I take it then by your last post that you acknowledge that 2.5 btus will not raise a pound of AAC by 10 degrees as you asserted, that in fact AAC is four times LESS effective as heat storage than water rather than four times more.....

No matter. You write,'Or mayhaps by "more effective" [you are] meaning "it takes less energy to heat it [water] up" ?? (Which isn't necessarily a good thing if you're intending for the thermal mass to moderate the temperature of the room.)' I am moved to shout "Yes, you've got it. Thank G*d Almight YES at last." It follows then, if the rate of transfer is more important than absolute heat capacity, that AAC is in fact 25 percent more effective than concrete.

Yes, the insolation that counts is coming through the windows. The slab doesn't have full coverage until midday. It would take a couple hours after that to gin up a delta T to drive heat transfer. Ergo, what we want in passive solar is all the mass that can heated to diurnal significance in six to eight hours (continuing after sunset because the heat load on the slab's surface would exist at the point.) Go ahead and invoke any exigency you'd care to add to my concrete plate illustration because, inevitably, it means that conduction through concrete is even less effective than the heat transfer math would suggest.

You know exactly how I came up with 72 and 78 degrees, Dana. And the millidegrees are more heat transfer math. I won't mention that you insisted on math only when I speculated on how AAC could have a higher specific heat than concrete, but you want all the variables considered, plus a lab, when the math works for me. (OK, I just have.) Innocent bystanders should check out the nasa webpage I referenced earlier. The formula is there.

My bet is that more heat would be radiated upward from my concrete slab than conducted downward in the few hours in question. AAC walls are a second line of defense for me. My third line, if necessary, will be a heat pump hot water heater mounted in a chase taking air off the great room and turning insolation into water storage.

Your description of passive solar high mass as an analog problem is apt if somewhat simplistic. Assuming my tuned walls work perfectly on Dec. 23, that is no guarantee that they won't fall far short on Jan.3, or split the difference on Feb. 17. You'd have to average it over 365 days. Well, you'd also have to accept equivalency measures, which leaves out the flat earthers.

Frankly I have no idea whether my 8 inch AAC walls will work, but I believe I have thought it through well enough to have contingencies in hand. I know that 8 inch concrete walls will not work in So Central Pa. If you still think they will, then you are hopeless as well as clueless.





You know exactly how I did the math for my concrete plates. If you don't, please refer to the Nasa webpage I referenced earlier that

> making water 4x more effective (per lb) at storing heat than AAC
> AAC is four times LESS effective as heat storage than water

Good, we can all agree on this.

Yes we can Jonr. That said, the folks who thought Christopher Columbus would sail off the end of the earth probably agreed with him on sail geometry.

Todd you amaze me- how does...

"If you raise AAC 10 degrees you've added 2.5 BTUs"

....from my previous post add up to your interpretation:

"I take it then by your last post that you acknowledge that 2.5 btus will not raise a pound of AAC by 10 degrees as you asserted"

Really?

Call me clueless, but I thought I just said that adding 2.5BTUs to a pound of AAC totally DOES raise it's temp 10F, and I have a hard time figuring out how the converse could be construed.

>>No matter. You write,'Or mayhaps by "more effective" [you are] meaning "it takes less energy to heat it [water] up" ?? (Which isn't necessarily a good thing if you're intending for the thermal mass to moderate the temperature of the room.)' I am moved to shout "Yes, you've got it. Thank G*d Almight YES at last." It follows then, if the rate of transfer is more important than absolute heat capacity, that AAC is in fact 25 percent more effective than concrete. <<

Uh no, it doesn't take less energy to heat a pound of water water up than AAC, iit takes more ( 4x more. )

Rates matter for passive solar storate, sure, (not necessarily more than the total storage capacity) but both rates and capacity are to the disadvantage of AAC vs. concrete. With the much higher R of AAC you're raising the surface temp much faster than that of concrete, and storing less in the interior mass. As a storage medium that's the opposite of what you want. With an equal volume of AAC vs. concrete you end up with higher surface temps during the gain phase, heating up the space rather than moderating the temp, and you have 1/4 the total storage capacity for the periods after the gain phase. To keep the temperature swings bounded in a passive solar design the rate at which the heat can get into the thermal mass of the storage has to be similar to the incident heat flux. Using an AAC slab would be like putting a substantial rug over a 2" concrete slab or something- the bulk of the heat gain is to the room, and not the storage medium.

Also,

>>You know exactly how I came up with 72 and 78 degrees, Dana. And the millidegrees are more heat transfer math. I won't mention that you insisted on math only when I speculated on how AAC could have a higher specific heat than concrete, but you want all the variables considered, plus a lab, when the math works for me. (OK, I just have.) Innocent bystanders should check out the nasa webpage I referenced earlier. The formula is there. <<<

Where to begin?

The text of the NASA thermodynamics primer page:

-------------------------------------------------------------

Thermodynamics is a branch of physics that deals with the energy and work of a system. Thermodynamics deals only with the large scale response of a system that we can observe and measure in experiments. In aerodynamics, we are most interested in the thermodynamics of propulsion systems and high speed flows.

The Zeroth Law of Thermodynamics introduces the concept of thermodynamic equilibrium, in which two objects have the same temperature. If we bring two objects that are initially at different temperatures into physical contact, they eventually achieve thermal equilibrium. During the process of reaching thermal equilibrium, heat is transferred between the objects. The amount of heat transferred delta Q is proportional to the temperature difference delta T between the objects and the heat capacity c of the object.

delta Q = c * delta T

The heat capacity is a constant that tells how much heat is added per unit temperature rise. The value of the constant is different for different materials. Heat is always transferred from the object at the higher temperature to the object with the lower temperature.

For a gas, the heat transfer is related to a change in temperature. The temperature, pressure, and volume of the gas determine the state of the gas. Heating a gas changes the state of the gas. But the state of a gas can be changed in a wide variety of ways. On another slide, we show how work done on a gas also changes the state of the gas. The amount of work that a gas can do depends on both the initial and final states and on the process used to make the change. In the same way, the amount of heat transferred in changing the state of a gas also depends on the initial and final states and the exact process used to change the state. Different processes result in different amounts of heat transfer and work. The effects of both heat flow and work are combined in the First Law of Thermodynamics.

There are some thermodynamic processes in which there is no heat transfer. Engineers call this type of a process an adiabatic process and there are simple equations which relate the pressure and temperature of a gas for an adiabatic process.
--------------------------------------------------------------------------------------------------- end clipped text-----

Again, show me the math on your 1lb square-foot plate of 70F concrete married to an identical plate of 80F concrete end up at 72F & 78F after one hour based on your interpretation of that page, or any of the hyperlinked page in the series, 'cuz I just don't get it.

>>My bet is that more heat would be radiated upward from my concrete slab than conducted downward in the few hours in question. AAC walls are a second line of defense for me. My third line, if necessary, will be a heat pump hot water heater mounted in a chase taking air off the great room and turning insolation into water storage. <<

That's exactly the problem with AAC as thermal storage- during the high-insolation period the temperature of the AAC surface rises, and re-radiates the heat into the house rather than being conducted into the slab. The temperature difference between the sun and your slab is HUGE, making for a very large radiated heat transfer into the slab. And hopefully you're using concrete & not AAC as the storage medium.


>>Frankly I have no idea whether my 8 inch AAC walls will work, but I believe I have thought it through well enough to have contingencies in hand. I know that 8 inch concrete walls will not work in So Central Pa. If you still think they will, then you are hopeless as well as clueless.<<

Since I can't recall weighing in as to whether 8" AAC would or wouldn't work in So Central Pa, so maybe I'm just I'm just clueless.

But now that you mention it, I think they WILL work in So Central Pa, about as well as 2x4 16" o.c. stick built with cellulose or open cell foam cavity fill during the winter, and even a little better during the shoulder seasons. So maybe I'm really hopeless too? :-)

I may be both clueless AND hopeless, but I have a track record of designs that actually work. ;-)

I have enjoyed this back and forth discussion. But now it is beginning to remind me of the line in the movie City Slickers. Remember the long drawn out discussion about programing a VCR and the retort that stopped it: "The cows know how to use the VCR by now." Now that was a funny line.

Gotta agree with Alton, except for a point of honor. I did not manufacture the heat transfer math error I asked Dana to recant. Readers will find it in his post of 9-7 at 6:45 p.m. It wasn't a typo because he used the same flawed formula (did it backwards) in a post on 9-6 at 6:18 p.m.

In his defense, my sloppy language, writing specific heat instead of specific heat capacity, started him down the wrong track.

I think (but I'm not sure) that there is agreement that one should not use AAC for floors. There probably is not agreement on my belief: "just use active storage with water".

Dana's formula use in his 9/6 post is exactly correct. I suggest an experiment if anyone thinks otherwise.

Posted By Alton on 09 Sep 2011 09:00 PM
I have enjoyed this back and forth discussion. But now it is beginning to remind me of the line in the movie City Slickers. Remember the long drawn out discussion about programing a VCR and the retort that stopped it: "The cows know how to use the VCR by now." Now that was a funny line.

The cows maybe, but the oxen still have it upside down!

Alton, you will see in chapter 4 of this UK building code for AAC that the Brits use pretty much all the permutations of wall stackups to add insulation to the block. http://www.aircrete.co.uk/pdfs/guide_bestpractice.pdf

My first reaction was don't mess with the architectural possibilities of a very, very workable building material. Then I reconsidered. You can still get chamfers, chair rails, quoins, pilasters and such, by switching to 12 inch block from 8 inch, or attaching 4 inch block with thinset and screws. At 12 inches, the ornamental parts of the wall are already R15. Add foam board and metal lathe to the flat surfaces in between, and cover in stucco.

Merline Van Dyke an engineer in Lakewood, Colorado has a model he works with capturing the earth temperature.

You need to go somewhat deeper in Colorado.

I am finishing building my first thermal mass home and it is mind blowing how comfortable it is on hot and cold days.

During construction while I was keeping the cement from freezing my heating costs dropped by 70% after adding the thermal mass to the inside with the thermal break to the exterior.

Oak Ridge National Labs has a study of 16 different wall systems and energy efficiency.

http://www.ornl.gov/sci/roofs+walls/research/detailed_papers/thermal/index.html

Check out CONCLUSIONS

(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.)
Just built a roof/sun deck that is R80 with thermal mass (shot crete)  and EPS the thermal break. Clear spans 30' flat or slight pitch for drainage.

Yea we talked about ORNL studies on page 2 or 3. I remember all the studied walls having some sort of exterior insulative shathing (higher than R 1.25 per inch).

This self-appointed internet expert ( more of a intermediate really) just had published, Cost Effective Passive Solar Design in GBA. I think it has the potential to be one of the most popular articles on Passive Solar Design on the web. Just for Todd, I called out AAC and not including thermal mass inside the insulated space. Your welcome.

http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/cost-effective-passive-solar-design

I wish Ribsys would stop pushing Radiant Barriers in every possible situation on this forum. Its really annoying.

Greetings,
QUOTE> I wish Ribsys would stop pushing Radiant Barriers in every possible situation on this forum. Its really annoying.

Ans: Another thing that's annoying is people pushing substandard insulation materials that do not save the energy RBs do and ignore their material's short comings. RBs are suitable for most any insulation project you might have. Do I have to apologize for RBs being so versatile and effective? Too bad your recommend material doesn't meet this high standard.

Whoa, your blog post will be big news to Jim Sargent, who built this passive solar house using R-14 Durisol block http://www.buildingscience.com/documents/case-studies/cs-anderson-sargent-dallas-show-house/view?searchterm=anderson+sargent
Durisol is a hybrid ICF, which like Apex and rastra -- and, yes, AAC -- are self insulating cementitious blocks.
Were Sargent to post on this site he would be in my view an appointed Internet expert , being DOE's Builder of the Year in 2006. Assuming you are not a DOE builder of the year, your opinions are roughly as valuable as those of these folks building AAC passive solar houses in North Carolina: http://www.sungardenhouses.com/

Gotta say tho, my respect for GBA has taken a major hit.

what SCIPS did you use? Any info to post on your project?

Posted By toddm on 29 Sep 2011 05:31 PM
Whoa, your blog post will be big news to Jim Sargent, who built this passive solar house using R-14 Durisol block http://www.buildingscience.com/documents/case-studies/cs-anderson-sargent-dallas-show-house/view?searchterm=anderson+sargent
Durisol is a hybrid ICF, which like Apex and rastra -- and, yes, AAC -- are self insulating cementitious blocks.
Were Sargent to post on this site he would be in my view an appointed Internet expert , being DOE's Builder of the Year in 2006. Assuming you are not a DOE builder of the year, your opinions are roughly as valuable as those of these folks building AAC passive solar houses in North Carolina: http://www.sungardenhouses.com/

Gotta say tho, my respect for GBA has taken a major hit.

It's a kewl house and all, but also in a cooling dominated climate (the benchmark comparison calls out 32% of total energy use as space cooling, 20% as space heating."  "Passive solar" is nowhere mentioned in the referenced document, and is a term usually only applied to heating-dominated situations where excess wintertime solar gain is designed into the structure, and balanced against an internal thermal mass.  There is some wintertime solar tempering designed into the glazing type, but it's no more "passive solar" than any other solar-tempered house- south aspect glazing does not appear to have been oversized and mass added to make up for it.

And most of the usable/useful thermal mass in that building is in the very substantial slab.  R14 Durisol would be isolating the cores of wall-concrete from the space with at least R5, and the thermal mass of the slab probably equals or exceeds that of the walls despite having lower overall area. (We'd need a better view of the overall plan to put a real number on it.)  If R5 is considered negligible isolation from the mass, consider also that R5 is what's under the slab, so the thermal mass of the earth below the slab would have to count too.  The mass in the walls will reduce the peak heat gain through the walls, but won't participate TOO much in buffering the whole house.

Given the density of AAC relative to the concrete in the cores of Durisol, the thermal mass of an R14 Durisol wall will be several times that of an R14 AAC wall, just as an ICF wall  is.  (Again, we'd need dimensions to put a hard number on it.)  Durisol has far more in common with conventional ICF than it does with AAC.

Sungarden also uses concrete slabs as the thermal mass of their true-passive-solar designs.  On their website they state with clarity & accuracy:

"Without a thermal mass, large south windows will cause intense temperature swings and overheating. (AAC and lightweight concrete are not suitable for creating internal thermal mass: they are too light.) "

(Emphasis mine.)

Our "intermediary" insists that it can't be thermal mass if it isn't sheathed in insulation, quoting you, so it doesn't really matter whether the Sargent house is passive solar or solar tempered, does it? The BSC write up says repeatedly that Sargent wanted Durisol for thermal mass. Self insulating is also self explanatory.

As for Sun Gardens, I agree that AAC walls alone will not work. My argument all along is that concrete slab alone won't work on a bright 50 degree January afternoon, with an emphasis on the 50 degrees, in a house with a southern exposure that is 18 percent glass, as mine is. So, I have 50 tons of regular concrete in a slab on grade, 90 tons of AAC block in 2,500SF of walls, 10 tons of mortar in vertical cores and bond beams in those walls, and 6 tons of steel, mostly in w8x21 beams carrying the second floor, plus the usual furnishings. (Trust me, at the end of a work day, there is no such thing as lightweight concrete.)

My additional argument is that, in the relatively few hours that acute overheating occurs, the 2500 SF surface area of additional cementitious thermal mass, plus an air handling system to move heat to it, matter a great deal more than how dense or deep that thermal mass is. I agree with Knight that specific heat capacity means more than specific heat -- apologies again for clumsy language. You will find surprisingly similar numbers for concrete and lightweight concrete, depending on the moisture content of the latter. (Yes, the brits feel compelled to specify the moisture content of the AAC in question.) Again, in the inch or two of AAC wall surface that matters, that is also exposed to 50 percent RH in a small tight house, I expect acceptable results when I need them. I am heating with free wood, so R10 isn't a big hit when I don't need buffering. That said, DBME matters a great deal more to me after it translates into that many fewer weeks of schlepping wood.

I may be wrong, but the windows came first in my homage to Frank Lloyd Wright, and the view from them is spectacular. If I want to live in a 10x10 windowless box and heat it with a candle, I'll give you a call.

Oops, writing too quickly. Make that 25 tons of regular concrete slab and 45 tons of AAC wall.

The terms "specific heat capacity" and "specific heat" have nearly identical definitions and wholly related- the former is an arithmetic function of the latter. The specific heat capacity of an OBJECT is the amount of heat required to raise it 1 degree, whereas the specific heat of a MATERIAL is the amount of heat necessary to raise a standard mass (pound or kg) one degree. To the heat capacity of the object can be calculated from the specific heat(s) of and mass(es) of the material(s).

I'm not sure what means "...it can't be thermal mass if it isn't sheathed in insulation...". I've previoiusly posted statements along the lines that thermal mass of the materials outside the insulation doesn't participate nearly as much as the thermal mass that is fully within the insulation- is that what you were getting at?

The definitions of "solar tempered" and "passive solar" are distinct.

In that BSC document about the Sargent house there is exactly one reference to thermal mass (p.17), where it does mention the Durisol walls in the same sentence as the slab.:

"Thermal mass – the block walls and first floor concrete slab have lots of
mass interior to the insulation. The walls and floor act as a heat sink,
storing heat for later release, smoothing out daily high and low
temperatures during both heating and cooling."

That's it.

The only other reference to mass at all in that document is on p.14:

"– High mass interior and exterior walls help keep
temperature constant"

Note, they're using on the mass of the interior walls as well as a temperature moderator, not just the isolated mass within the exterior walls. The surface area & mass of the interior walls is also significant, and unlike the concrete in the Durisol, isn't thermally isolated. The wallboard on the exterior walls will have a greater effect on peak cooling loads from windows & roof than the isolated concrete.

The interior side of the thinnest Durisol is ~ R4. With R14 Durisol they have the bulk of the insulation on the exterior side, so that mass WILL participate more on average than AAC, but less than AAC for peak gains from windows & roofs, and way less than that of the slab despite far greater surface area. The interior finish gypsum (all walls, including interior walls) have a far greater effect.

You'll get some benefit in peak cooling load reduction from window or ceiling heat flux from the thermal mass of the interior ~2" of AAC- far more than with Durisol which isolates all of the mass (albeit higher mass than AAC) from the interior. But the thermal mass of that interior 2" of AAC is about the same as 3/4" gypsum wallboard- enough to count, but not nearly as much as the rest of the story (such as your 25 tons of slab.) The R value of the AAC being an order of magnitude higher than that of the concrete limits it's ability to participate in the peak loads despite having much more surface area than the more-conductive concrete (or gypsum.)

High-R walls and high glazing factor passive solar are not even remotely at odds with one another. Most PassiveHouse designs in US zones 5 & 6 have a lot of south facing glass & great far more than "conventional" homes in heating dominated climates, but they're also sporting R50-ish walls, and moderate to high interior thermal mass (usually slabs + gypsum) to make best use of the solar gains. eg: http://www.gologichomes.com/featured-projects/passive-house.html

Agreed, aggressive glass and high R walls aren't at odds IN MAINE which is where the go logic house is. Here on the Mason Dixon line, where Jan 15 could be 10 degrees or 50 degrees, it's not quite the same.

What I meant by "can't be thermal mass without insulation" is that you gulled Brian Knight into believing something stupid by talking out of your hat on thermal mass. In fact, the thermal mass outside the insulation is key to the dynamic benefit of massive systems. All you need is sufficient mas so that heat and cold fluxes never reach the inside on a 24-hour basis. Insulation is optional, unless you think Indians in the prehistory Southwest went down to the local Adobe Depot to buy EPS.

Yes, talking out of your hat. From the start:

"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."

Which is why we find adobe houses in Santa Fe rather than Saint Paul, and three-foot thick masonry walls in Athens rather than Oslo? What's more, your statement is a complete butchery of DBMS. That Carlsbad Caverns is a constant 56 degrees has nothing to do with solar load.

"AAC has worked fine in Europe, where they have much lower average dew points than the eastern or southern US."

Yup, its use in Europe is limited to sunny, dry climes -- like Great Britain.

"In Canadian-cool climates I can imagine spalling issues occuring in AAC from interior vapor drives without at least some exterior R factor (&/or interior side vapor retarder) too."

To improve freeze thaw resistance of concrete, one adds foaming agents to entrain air -- unless the concrete is AAC-4, which is already 80 percent air. It floats. Seven days later it still floats. While that was the length of my experiment, my block manufacturer says the blocks can't be completely saturated, even though air eventually replaces the hiydrogen that puffs them up like marshmallows.

"A 2" non-structural slab will have the same thermal storage capacity per square foot of 8" of AAC, but far more passive solar heat storage capacity per square foot than 8" AAC, since it's fully inside the thermal envelope- stored where it counts (indoors) rather than in a progressively lossy wall."

Out here in the real world, my alternatives to a lightweight concrete system were concrete block or 8 inch poured walls. Both flunked my DIY criteria, but the poured wall also flunked my passive solar test. When I pointed out to you that average daily insolation through my windows would raise the temperature of solid concrete walls by .003 degrees, you doubted my ability to measure that temperature change. Exactly my point.

Sargent probably had carbon footprint in mind when he chose durisol over concrete, but he got to the same place.

To posit nonstructural anything reminds me of the baseball tournament at the annual economics convention. The pitcher starts the contest by calling out from his seat, "assume I have thrown a curve ball breaking on the inside corner of the plate for a strike."

"Concrete has only 80% of the thermal mass of AAC, PER POUND."

Umm, a pound of concrete doesn't weigh a pound in MA?

We'll skip the errors in the thread that were addressed at the time.

I admire your knowledge and your willingness to help, as do all of the regulars here. But when folks like Knight take your word as gospel, you owe them a greater duty of care.