire assembly much better, grout the whole height of the wall at one time with a boom (and do it right), vibrate to consolidate, etc., - you really need a crew of people who know what they're doing. Yes, stacking styrofoam is a heck of a lot easier than stacking concrete blocks, but filling the styrofoam the right way doesn't seem all that easy.

But I am totally ready to change my mind and move to ICF. It's just important in my case to keep the bottom line down and have the ability to do most of the work myself (or with the help of my inexperienced posse - in other words, my in-laws!). I have an open mind about it - that's why I'm here. And I very much appreciate the discussion and the chance to learn. Thanks everybody.

Ok, so back to thermal mass. From what I have read, people in concrete houses are getting dramatic reductions in their cooling costs in climates like Louisiana. Maybe it's not due to thermal mass. Maybe it's just a tight envelope and good insulation, or a combination of all that plus thermal mass. But I keep thinking of those cathedrals in Europe with two foot solid stone walls - walk inside of one in the middle of a baking hot summer and you'll need to put your jacket on. There's no AC and there's no insulation inside or outside, but still it's cool.

True, to do ICF right is not so simple as block manufacturers would have you believe. But it results in a wall of solid rock, which you can not say of CMU. Stacking and cutting are not hard, as long as you're not drunk. Bracing requires some analysis; but make sure you rent a newer system like Giraffe to make it easier to carry and adjust. Get a digital level, which beeps at 0/45/90/135 degrees. Filling with concrete is not hard as long s you follow some basic principles to prevent blowouts. Consolidating is easy, and the machine is rented. I do smaller jobs alone, and hire a 'vato' on pour day.

A "VATO"??? What's that?

I believe that's a "dude." Sounds pretty cool don't it?

By the way, it's solid rock when the CMU have interconnecting cells and the voids are fully grouted...

There is more to the cost than comparing block to block, as you have noted part of it. On the typical housed that I do the A/C system is a ton less, and your electric bill will be cheaper also.
Even though you intend to fill all the cmu cells the wall strenght will still be inferior to ICF.

A vato is indeed a 'dude'. (an illegal one)

CMUs filled in are not what I'd call solid rock. We have earthquakes up here in the PacNW. CMU is no comparison with an ICF wall, for strength. Also CMU does not store thermal energy anywhere close to solid concrete. It insulates slightly better though.

Hang on Jim C, do you mean the A/C in an ICF house is a ton less than the system in an equivalent CMU house, or an equivalent stick frame house?

I built my home with Durisol ICFs which promote their thermal mass properties. I somewhat buy it, but somewhere else in this forum someone referenced a study which showed that the thermal mass effect IS only good during those day-night temperature swings. Once you have consistently hot or cold temps you have to rely on the insulative properties and envelope more so than the thermal mass effect (speaking from experience in a mixed climate).

I had considered the cmu/rigid insulation method Jelly brought to discussion, but felt the ICF would be eaiser for a DIYer. I still may consider it for our planned second phase, but the one extra cost not haven't considered (or at least considered aloud here) is the interior finish. I personally wouldn't want to have plastered cmu for an interior, and the electrical wiring becomes problematic (or ugly) with attached raceways. Thus you have to furr the interior and the possible methods are problematic and likely costly (in terms of the overall system) as well.

I'd like to hear from Jelly how he or she proposes to do the interior, and what that does for his or her budget.

I think you have a good basic understanding of the situation, but the ICF wall will thermally isolate the interior of the building from the outside.  Even if the temperature does not fall significantly during the night the concrete in the wall and the concrete in the footing will work as a heat sink to pull the heat energy into the soil.  The HVAC system will be dealing only with the interior conditions.  Therefore the heating load will be considerably less than with other wall systems.  If the concrete is exposed on the interior and the air inside the house is warm you could have condensation on the bare concerte wall.

Good question skia_d, the interior finish is indeed something that needs consideration in the cost comparison. Wiring can be run in conduit inside the CMU before the voids are grouted (there are knockouts and channels already in the block for conduit and rebar). The conduit can be run vertically up to the ceiling where you can make long horizontal runs, but it can even be run horizontally in the block. And gypsum board (preferably the one without paper I imagine) can be mounted directly to the CMU interior either with a nail gun or tapcon screws if need be.

I understand in an ICF wall the wiring is run in channels cut in the foam with a hot knife? That would make changes or remodeling in the future much easier. And gypsum board is hung on the ICF interior by nailing into plastic that's embedded in the foam? For anything heavier (like cabinets or something) do you go through the foam into the concrete with tapcons?

Did I mention this is a great forum? :)

Jelly, utilities can be run by cutting the foam with:
- router;
- chainsaw with nosewheel;
- special hotknife that only we pro's have.

We always recommend that electrical be run from above. It's just better.

Cabinets we cut to the concrete and fasten either 1"x2" or plywood directly to the concrete, for fastening of cabinets or ADA grab-bars.

I'm on my third home built with the "Epic" wall system by Weaver Precast in Orlando.  Check it out at LINK.  It's 5,000 PSI concreat on the exterior, 4 inches of foam on the interior, with the furring strips alreacy embedded.

These walls are put up 4 hours.  The next 4 hours, the trusses can be put on.  Yes, they may be a little more expensive, but my savings is in time and my energy and insurance expenses are a hugh savings.  And I know I have a very sound home.   

Check it out and if you have any questions, let me know.

Lynda