I suggest you get a couple of small coolers and do some experiments.

bpnkrtn-
You said: "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."
Yes, the most important things that you can do to reduce cooling and heating energy use in central Texas are to:
1. Use a CRRC rated cool-roof as mentioned by Dana1.
2. Use significant roof overhangs to avoid direct solar gain through windows and to shade the walls. Use http://www.susdesign.com/overhang/ for quantifying, but I would guess something like 24" might work.
3. Use deciduous trees around the house to reduce direct solar gain through windows and on the roof and walls.
4. Use at least R-30 (code minimum), but preferably more, in the attic to isolate the very hot attic from the house.
5. Make use of big covered porches if you can, a common approach in older homes in central Texas (e.g, Johnson City) before air conditioning took over. These porches cool down more rapidly in the evening than the inside of the house, provide access to cool breezes, and also act like roof overhangs to limit direct solar radiation in windows.

Concerning item 3, an important topic not covered in this discussion previously, live oaks Quercus virginiana, and Q. fusiformis) were traditionally planted in central and Texas for shade, but they can no longer be recommended in central Texas due to oak wilt. They also maintain leaves through the winter, reducing solar gain when you might want it. White oaks are generally less susceptible than red oaks to oak wilt. I had excellent results in San Antonio with chinkapin (or chinquapin) oaks (Quercus muehlenbergii) that are in the white oak family, and grow more vertically and faster than live oaks. Pecans can also make good shade trees but tend to be messy with nuts and broken limbs. Look around at the shade trees that survived around pre-air-conditioned homes in Blanco, Johnson City, New Braunfels or wherever you are going to build. Shading with vegetation should be part of the your energy plan.

On a different note, although cool roofs are recommended, the light colors show stains more easily, and are not allowed in some subdivisions for esthetic reasons.

And on yet a different note, the idea of thermally heat sinking the indoor house temperature to the ground (Jesse Savell idea mentioned by Alton) might work in central Texas where ground water temperatures are around 76 F, but is a poor idea if you don't want to live at a temperature near the local ground water temperature, which very widely in the U.S. and are mostly colder than a desired indoor temperature.

Greetings,

If you use RB sys as I recommend then you can use any type, color, etc roof sys you want. Be sure to use ridge/soffit venting. Ck: cor-a-vent.com

You say to use at least "R" 30, but, there is no installed. winter-summer tests that I know of that says you will get your money back on going over "R" 19. Using too much bulk insul in attic can store more heat energy and raise energy costs.

When using trees the trees should be far away from the building as to not cast shadows on the walls/windows.

If you are going to use ground temp water for cooling, the best method is radiant floor sys. However some people cannot tolerate the cooler floor temps. If your going to use ground temp sys, air or water you should use RB insulation for walls and ceilings for best results.

For shading windows that do not lend themselves to vegetation or overhang shading use vented alum awnings.

Posted By rbisys1 on 04 Sep 2011 03:18 PM
Greetings,

If you use RB sys as I recommend then you can use any type, color, etc roof sys you want. Be sure to use ridge/soffit venting. Ck: cor-a-vent.com

You say to use at least "R" 30, but, there is no installed. winter-summer tests that I know of that says you will get your money back on going over "R" 19. Using too much bulk insul in attic can store more heat energy and raise energy costs.

When using trees the trees should be far away from the building as to not cast shadows on the walls/windows.

If you are going to use ground temp water for cooling, the best method is radiant floor sys. However some people cannot tolerate the cooler floor temps. If your going to use ground temp sys, air or water you should use RB insulation for walls and ceilings for best results.

For shading windows that do not lend themselves to vegetation or overhang shading use vented alum awnings.

I have a question about Radiant Barrier retrofit in an attic that is not really appropriate for this thread. Is there a way to ask you directly? Email perhaps ... or should I start a new thread?

Greetings.

Contact me at [email protected]

I'll give you my phone #.

Springtime, are you really that dense? If the specific heat of concrete is 0.20, which is to say that it captures 2 btu/hr of every 10 btu applied to it, where do you suppose the other 8 btu go? Hint: they are still inside the house. My mother in law comment was meant to smack of exasperation. Like a self-appointed Internet expert who decides that mass only works in Phoenix and Bakersfield and then challenges people to prove his half-**sed theory wrong.

That's an odd definition of specific heat you have going on there toddm. The common definition of specific heat is the amount of heat energy required to raise the temperature of a fixed amount of mass of the material one degree.

Concrete's specific heat (in English units) is about 0.20 BTU per lb per degree Fahrenheit. It doesn't "capture" 20% of the heat flux, rather, it takes 0.2 BTUs of heat to raise a pound of it one degree F. Yes, it can store heat, but only relative to it's mass and temperature rise. If it's "storing" 2BTU/hr per pound(or square foot or whatever you meant by that example) under a flux of10 BTU/hr, that could be good if you're trying to reject heat during the cooling season, but if the heat flux is from the interior (say in a passive solar design), the extra 8BTU/hr would lead to serious overheating.

Concrete as AAC has an R value about two orders of magnitude higher than standard concrete, which slows down the transfer of heat into and out of the material considerably. And sinces it's 1/5 as dense as standard concrete, even under the best of conditions it can only "store" 1/5 as much heat for the same material volume & fixed rise in temperature. As such it's a LOUSY form of thermal mass for moderating temps in a passive solar design, even if it's pretty good at deferring and lowering the peak loads relative to mid-day-heat coming through the walls (which is still but a fraction of the total cooling load) during the cooling season.

Dana1-

Thanks for addressing the obvious erroneous misunderstanding about the definition of specific heat.

OK Dana. You got me. I should have written specific heat capacity. In my defense even NASA uses the shorthand: http://www.grc.nasa.gov/WWW/k-12/airplane/heat.html

Now if you use the heat transfer formula on that page, which appears to be meant for school children, then to raise a unit of concrete by 2 btus in temperature requires 10 btus. And don't make me dive into that dragmit mess and find an example of you doing just this math.

In short, concrete is a relatively poor conductor of heat -- hence thermal lag. Also hence: the correct focus in passive solar should be on rates of transfer vs. absolute storage capacity. The people who say a slab is enough had better be conservative on glass as well because the other 8 btu are in fact still in the house.

AAC's R value slows the transfer rate THROUGH the block. The inside surface is a different story: more area perhaps because of its porosity; a higher moisture load perhaps. Anyway 10btus times a specific heat of 0.25 times one unit of mass = 2.5 btus that, yes, we might say has been captured by the wall.

"...to raise a unit of concrete by 2 btus in temperature requires 10 btus."

What means "...to raise a unit of concrete by 2 btus..." in your language? Did you mean to say:

To raise a pound of concrete with at specific heat of 0.2BTU/lb-F two DEGREES takes 10BTUs? (Which works in my arithmetic.)

Moisture permeation or porosity or air permeability of the material irrelevant and are not part of the equation (not unless you measure the mass of the moisture and add it in as a secondary effect.) The specific heat of water is 5x that of concrete per pound, but a pound of saturated concrete contains only a few grams of water.

"...10btus times a specific heat of 0.25 times one unit of mass = 2.5 btus"?? You may have read the NASA thermodynamics primer, but from your explanation it's not clear that you've fully grasped it. Try to at least add/divide/multiply by the correct units, eh? To store 2.5BTUs in a pound of material with a specific heat of 0.25BTU/lb-F takes a temperature change of 10F.

The thermal conductivity of concrete is significantly less a tenth that of AAC, but it has 5x the thermal mass of AAC for a fixed volume. The thermal mass of the portion of the AAC that's say, 6" away from the interior is completely irrelevant from a passive solar storage point of view since it's isolated from the interior by R5+, and in an 8" AAC it's less than R2 from the exterior- it'll release the vast majority of any stored heat to the exterior. But in a poured concrete wall or slab with exterior-side insulation, the concrete 6" away from the interior less than R0.5 away from the interior, and R(x- whatever is installed) away from the exterior- most of heat related to the temperature rise of the concrete will be retained by the interior, but without a large temperature change. A 6" poured concrete wall or slab has about 300% the thermal mass of an 8" AAC wall of equivalent area, and ALL of that thermal mass is inside the thermal boundary, releasing the vast majority of that heat back to the interior. It's 5x the mass, (4x the thermal mass) of AAC, but a tiny fraction of the R.

The thermal lag is a function of both thermal mass and R. Concrete has low R, high mass, AAC has moderate-R, moderate-mass. A 2" poured concrete ICF wall with an inch of EPS on either side would be a good rough-comparison to an 8" AAC wall from total thermal point of view, including thermal lags & heat storage.

> concrete is a relatively poor conductor of heat
> thermal conductivity of concrete is significantly less a tenth that of AAC

Hey, you agree on something. But I don't....

Right you are, jonr!  (And rong I wuz- didn't I mean MORE conductive than AAC? )

Poured structural density concrete is ~R0.08/inch.

Taking  toddm's R1.25/inch as fact,  that's about 1/16 the conductivity of structural concrete, which means concrete is MORE than an order of magnitude more conductive, not less than a tenth.
Now if I can just get him to agree that BTUxBTU/b-°F is equal to  BTU²/lb-°F not equal to BTU/lb of stored heat, then we'd be getting somewhere.

(Or that °F x BTU/lb-°F in fact IS equal to BTU/lb, the amount of heat stored.)

It doesn't take a differential equation description of heat transfer within AAC to judge it's (ill) suitability as a thermal storage medium compared to concrete.

Meant to write degrees F. Wrote btus instead. I have a house to build. It isn't a huge mistake since the specific heat coefficient whittles away in both directions. (i.e. a pound of concrete that cools by a degree gives up 0.2 btus.) That said, how is this statement in your prior post anything but a whopper?

"Concrete's specific heat (in English units) is about 0.20 BTU per lb per degree Fahrenheit. It doesn't "capture" 20% of the heat flux, rather, it takes 0.2 BTUs of heat to raise a pound of it one degree F."

By my math and Nasa's it takes 5 btus to get that job done.

Gotta love your misdirection with differential equations though, and the helpful summarization: "It doesn't take a differential equation description of heat transfer within AAC to judge it's (ill) suitability as a thermal storage medium compared to concrete."

Remember me? The guy who called you clueless to consider passive solar as an exercise in heat storage? Who showed you how daily heat storage in concrete walls could require three decimal places (.003 degrees F in my example)? Who described his use of mass in passive solar as defensive (i.e. as mitigation in acute overheating situations)? Who wrote "the correct focus in passive solar should be on rates of transfer vs. absolute storage capacity." So, one last time, if AAC's specific heat capacity of 0.25 is not superior to concrete's 0.20 let's hear why not.

And as an aside to anyone contemplating passive solar: This site is a commercial site, run by and for people who have something to sell, be it building products or advertisements of same. These people would see passive solar as slim pickings, and AAC as a product that hasn't paid any bills to date. Builditsolar.com is a far better source of information.

It takes 2.5 BTUs to raise the temp 10F, storing 2.5BTUs, not 5. Specifit heat is a constant, not a dynamic element with hysteresis. Specific heat doesn't "whittle away" in any direction- it's a direct property of the heat storage capacity of a material.

I seriously can't fathom how you've managed to misconstrue the NASA thermodyamics primer. Specific heat isn't rocket science, it a basic definition. To model the dynamic response of AAC accurately may require differential equations, but specific heat of a material is a constant (or nearly so, within the modest temperature ranges we're talking)- it's how much energy it takes to raise a fixed mass a fixed temperature, and conversely, how much energy is released when the fixed mass is lowered a fixed temperature. There are no dynamic losses, no rates to calculate, just a beginning & end temperature states of the fixed mass. Its the same amount of energy whether it takes 6 nanoseconds or 6 centuries for that temperature change to occur.

Actually I don't remember your passive solar calculation exercise, or being called clueless by you (you been talking to my wife or something? :-) ) I guess I don't insult easily enough.

>>>So, one last time, if AAC's specific heat capacity of 0.25 is not superior to concrete's 0.20 let's hear why not. <<

The first part of it is, AAC's specific heat of 0.25 is superior to concrete's 0.20, concrete has only 80% of the thermal mass of AAC, PER POUND. But a concrete wall of identical thickness as an AAC wall has 5x as many pounds ( 500% the mass.) For a fixed volume or wall/slab thickness raised the number of degrees F, the concrete has stored 80% of 5 times as much heat as the AAC. That's 4 times as much absolute heat storage capacity in a concrete slab (or ICF) vs. an AAC wall of equal thickness.

The second part is (as previously posted), without insulation to the exterior the thermal mass of the exterior portion of the AAC isn't participating in the storage significantly. Under a heat load condition it's giving up most of any heat it gained from the interior during to the great outdoors, not the interior. With symmetric ICF it's splitting the difference, but with an interior slab or concrete partition wall, the vast majority of the stored heat is given up to the conditioned space.

Don't confuse me with being a site operator or a commercial entity with goods & services to sell here- I have no commercial interests or personal ties here. (Indeed I have more connection with Gary Reysa at BuilditSolar than I do with anybody here, not that it's a huge connection.)

Dana1, I followed your thinking until the penultimate paragraph. This paragraph has only three sentences. However, I get lost on the second sentence at the word "during".

Probably "during the day". I agree with Dana.

The "return" pinky is quicker than the self-editing eye at that time of day.

jonr's interpretation is right: "during the day" or  "during periods of solar gain" is where I was going...

An 8" ACC wall has the same thermal mass as 2" of structural density concrete, and about the same R value as 2.5" of EPS, but with that mass distributed continuously within the R it'll dynamically underperform 2.5" of EPS on the exterior of 2" of concrete.  Dynamic thermal performance would be similar to...

1" EPS|2" concrete|1" EPS

...the world's skinniest ICF.  (And not very useful as passive solar storage.)

Dana1, Thanks. I always appreciate your posts.

Clearly Dana one of us is working the heat transfer formula backwards. I don't think it is me. My reasoning.

A. Pound for pound, water is considered the best storage medium in anything I have read.

B. We know what that standard is from the definition of a btu (Energy sufficient to raise one pound of water by one degree in one hour.)

C. You have 2.5 btus raising a pound of AAC by 10 degrees, making it four times more effective than water (and turning jonr's world upside down.)

D. Specific heat capacity is based on water at a value of 1.0. A priori, a specific heat of 0.20 for concrete means it is five times less effective, which is to say that it requires five btus to raise a pound of it by one degree in one hour. The coefficient, one might say, whittles away at the results one might get with water, (Eh, jonr.)

Now let's try one more exercise to make my point that transfer rates are more important than absolute heat capacity.

Let's take two concrete plates that weigh one pound and measure one foot square. Heat one to 80 degrees F and one to 70 degrees F and place the hotter one on top of the other. Assume that the only heat transfer is down (which obviously is not the case) At the end of hour, what's the temperature of the bottom plate? The top plate?
(72 degrees and 78 degrees.)

BTW, the plates in concrete would be 0.08 inches thick, in AAC, 0.4 inches thick. Of the 10 hours of insolation on a Jan day heat transfer would be substantially backloaded because of the position of the sun and the formula's sensitivity to temperature difference. With AAC's thermal lag of 8 hours, peak insolation wouldn't work its way outside until, say, 1 a.m.

* Ask Gary Reysa if a passive solar house can have so much mass that diurnal insolation gets lost in the noise. (As in a .003 degree heat gain for my house in eight inches of concrete vs 8 inches of AAC.)

A: Water isn't the highest specific gravity material or necessarily the best but it's pretty cheap thermal mass, and is easily moved (into insulated tanks, f'rinstance. It's pretty good stuff.

C: If you raise a pound of water 10 degrees you've added 10BTUs. If you raise AAC 10 degrees you've added 2.5 BTUs. Water has 4x the thermal mass of AAC, making water 4x more effective (per lb) at storing heat than AAC, not the other way around (turning jonr's world completely right side up to match the rest of us! :-) ) Or mayhaps by "more effective" meaning "it takes less energy to heat it up" ?? (Which isn't necessarily a good thing if you're intending for the thermal mass to moderate the temperature of the room.)

D: With a specific heat of 0.20, yes it has 1/5 of the specific heat of water (just as AAC with a s.h. of 0.25 has only a quarter that of water, making it only 1/4 as effective as ACC per lb.), but whether it takes an hour or a year or a century, it's the same amount of energy- the rate simply doesn't matter from a thermal storage point of view, but even if does from a heat-rejection point of view.

Show me the math on the concrete plates. In my world in an hour they'd both be substantially affected by the (unspecified), in your thought experiment. Making an assumption that the heat only moves down renders it an Alice in Wonderland exercise in the first place, but I'd be curious to see how you came up with the numbers.

The primary heat gain in a passive solar building design is insolation through the glazing, not insolation on the exterior of the wall assemblies. Yes, it's possible (even easy) to design sufficient thermal mass into a building that during typical diurnal temperature swings within the building can stay within any arbitrary range. BTW: The temperature gain within the house isn't a measure of heat unless the thermal mass is unknown, it's just temperature. I'm highly skeptical that you have instrumentation that will measure temperature differences of 3 milli-degrees F over the period of a day though. (I know I don't, and I have an engineering lab at my disposal!)

The thermal lag is dependent on both R and thermal mass, and the location of R to that thermal mass. With AAC, daily temperature swings model pretty much like a lossy transmission-line, a string of tiny R/s & masses (distributed resistance & capacitance,using an electronic transmission line analogy). With some assumptions about the periodic changes of energy input and withdrawal on the exterior it's possible to tune the thickness of the wall to null the amplitude of the variable inputs/withdrawals) to any arbitrary number, but it ultimately isn't very different from the 3-element filter simplification (the world's skinniest ICF example). But 8" or even 12" AAC is still a lot lower (thermal) mass, lower R, and lower performance than any real ICF.