Save 50% of what?

Obviously you save 100% for the 8 hour period that the AC is turned off. But then later you have some thermal mass that you need to cool down.

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??

Not much at all if you have little mass (ie, 0 with zero mass, but that never happens). And let's not forget that most people don't have variable speed compressors. So they can safely ignore the COP vs output argument and save 10-15% per year with setbacks.

a setback strategy costs more than keeping it at constant temp

So one should leave those inverter mini-splits on normal temp when on vacation? No, the right answer is it depends on times, temps, mass, etc.

If your vacation is only 8 hours, yeah, leave it on, eh? :-)

If thermal mass that's cooled down with a COP of 3 could have been kept cool with an average COP 8 it's hard to make the case. Unless the house is getting SO hot that the gain is dramatically lower (the zero mass reductio ad absurdum) it's a losing proposition to use setbacks. There's no such thing as a zero mass house (or anything even close), or even zero-mass air (especially when there's a latent load). I'm still buying the mini-split performance as tested by Ecotope. YMMV ;-) It's the very high part-load efficiency that makes them so dramatically more efficient than traditional AC systems, not their full-on COP.

I think we'd already agreed that single-speed systems will benefit from setback approaches. With dual speed systems it kinda depends...

To answer Hugh's question: NO. Not enough to bother with anyway. Your cooling dominated climate will not benefit enough to justify any added costs of additional thermal mass. If someone has a study that says otherwise, bring it forth. This is why most energy modeling software gives it little to no value.

The only place Thermal Mass might make economical sense is the high and dry, desert southwest where it always gets much cooler at night. In your climate, heat is only going in one direction: Usually Outside to Inside. Take any additional money you would spend on thermal mass and put it into air sealing and insulation. You said it ...Air sealing to control the humidity is the best strategy along with better insulation which is the problem with ACC and Rastra.

AAC block of 9" is barely over R9. Does that even meet the code minimum in your area? AAC is great for durability, not so much for saving energy in most US climates. I hope everyone can agree that its possible to have both.

Posted By Springtime on 24 Aug 2011 05:58 PM
To answer Hugh's question: NO. Not enough to bother with anyway. Your cooling dominated climate will not benefit enough to justify any added costs of additional thermal mass. If someone has a study that says otherwise, bring it forth. This is why most energy modeling software gives it little to no value.

The only place Thermal Mass might make economical sense is the high and dry, desert southwest where it always gets much cooler at night. In your climate, heat is only going in one direction: Usually Outside to Inside. Take any additional money you would spend on thermal mass and put it into air sealing and insulation. You said it ...Air sealing to control the humidity is the best strategy along with better insulation which is the problem with ACC and Rastra.

AAC block of 9" is barely over R9. Does that even meet the code minimum in your area? AAC is great for durability, not so much for saving energy in most US climates. I hope everyone can agree that its possible to have both.

The R9 of  9" AAC  does in fact meet code in much of US Zone 2.  Typical 2x4 stick-built with R13 batts has a comparable U-value to 9" AAC at 75F mean temp, but underperforms AAC during hot weather due to higher radiant heat transfer through low density fiberglass- a far bigger factor than mere mass.

I think we're on the same page in re the costs of higher mass construction vs. higher-R construction.  Yes, mass can help, but that alone is not sufficient reason for going there, and it's no substitute for insulation.  ICF is a fine way to build, with many many benefits, but the thermal benefits of that masse aren't enough to spend the money.  Where raw thermal performance is the design goal there's better bang/buck.

Just for kicks, let's say a house has drywall all over (~3 tons) and not much else. You turn the 3 ton AC off for 8 hours and you save 6 hours of run time (assume it averages 75% duty cycle during typical hot weather with no occupants). You come home and there is about 36,000 btu to be extracted from the +20F drywall over the course of the evening. That's 6 hours run time vs 1 extra hr. Even with a 3 vs 8 COP (bogus figures *), you are still way ahead. You can add in air (almost nothing) and furniture, but I suspect that 8 hour turn off still looks good. Extremely so with a typical single stage heat pump.

Take a well insulated house that runs with 2 tons of inverter AC and a short 3 hour turn off and it doesn't look as good. Same for a house with 150,000 btu/20F stored in the walls and floors.

That's right - a high mass house could eliminate 100% of the gains from setback.

* - The key thing to note about Ecotope's claims of huge efficiency improvements with inverters at partial load comes the fact that they don't account for latent loads - "Note only sensible cooling was evaluated". The important effect on COP is delta-T (which makes cooling a hot house *more* efficient).

How about Thermal Mass in a Heating climate of a Passive Solar Design(Air-tight and continuously insulated of course)? My feeling based on Energy Modeling and personal experience is that there is not much help there either.

A big factor in that is how much over and under heating you can tolerate. If you want 72F all the time, then the internal passive storage is pretty much worthless. Active storage is a completely different (and better) thing.

Too bad Thermal mass isnt very good at de humidification.

Just read proposed ASHRAE 189 Minimum performance standard for Green High Performance buildings (low rise residential exempt) and Zone 2 is calling for an increase in Minimum R for Walls to 25. I would feel bad about not meeting the Energy minimum that ASHRAE and LEED calls for and labeling it Green. Is LEED finally getting tough on energy?

Surely AAC proponents have a way around this? Most ICFs and SIPs used in Tx dont achieve actual R25 either.

Posted By Springtime on 25 Aug 2011 03:15 PM
Just read proposed ASHRAE 189 Minimum performance standard for Green High Performance buildings (low rise residential exempt) and Zone 2 is calling for an increase in Minimum R for Walls to 25. I would feel bad about not meeting the Energy minimum that ASHRAE and LEED calls for and labeling it Green. Is LEED finally getting tough on energy?

Surely AAC proponents have a way around this? Most ICFs and SIPs used in Tx dont achieve actual R25 either.

Not even the Building Science Corp folks see an economic argument for R25 (whole-wall R) in zone-2, even though they advocate R50 roofs there:

http://www.buildingscience.com/docu...mate-zones

(See table 2, page 10.)

Minimalist ICFs run ~R16 ish, which would meet the nominal R15 BSC wall recommendation for zone 2, but the up-charge for R20 isn't huge.

For the difference in energy savings between R15 and R25 walls in zone 2  it may even be more beneficial to spend the money on grid-tied photovoltaics. (Especially at the current level of subsidy for the latter.)

An 8" AAC is an R10 wall, give or take, just like 16" o.c. 2x4 timber framed with R11-R13 batts.  I wouldn't be calling that particularly green anywhere even if it meets code-min.  Either 10" or 12" AAC might be more interesting/useful for moderate-R residential construction in zones 3 or lower.  The theramal benefits of mass buy far less than presented by the marketeers cherry-picking data, but it really comes down to cost/benefit comparisons of thermal & structural aspects. I've never priced AAC for any project, don't know what it is relative to stick-built or ICF.  I suspect the real market is as a replacement for CMU construction, where the thermal differences are much more distinct.

Mass requires a different mindset than insulation because its benefit is calculated over 365 days rather than extremes of summer or winter. While Brian Knight is right that temps flow in one direction, and the wrong one, at extremes, there are many more weeks in the year that they flow both ways and save energy. Modeling my house design in low- and high mass configurations in UCLA's HEED, the high mass version extended the period of "No HVAC required" by seven weeks. There were many more days when the mass-buffered temp limited HVAC runs even if they couldn't be considered comfortable alone. That's why ORNL developed multipliers for mass walls in the 1980s so consumers could compare the relative benefits of insulation and mass as apples and apples.

The manufacturer of my block, which are nominal R10, estimated that they would be the equivalent of R17 in the Washington DC climate after accounting for mass effects. They are using ORNL's coefficients for that climate so I'm not sure I'd decribe the numbers as cherry picking. What's more, it is perfectly kosher to effective R values in RESCHECK, the modeling software that enforces the energy code model. I did so to get my building permit. Pa now requires R19 walls after adopting 2009 IRC, so approval today would require 10 " AAC blocks rather than my 8 inch. 12 inch blocks (nominal R15) would pass Knight's Ashrae muster in many locales, but it would be a squeaker where I live. (I have no idea where Knight buys 9 inch blocks.)

But I share his disappointment that mass can't deal with humidity problems. Too bad super insulation can't either. In the dark side of energy efficiency, green homeowners of all stripes are asking themselves, "OK, now that the airconditioner barely runs, what we do about humidity?" Super tight, contrary to Knight's musings, is not an answer. Nor is an ERV. By my math, the minimum required rate of .35 air changes per hour in my house, given 80 degrees and 80 percent RH outside, would introduce 10 cups of water per hour, which would keep a large dehumidifier running full time. The ERV handling exhaust in my baths and kitchen, meanwhile, would be ADDING humidity to this airstream at some point in a long hot shower. Cooking. Ingress and egress. The dishwasher. Laundry. All of this has me shopping for a Daikin Quaternity mini split, which can dehumidify without adding or subtracting from indoor temps. How nice it would be to mitigate humidity by drawing in dry high-desert air. Super insulators take note: Durango, Colo., is where you should be. With a nod to Dana, Texas won't do until you are well past the 80th meridian.

I am not sure whether Knight's problem with passive solar high mass is the solar part, the mass part or both. I've made no bones about the fact that passive solar is a tossup in gray Pa, except for the record January day, reported in Climate Consultant, that insolation would have poured 40k btu/hr through my windows. On that day you'd want lots and lots of mass.

At any rate, the windows came first in my quest to realize Frank Lloyd Wright's claim,, 80 years later, that the unwashed masses could commit fine architecture if they were willing to work for it. The mass came second, and the AAC came third, as the most DIYable of concrete systems. (FLW's last mass marke offering, in the 1950s, was a drystack cmu that anticipated ICF.) So, no, I didn't choose AAC as the biggest efficiency bang for the buck. Does anyone build a house that way? I am delighted to say, with the exterior 99 percent done, that AAC was the perfect choice for my vision. Yours will vary.

Jonr, you don't explain in your recovery math, why a high mass house would warm at 4 times the rate of the low mass house (i.e. gain 150kbtu in 8 hours vs. 36k btu for the dry wall special.) Perhaps you meant to say that inhabitants would find the mass house 5 degrees warmer than they left it, vs 20 degrees for the low mass? Is there perhaps a disconnect in your logic? On the one hand you insist that passive solar is unacceptable if temps stray in the least from 72 degrees as a constant, and yet, in your fallacious knock on mass as setback interruptus, it is perfectly acceptable for Texans to come home to a 92-degree house.

That's not at all what I said and yes, the high mass house will tend to not warm up much - which is why setback doesn't work so well (or not at all $ wise) with high mass. And of course setback is automated to bring the temp back down before one gets home.

Now that's a plan, jonr. Shut off the AC at 7 am when it is working close to its peak efficiency for the day; fire it up full blast at 2 pm to labor through the day's worst conditions. (From personal experience, I know that recovery on a 100 degree day would take hours in what would now be considered a code min house with a properly sized single stage AC.) If only there were a way to delay the heat load on the house.

Wait. There is. Let's say we shut off the AC at noon and fire it up again at 8 p.m. near dusk. With an 8 hour thermal lag on 8 inch AAC, we'd be starting five hours before peak impact. Let's say indoor temps would swing by five degrees, so we turn down the thermostat by 2.5 degrees to set the range at target plus or minus 2.5 degrees. Adding more mass, of course, would tighten the range and allow for later startups. Is it possible that mass can improve setback savings?

Posted By toddm on 26 Aug 2011 09:58 AM
Now that's a plan, jonr. Shut off the AC at 7 am when it is working close to its peak efficiency for the day; fire it up full blast at 2 pm to labor through the day's worst conditions. (From personal experience, I know that recovery on a 100 degree day would take hours in what would now be considered a code min house with a properly sized single stage AC.) If only there were a way to delay the heat load on the house.

Wait. There is. Let's say we shut off the AC at noon and fire it up again at 8 p.m. near dusk. With an 8 hour thermal lag on 8 inch AAC, we'd be starting five hours before peak impact. Let's say indoor temps would swing by five degrees, so we turn down the thermostat by 2.5 degrees to set the range at target plus or minus 2.5 degrees. Adding more mass, of course, would tighten the range and allow for later startups. Is it possible that mass can improve setback savings?

But I thought he was talking only the near-zero mass zero-R tent paradigm, no?

Even code-min homes aren't mass-free, and yes, delayed peaks due to mass effects can and DO improve net efficiency- particularly (but not exclusively) with variable or multi-speed compressors.  With more mass an higher-R pre-cooling can also boost net efficiency.

BTW: To hit R17-equivalence  for R10AAC I suspect the thermal mass of the 2x4 straw-man is below what people are actually building.  And even then, cost/benefit comparisons between AAC vs. another R17 (say, 2x4 + 1.5" of XPS) would be useful, just as in the ICF case.

Me thinks someone is having a hard time reading the latter part of the parenthetical "with zero mass, but that never happens". Calculations were done with 3 tons of mass and common R values.

If you are determined to do passive interior high mass with air source AC, then you need to forget about the savings from setback (the simple 'don't cool the house when you aren't there' idea) and yes, look at set-forward - ie, get some extra capacity, cool down further than normal in the morning and coast through the hot part of the day. You might find the temp and humidity swings tolerable.

Hey, that tent has SOME mass- it even has some R-value! ;-)

While mass has it's advantages, putting the bulk of the mass directly in the walls rather than fully inside the thermal envelope has no particular advantage. Stick framed homes with gypcrete/concrete radiant slabs or slab-on-grade construction have substantial interior mass, but there's no "equivalent-R" marketing hype around that approach. While it's possible to design ultra-low mass homes, it's not how people actually build these days, at least not most custom-built-comfort or energy-conscious homebuilding. I suppose you could make the case for offsetting the cost marble staircase and the granite countertops in a SIP structure on energy use...

In the cooling climate, if the standard code min is R30 batts between joists in the attic, I'd wonder how R13-cellulose code-min wall + R50-cellulose attic & cool-roof shingles stacks up in a cost/benefit of AAC + the straw-man's R30 attic w/standard mid-color composition shingles. I suspect the high-R low-gain roof would be better bang/buck, on annual energy use in most designs.

Factored into the ORNL "typical" straw man model stick built vs. AAC is a much higher infiltration rate than the typ used for AAC, based on 1970s vintage construction. It's like anything, it's possible to build that way, but it's not how it has to be done. I'd hazard typical 2011 stick-built code-min, even with poor implementation of air-barrier systems is much better than the model used.

ORNL's mass work is walls only, which are a relatively small part of overall heat loss/gain, so, yes, modern construction techniques could skew its multipliers. But AAC is quite demonstrably mass. (Hefted every block): In my house, 60 tons counting stucco vs 25 tons for slab on grade and maybe 25 tons for finish/furnishings. Better, the upstairs walls represent heat store well removed from the concrete in direct sunlight.

And the rest of the house is well above code: R60 attic, R25 foundation and super tight. (4 inches of xps stuffed between the ceiling joists, caulked or foamed top middle and bottom; two penetrations; bathroom/kitchen exhaust through ERV; unvented condensing dryer; a "sunlock" walkway that lets hot air into the house on ingress and egress.)

My DIY experience isn't overly helpful on cost benefit. 8 inch block a year ago was ~$2.40 SF -- $2.80 after shipping from Florida. Figure $4 for material after adding steel and mortar. I hired a moonlighting mason and acted as his helper so my $3 labor cost isn't realistic. Stucco cost $4.50 SF. Plan B was a site-built SIP: 2x6 OVE framing with xps stuffed between the studs and as sheathing, with the walls sealed with PU spray. The material cost would have been roughly equivalent but labor less because I could have DIYed everything but the foam. HEED liked this approach much better, but my aux heat is firewood, which falls from the sky around here, so operating costs would not have changed. Overheating would have been a bigger worry.

But stud wall construction would not be my house because I don't trust EIFS over wood. When it comes to architectural detail, AAC is EIFS on steroids with none of its baggage. My homage to FLW is prairie style with Japanese flourishes. I still look at the thing and say Wow. Feel free to say ugh. It's my house. Yours should be different.

I think Todd has legitimate reasons for choosing AAC but I have to question how the Thermal Mass properties increases an R10 block to R17? I would consider any information brought to light by ORNL, but I cant find the multiplier or study that this is mentioned in. It seems that all the work by ORNL's studies of thermal mass in "walls only" look at walls that have at least one layer of insulation and I cant find any whole or clear wall R value calcs of AAC. I would guess that any benefit gained by the mass wouldnt overcome the bridging caused by the grout/mortar connections based on their other studies but hopefully AAC folks keep these losses on the low end of the penalty range. Thinner grout lines/less conductive mortar?

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

I have been educated by this thread so thank you all. The take away I get from this particular study (if its the one mentioned by ToddM) is that if you live near Phoenix or Bakersfield it might be advantageous to spend additional money on thermal mass (inside an insulated envelope). ORNLs work BTW is what has led to my current, favorite wall system of SIPS, preferably made with polyurethane foam. I think much of their research supports SIPs across most US climate zones.

I dont really have anything against Thermal mass I just cant find much convincing research to make me want to spend extra money on it. I have spent extra money on it in the past for a Passive Solar Home but could justify it by getting beautiful interior stone work as an aesthetic benefit. I have often wrestled with upgrading to concrete floors on upper levels but outside of Radiant heating purposes, I dont think they work enough to justify the cost or hassle(but I love the look!). I have been in three recently completed high mass, passive solar homes in our climate with decent envelopes that were hot and very uncomfortable due to the humid air and lack of air conditioning.

Insulation does not dehumidify but it does a much better job of keeping hot air out and in, as long as its airtight. R10 would represent a thermal bridge in most well insulated envelopes.

As Iam sure ToddM will have a worthy reply, do you mind mentioning how you detailed your window headers? I understand that this can be a problem with AAC walls without drainage planes. Sounds like you have some healthy overhangs for one.. Also, you seem to imply that you didnt install a way of cooling with dehumidification? Didnt catch your climate either..

I don't want to pretend that like I have all the answers. What I can say today is that AAC is a designer's dream and that it is DIYable without special equipment (unlike SIPs.) I did the windows two ways. The 40 foot stretch of glass is post and beam bolted to a AAC knee wall. The windows are caulked and flashed as per Andersen's instructions. The second way is AAC pilasters and entablatures covering pressure treated bucks. Now water does cling to the bottom of bump-outs like these and runs down the glass anyway unless you cut a rain channel on the underside. A diamond blade width is sufficient to get the water to fall away. EIFS has this problem also, and rain channels are SOP. Hebel did this thermal study of AAC under ORNL oversight in 1999: http://www.safecrete.com/products/techmanual/pdf/thermal.pdf You will see a nominal R value of 8.3 and a equivalent R value for Washington DC of 14. I am assuming that the nominal value accounts for the thermal bridges of the mortar (minor at an eighth inch in thickness) and 3-inch concrete vertical cores every four feet plus horizontal bond beams at ceiling heights, and , in my case, steel post and beam carrying the second floor. Can I make up the difference in super super tight construction? Will passive solar work in south central pa in december? Stay tuned.

The windows