Howdy. I've been reading many threads on this forum since I stumbled onto it ... very informative with a minimal amount of typical "discussion forum BS." Outstanding! I read about mass in homes/buildings and stored heat/energy all the time in terms of climates dominated by heating loads. Does mass also help in climates dominated by cooling loads ... assuming a closed house relying on air conditioning? (Central Texas; cooling degree days = 3000+, and heating degree days of 1600). I've learned from this forum that tightness of the structure is probably the foremost concern; i.e., no air leaks in terms of energy efficiency.

Passive thermal mass can help whenever the temperature that the mass is exposed to varies above and/or below comfortable levels over a short period (typically 1 day). Once you factor in the negative effect on thermostat setbacks, it won't do much for you in your climate (vs someplace with greater day/night swings).

Massive walls are generally more useful in cooling dominated climates than in heating climates, since the daily swings in surface temps of sun-exposed buildings in those areas is often quite higher than the daily air-temperature swings. (I'm not sure it's doing much for you when it only gets down to 90F at night though.)

Using CRRC rated cool-roof materials with combination of high solar reflectivity and moderately high infra-red emissivity limits the surface temps considerably cooler, and can have a very good performance boost in excess of mere mass though.

Using a similar high reflectivity moderate-E approach to the walls helps too, but that's less important than the low-E coatings or exterior shading factors on the windows. In central TX even the north-facing windows can be getting almost 50% as much radiated heat gain of the south facing windows, depending on the reflectivity & emissivity of what's in the view(!).

In point of fact, massive walls are more useful in cooling climates than heating climates, as suggested by the fact that there are many more adobe houses in New Mexico than there are in New Hampshire. What you have going for you in Texas -- in spades this year -- is low humidity. As I am learning in Penna., a mass-buffered indoor temp of 78 degrees is a pyrrhic victory when the RH is 60 percent plus. Mass works to your benefit when daily temps swing above and below an average temp you'd consider comfortable. (i.e. 65-85 outside would be pretty pleasant inside if a mass envelope holds indoor temps near the daily average.) It is REALLY effective when outside temps swing in big extremes: (i.e 60-100 in Phoenix.) Mass has some benefit everywhere because you have look at its impact across 365 days of the year. Download Climate Consultant 5 here http://www.energy-design-tools.aud.ucla.edu/ plug in weather data from the nearest NWS station and judge for yourself. Its psychrometric chart will show you how many hours in a year that high mass should substitute for HVAC.

Pay no attention to jonr on the subject of thermostat setback because mass would be reducing cooling load in central Texas. Michiganders. What are you going to do?

Thanks for the input fellers. I have no training in these subjects, formal or informal, but do have a a lot of interest. We are of course going on 2+ months of 100+ degree weather, along with a record setting drought. We had some 100+ days in the spring ... if you could cal it that? Don't count on the low humidity in this area ... east Texas is deep south with high humidity (including swaps with alligators) while west Texas is the dry, low humidity of the southwest ... central Texas, is a mix, or in-inbetween. We can see some low RH, but can also see 60 to 80%.

I downloaded the Climate Consultant (was happy to see a Mac version!) It will take me a while to figure it out, but I will. It would seem to me that in this climate one should keep the sun off all walls as much as possible, i.e., expansive overhangs and vegetative shading ... and just accept the loss of a little benefit in the winter. I've used a sledge hammer on frozen cattle water troughs in the winter a couple times ... but I've gone swimming during Christmas more often.

A last note/thought: It would almost seem like the "inside foam" of an ICF would be a hindrance, that the mass (concrete) would need to be exposed on the inside (I don't think I've expressed myself very well here, sorry).

I've tried to research this subject myself, and most of the information available seems to pertain to heating dominated climates and passive solar designs, next to nothing about a hot, humid climate.

I took an HVAC design class when I was in college, and although I haven't done any load calculations in the twenty five years since then, I did notice that if infiltration was eliminated, typical residential wood framed construction in the examples that I calculated experienced peak heat gains at around 5:00 PM due to lag effect, in a central Alabama climate. High mass construction would increase the lag effect, so, a concrete block wall might shift that peak to say 9:00 PM. At 9:00 PM, the outdoor temperature would be less so the COP of your air conditioner would be higher (i.e. more efficient).

That sounds really great, except that most of the heat load in my climate comes from the roof, not the walls, and there really aren't many very good ways to build a high mass roof. If you could though, some combination of insulation and mass could shift the peak heat gains to off peak times when your AC would produce more cooling for less kilowatts consumed.

Another thought is the deep sub-soil temperature. Around here, at 12 feet deep, the soil is between 68-72 year round. If you constructed large footers at that depth and built walls that were insulated on the outside perimeter, you could create a structure that was coupled to that ground temp year round. Instead of passive solar, you would have passive geothermal. You'd still have to dehumidify though, and certainly need some air conditioning for days when you had high internal loads, such as when relatives visit and keep all the doors open all the time.

Obviously, there is some relation to mass, insulation, AC sizing and COP, and solar orientation, shading, etc, where all the lines cross which represents the most benefit for the least amount of money spent. One day, I may do a graduate thesis to model it. Until then, I am just going to pay the extra $5000 to get twice as much foam in my attic, which will probably come close to the same performance.

What is COP please? And thanks for the Alabama thinking!

Tigerfan6, You might be interested in this web site regarding insulated deep foundation stem walls by Jesse Savell in 1973. See http://infoark.org/InfoArk/Sustainability/Mother%20Earth%20News/70/MEN_CD/mendemo/menarch/archive/issues/048/048-116-01.htm. By the way, the Passive Solar-Variant Homes by Jesse Savell are patented.

COP is an HVAC term.  It stands for Coefficient of Performance.  A higher COP means your Air Conditioner is more efficient.  COP is determined by dividing the output of your air conditioner by the energy required to produce it, but that is BTU's of cooling divided by BTU's of electricity.  1 KW electrical = 3412 BTU's electrical.

EER, as used in the US, is more or less the same thing except that the EER number uses btu's of cooling divided by watts of electricity.  SEER is an attempt to take the EER number and adjust it for real world conditions, except that every place in the world is different, so it really doesn't mean much.  EER and SEER came into use sometime in the 1990's.  COP was the measurement used before that.

So an Air Conditioner with an EER of 10 and an output of 3 tons (36,000 BTU's) uses 3600 watts of electricity.  Its COP is 2.93.  In practical terms, you take the cooling load and divide it by the COP and you get the electricity it takes to operate your air conditioner.

However, just like the "your mileage may vary" disclaimer, it is important to remember that numbers like COP and EER are just calculated values for standard conditions.  I think EER is calculated for a conditioned space temp of 78 and an outdoor temp of 85, it's simply a way of sizing an air conditioner.  On a day when it's 102 outside and you want it 72 inside because the humidity is high, your COP is going to be a lot lower, just like your car doesn't get good gas mileage when you are towing a trailer, but it is still designed to do that. 

The significance of COP as pertains to my post is that the lower the outside air temperature is, the less energy required for our air conditioner to transfer heat from inside to outside, so its COP increases.  There is some benefit to shifting cooling loads to times when outdoor temps are lower, but I am not sure that it is practical to try to do that rather than just adding more insulation or shading and reducing the cooling load to begin with.

Interior thermal mass to reduce cooling is great when you like opening and closing windows every day (always check the temps and humidity first) , having uncomfortable humidity (or mold on the walls), tolerating temperature swings, getting up at 4am because the house is too cold and finding rain on the floor. All for single digit savings (in most cases).

And as everyone except toddm knows, thermal mass does reduce the effectiveness of thermostat setback (which for cooling means higher) in any climate, cooling or heating. Exactly how much depends.

That being said, if I lived in Phoenix, I would consider a high mass house (either exterior or with automated ventilation to avoid the above issues).

Yes, bpnktrn, the interior foam of ICF walls limit its mass effect, but other approaches do away with the foam sandwich altogether. Many of these so-called hybrid ICFs are based in the SW. Apex and Rastra come to mind. While autoclated aerated concrete is not technically an hybrid ICF, construction and mass benefit are similar. Xella, the North American arm of German world leader Hebel, is based in tx and operates a plant in mexico. Surely there are Rastra and AAC houses in Austin.

Tigerfan, you are underestimating thermal lag. My 8 inch AAC walls have a lag of 8 hours. AAC comes in 10 and 12 inch blocks. Peak heat never really penetrates but advances and withdraws in the wall in a diurnal sine wave. An adobe house in Tucson doesn't need air conditioning. Bu the givens are low humidity and cool nights. Humdity is key. Even if night temps hover around 80, thermal mass with night flushing would almost keep bpnkrtn comfortable in several months of 100 plus heat. (I lived through 61 days of 100 plus in Dallas in the mid 80s. IIRC, the RH stuck in single digits.)

So, jonr, "uncomfortable humidity" isn't a particular problem for the OP, who would probably dance a jig at this point upon finding rain on the floor. I have pointed you many times to this study referenced by ornl:

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%.

I chased down the study and shared its conclusion that, while the setback penalty was greater as heating degree days increased, it wasn't a deal killer even in Madison, Wis. So far, your contribution of research as opposed to opinion is bupkus. Gotta say I resent being cast as the one suffering a snit fit.

the setback penalty...wasn't a deal killer

Ah, admitting that it does reduce effectiveness. And I also chased down that same research and found that you keep misapplying it (see previous topic, no sense it going through that again).

Gotta say I resent being cast as the one suffering a snit fit.

Seems like a good description of your snide remarks in post #4 above.

OP here. Just a note ... at 9 AM this morning, temps are closing in on 90, and RH is 62% ... will be 100 by noon to 2 PM if the last couple months are to believed . Rain on the floor would indeed be cause of great celebration!

I almost hesitate to ask here, but I don't really know what "thermostat set-back" means/refers to.

tigerfan6 ... thanks for the acronym lesson ... it really did help my understanding!

toddm ... I'm googling Rasta and apex this morning ... learning about AAC (I'm in the remedial stage here, watching some youtube videos on AAC blocks and panels, and if that doesn't explain my lack of of knowledge in these areas I don't know what would .)

Thanks again to all for the help/education.

toddm: "So, jonr, "uncomfortable humidity" isn't a particular problem for the OP, who would probably dance a jig at this point upon finding rain on the floor."

Uncomfortable humidity isn't about rain, it's about the dew point. Dew points this summer in say, Ablilene have been mostly in the mid 60s bumping into the 70s fairly often this summer. While that isn't torrid as a gulf-coast swamp, it's well above what's comfortable for humans, an increases the potential for mold on building materials, and fungal infections on humans. Uncomfortable humidity very much IS a problem in central TX, despite the drought.

By contrast, in rainy Seattle summertime dew points rarely exceed mid-50s, and hitting mid-60s makes page-2 in the newspapers.

what "thermostat set-back" means/refers to

You can get thermostats that change the temperature based on the time of day. For example, during the cooling season, you might have a higher setting during working periods (when nobody is home). Or during the heating season, a lower setting at night (when you have all those blankets to make you comfortable). Savings vary - maybe 10% - and the ROI and ease of use are fantastic (especially as compared to thermal mass).

http://www.energysavers.gov/your_home/space_heating_cooling/index.cfm/mytopic=12720

With variable speed split-system AC (mini/multi-splits) a setback strategy costs more than keeping it at constant temp, since the compressors run signficantly more efficiently at part load, and it has to crank to max during recovery from setback, more than erasing any savings during the setback period due to lower-efficiency operation. (A similar condition can occur with some more traditional 2-stage AC units too.)

But for single-speed oversized AC systems, setback can be a worthwhile strategy. But the higher the R value even in moderate interior mass homes, the less difference setback makes.

, more than erasing any savings

It depends. Turn off the AC for 8 hours while you go to work and save say 50% for that period (depends on weather, mass, insulation, solar gain, infiltration, etc). Just before you come home, it runs for 20 min at 20% less efficiency (ignoring that the indoor air is now hotter than normal which offsets this some with higher efficiency) before the temp gets into a tolerable range and it can move to higher efficiency. These numbers work very well even with the extra cooling needed after you come home.

A reasonable check is how long it takes before your house becomes uncomfortable with the AC off. For me, about 3 hours to heat up, 10-15 min to cool down, no efficiency hit (single stage, so an actual efficiency increase).

Posted By jonr on 24 Aug 2011 01:19 PM

, more than erasing any savings

It depends. Turn off the AC for 8 hours while you go to work and save say 50% for that period (depends on mass, insulation, solar gain, infiltration, etc). Just before you come home, it runs for 20 min at 20% less efficiency (ignoring that the indoor air is now hotter which offsets this some with higher efficiency) before the temp gets into a tolerable range and it can move to higher efficiency. These numbers work very well.

Save 50% of what? 

And HOW oversized would it need to be to make up from a setback that saved 50% on AC power over the 8 hours in only 20 minutes??

Inverter drive mini-split COPs can vary by more than a factor of 2 or 3 (not 20%) between low to mid compressor speeds and max speed (in either cooling or heating mode.)  The rated COP at various temps in the specs are usually for a mid-range loading on the compressor and interior unit- they usually beat that by quite a bit at low speed, and exceed it at higher speeds.  If it's rated say, 4.0 at mid-range at a 100F outdoor temp, it can exceed 7 at lowest speed, and run less than 3 running flat-out. See the discussion on p.5 & 6 of this document.

Assuming the 8 hours in question are the 9-5 shift, the more heat gets pumped out earlier in the day at a high COP due to cooler outdoor temp, enhance by higher COPs due to lower compressor speed at part-load, resulting in a VERY high average COP. In hot afternoon it takes a big hit in COP due higher compressor speed and another hit in COP due to higher (average) outdoor temp.  Letting it track the load uses less overall power, even if it's pumping out twice as much heat overall (and 2x the total heat would be a very extreme case, IMHO.)

If you were air-conditioning an uninsulated tent with an oversized mini-split you might be able to make setbacks pay, but in real houses it's a losing proposition.

I may be remembering wrong, Dana, but by august in Dallas the combination of intense sun and no rain had baked things to the point that there was nothing much to provide humidity, unless it got blown in from Ks. I remember the tv weather guys warning farmers about excessive drying conditions. But Abilene should be similar if not drier than dallas. Happily Climate Consultant takes out the guesswork.

Jonr, if you can't say definitively that the OP would be trading significant setback efficiency for high mass, other to say that "it depends," what's the point, especially when the research says otherwise? OK, I know the point. You don't like thermal mass. You are entitled to your prejudices. Happily, the OP doesn't have to sort out this one either. Climate Consultant weighs setback as one of its energy strategies. OP, please advise us after you crank the numbers.

Posted By toddm on 24 Aug 2011 02:28 PM
I may be remembering wrong, Dana, but by august in Dallas the combination of intense sun and no rain had baked things to the point that there was nothing much to provide humidity, unless it got blown in from Ks. I remember the tv weather guys warning farmers about excessive drying conditions. But Abilene should be similar if not drier than dallas. Happily Climate Consultant takes out the guesswork.

Jonr, if you can't say definitively that the OP would be trading significant setback efficiency for high mass, other to say that "it depends," what's the point, especially when the research says otherwise? OK, I know the point. You don't like thermal mass. You are entitled to your prejudices. Happily, the OP doesn't have to sort out this one either. Climate Consultant weighs setback as one of its energy strategies. OP, please advise us after you crank the numbers.

Pulling up the weather history and graphing just the dew point, moving the cursor to center on July 2011 and including June-August data in the view, I'd hazard the average dew point in Dallas TX this summer has been about 65F, with MANY excursions into the 70s.

http://weatherspark.com/#!dashboard;a=USA/TX/Dallas

Yes, the average for Abiline was a few degrees drier, but still in the 60s for an average, also with many excursions into the 70s:

http://weatherspark.com/#!dashboard;a=USA/Texas

(go to the "select graphs" pulldown on the right, turn off temp, etc, turn on "dew point")

Hotter'n'hell air is capable of pulling MORE humidity out of the soil, lakes, rivers, etc., raising the dew point, even if the outdoor relative (to the higher temperature) humidity is low. High temps are drying of the soil & vegetation due to the lower relative humidity of the air and higher soil/material temps increasing the vapor pressure between soil/veg and the surrounding air, but that generally raises, not lowers the dew point (from sucking all that moisture out of the ground & plants into the air.)  Dew point is a measure of the absolute humidity of the air.

The dew point averages for central TX this summer are below the 25 year averages, but they are by no means in the comfortable-healthy range.  Pulling 65F dew-point air into a 75F house raises the interior RH to over 70%, with significant mold and human fungus infection potential.  (ASHRAE says 65% RH is the max allowable for human comfort, most health professionals recommend 50% as a summertime max RH.)