I know, it's a mouthful.  first as a long-time lurker, thanks to all posters, regular and irregular.  I'd like like some feedback on two issues:  contemplated residence calls for exceptionally thick a la Mediterranean walls (3').  After casting about, I've settled upon the notion of using essentially an interior block wall and an exterior block wall with a layer of blue/pink foam between the two.  Even with enough foam to give an r-value of oh let's say 60, I've still got a foot to fill.  The fill can be concrete, gravel, sand, what have you.  I believe that the block (core-filled) should be structurally sufficient by itself so the question is what kind of material on the inside of the insulation envelope would provide the most benefit in terms of thermal mass?  (I understand that based on non-exposure to the exterior, I will miss out on the "flywheel" effect, but still, is there is a more or less optimal fill?). 

Second, given essentially a concrete sandwich wall, is there a way to tie the two walls together without creating a thermal bridge through the insulation?

Location of project:  houston, Texas.  for those who want to scratch their head and wonder aloud over design issues, I anticipate receiving no federal funds and my vices are few, I'll spend my money as I please.  Which is to say, I know the numbers. 

But I really am interested in your answers and input.  Thanks very much.

jelston, sounds like the perfect project for the SABS™ is a multi-patented, composite building system that utilizes Expanded Polystyrene (EPS) as the core material for all structural members – walls, roof, floor – that is sprayed with a composite coating made up of a precise blend of sand, cement, glass fiber and other additives that, together, create a building shell that meets or exceeds all testing protocols and load requirements of the ICC-ES. http://strataus.com/

thanks for the link and it looks like an interesting system worth considering.  i would still like to know what the options are for tying two essentially concrete walls together without thermal bridging.  Essentially, if I am getting the jargon correct, the basic structure I am talking about is a composite type wall, so I am thinking of a composite (non-rebar) wall tie.

Solar Crete has one here http://www.solarcrete.com/wall-tie.php.  I have not contacted them yet so I am not sure about price.  I was hoping there might be an alternative, maybe even off the shelf that I have not been able to think of. 

Still wondering about thermal mass material.  Concrete does not sound like a bad choice given structural advantages and at about $70/cu yd (I think what I most recently saw it going for here, it's not too bad.

cheers,
jke

Jelston,

A double block wall with insulation in the middle should be very energy efficient and tight.  However, the cost might be more than you anticipate unless you do the labor.  If the walls are above grade and they are not in a high seismic area, then building an un-reinforced wall can be done.

Labor is not as much an issue: a) I prefer to not go to the gym, my wife does that; b) I have lots of clients who already work off parts of their bills doing work for me (I do lots of family, criminal and bankruptcy law and even if my clients aren't fit, their children (who are late teeners, or in twenties) wind up being available.
But seriously, I was thinking in terms of filling the cores both with concrete and steel. tying them together may simply not be necessary provided the base is sufficiently stable. Houston is not known as a significant seismic zone although there are some faults running through the area and we are built upon a swamp with the mosquitoes to show for it.
My thinking has been based on the fact that most of the icf blocks of the rastra variation are not much more complicated than simply stacking block a la VOBB and filing with steel and concrete (while providing for some horizontal beams). I start veering into three foot wall thickness when I get into approaching superinsulation thicknesses as well as my personal preference for having masonry interiors and exteriors. I love straight stucco on cinder block but block is of course horrible for insulation even if you stuff the cavities (that's just putting earrings on a pig!).
So I get left with the concept of sandwich walls, whether poured, block or what have you, whether they should be tied together (depends how independent they are structurally), how they should be tied together, and what kind of fill ( in addition to the foam board can be used) in the cavity.

jelston, when you say block walls, do you mean an exterior CMU block wall and a separate interior CMU block wall? If that's what you mean I'm not sure you would really have to tie them together at all. The layer of insulation between them would be sufficient. I might not be clear on what you're doing though.

I am sure that i am the one not being clear.  but your assumption is exactly correct.  tying them together became a consideration when i began to think of filling my extra space (thermal mass) with something other than self-supporting concrete on the interior of the insulation envelope.  Solarcrete has those nifty "special polymer alloy" composite wall ties which allegedly create a thermal break between the two concrete skins.  Seems to me that is the same concept i am looking at here.  Whether it is significant enough to worry about, however, is a different issue. 
Secondarily, if the fill material is non-self-supporting, then the question is whether a cmu wall will withstand the lateral load (which is of course an engineering questions for the one who seals the plans.  My question about fill is really whether or not thermal mass advantages are substantially affected by choice of material provided the choices are limited to reasonably dense materials such as concrete, clean dry sand, gravel, river rock, etc. 

Jelston,

Where will you run the electrical wiring?  If I were going to do two walls with Dow Chemical Styrofoam (Blueboard) or that pink board in between then I would consider building the outside wall, attaching the Styrofoam to it, attaching the electrical wire to the Styrofoam with long enough leads to come through the interior wall and then installing the electrical outlets as I layed the inside block wall to create a 1" hollow space between the Styrofoam and the inside block wall.  You can fill both walls with concrete and rebar if needed for mass or strength.  The one inch space would serve as an additional thermal break and also as a rain screen.  Ok, everyone, what have I left out or how dumb is this approach?

By the way, I would second the opinion to use the SABS system.  I belive in your climate the SABS system will outperform the double wall concept.

Unfortunately SABS isn't something that you can just go and put an order in for. SFAIK they are only talking to major builders who are putting in whole new subdivisions or similar large development.

jelston, I assume you're considering the double CMU wall because you want the visual thickness. But if you're just interested in doing that for energy efficiency then I think there are other methods which would work much better.

But assuming it's a style choice, then I'm not sure you would need to add any extra mass to the interior CMU wall (which would be inside the thermal envelope if the space between interior and exterior walls is filled with foam). If you build it according to prescriptive design methods, then you're going to have a solid filled core every 4 feet and at every window and door opening, and solid grouted lintels. That's a lot of concrete right there, not to mention all the concrete in the CMU units themselves (lots of thermal mass).

You might consider instead going with a larger block size rather then the usual 8 inch thick units, and building just one wall and insulating that externally. Just for example, let's say you use a 12 inch thick block, add 4 inches of foam board to the exterior, fur out the interior walls with 4 inch steel studs, and depending what exterior finish you use then you've got approximately a 20 to 26 inch thick wall.

Dunno if you are after the romance of the Mediterranean villa or you want its thermal performance. If it is the latter you should ponder one major climatic difference in Houston. Your relative humidity is much higher. Throwing the windows open at night, as the Greeks do, won't work as well for you. Don't want to discourage you. The combination of insulation and mass works in hot humid climates according to the literature. Just think wet bulb rather than dry bulb.

As for the selection of fill material, specific heat is the yardstick you need. Here is a partial list: http://www.engineeringtoolbox.com/specific-heat-solids-d_154.html You'll see that most of the materials you mention are clustered around .2 btu per lb per degree F (Water=1 btu/lb/degreeF)

and essentially i guess i am thinking of the outer cmu as being the exterior finish so that a stucco can be applied. here in houston, there have been lots of nightmare stories of synthetic (!) stucco applied over the foam board (BTW, stucco will be the exterior as well as the interior finish). the possibility of basically furring out is still being kicked around in the thing i call a head, i just like the concept of the composite walls so much.

but i think your point, gently made is that given a certain degree of insulation, thermal mass within the envelope is largely unnecessary. and since in houston where we have 10 months of summer and two months of he!!, staying inside the envelope is important.

i was planning on running the wiring probably in conduit in the cavity in the extra space i will have even after allowing for block and insulation.  I've a wired a couple of block houses (prior lifetime) and it just was not that difficult.  Additionally, I'm allowing for crawl space so the access will be relatively simple. 

I would appreciate if you would educate me on the term "rain screen" and also the reason for location on the interior side of the foam board insulation.  I thought that the blue/pink board while not water proof was fairly impermeable.  Of course, block and concrete are.  Thanks in advance. JKE

Jelston,

A rain screen is just the open space behind the facade for any penetrating water to use to fall to the ground.  It works best when air can flow in the open space to dry out the material from the backside.  However, in your case the 1" air space between the two courses of block would not have any openings for air flow.  My concern is that concrete block is very porous and water driven by hurricane has been known to pentrate block.  I primarily suggested that the foam board be attached to the inside face of the outer wall because it would act as a retarder for the penetrating water and also it would be more continuous.  If it was on the outer face of the inner wall, then there would be holes in it at every duplex outlet and light switch.  No insulation for several square feet when you consider all of the boxes on the exterior walls.

I would be almost willing to design your home for free if you did not use this system.  I think this system would have more disadvantages than advantages.  I believe you will get a much stronger wall with reinforced concrete.  I may be wrong, but I believe that a 3" polyurethane metal SIP wall would be more efficient without ever having any hint of odor from moisture in the block.  I also like the idea of a light-weight wall in hot-humid regions.

toddm:  thanks for the link, I'm no engineer but i'll go there and do a little self-learning.

Houston has a significant number of temperate days but you are absolutely right, it's not a throw upon the windows sort of place if the ac is efficient and affordable.  my thoughts on mass are along the lines of why i kept the fridge stocked to the gills with beer when i was in college, it's cheaper to keep cool a full fridge than an empty one (there may have been other reasons for the beer but i can't remember)

Alton's rain screen brings to mind an elegant old answer in the tropics. Vented top and bottom and separated by an air gap, an outer masonry wall creates a chimney effect as the sun beats on it, venting much of heat load before it warms the inner wall. Of course, foam board at R-60 works better.

At which point, jelston, your mass walls are probably more nuisance than help. R-60 effectively dispenses with thermal lag, so your walls won't be averaging anything. The walls will dilute internal heat load, but when it comes time to flush it, it's the same number of btus. Could be your refrigerator analogy holds and HVAC runs infrequently in long, efficent cycles. But you wouldn't need anything close to 18 inches of solid wall to get this effect, and too much mass may require the opposite approach: slow and continuous conditioning to match the wall's reluctance to give up its heat.

This approach would give you superinsulation in wide walls with a touch of mass: Reinforced masonry wall; air gap; foam board; low density concrete panels. You can parge stucco on both sides.

In hot humid environments (I'm in Louisiana, so my climate is very similar to Houston), you would indeed benefit from thermal mass on the *inside* of the insulation envelope.

As a fellow Houstonian (for a few weeks a year) and a Med resident (Barcelona for 7 years and  now Milano), I am curious why you are motivated to use 3' walls?  Aesthetics?  Energy?

Bruce

only aesthetics, and once you get to 6 inches of foam (r-30; another 3-6 inches doesn't add too much).  Everyone seems to think of superinsulation just for the north to keep the heat in but of course here in Houston we're trying to keep the heat out. 

And let's keep in mind that walls that thick get to have lots of other stuff from wires to pipes to duct which will degrade the r-value some but not too much. 

but mostly the wife wants thick walls.

jeltson,
Have you thought about going w/ an icf product for your application. It would seem a pretty easy straight forward application for the systems. Achieving a core of 3' is very doable and insulated on both sides to contain temps.
Laying all these cmu's seems time and labour consuming.
Cheers!

True enough, Jelly, but how much mass is too much? I have to say I haven't found a satisfactory answer. Arithmetically, there can never be too much, because whatever you add will do that much more to temper extraneous weather events. Even so, from what I've read, heat or cold rarely penetrate deeper than the first four or five inches of the mass structure in the classic 24-hour cycle, although the heat flow in the wall continues to seek equilibrium. With R-60 in Jelston's walls, there won't be much flow from the outside. It seems to me he won't be adding much to the inside either in a superinsulated house. So what would 18 inches do that four inches would not? R-60 is the rub here. That much foam pretty much covers it.