A modest, single level home in San Diego, slab on grade exposed as the flooring with radiant heating.

To increase the internal thermal mass, i was thinking of using either CMU or concrete walls for the exterior walls with stucco inside and out. Insulation outside and Passive Solar design considerations taken into account. Wood framed partitions with sheet rock for all internal walls.

But let's say wood framing was used for the entire structure, is there a way to add thermal mass internally that would approximate the level of thermal mass provided by concrete walls?

Would a concrete floor (exposed slab) throughout with a tight envelope (high R-value wood framed walls, low u-factor windows) provide sufficient thermal mass to provide that thermal lag that keeps the home cooler in the summer and warmer in the winter?

There's no cheap way for a framed wall to have the same thermal mass as a concrete wall, but it may not matter if you design the house for low summertime thermal gain.

A CMU wall with as little as 2" of exterior EPS would meet code minimums in San Diego, but something like 4" might still be financially rational, since that's enough to hit Net Zero. In your climate the approximate "whole assembly-R" values (average performance after factoring in the thermal bridging of the framing) to be able to hit Net Zero Energy with an array that fits on the roof are roughly:

Walls: R20 (framed), R16 (mass wall)

eg: 2x6/R20 +1" to 1.5" EPS insulating sheathing (or 1.5" Huber ZIP-R) , or CMU wall with 4" of EPS on the exterior.

Attic/roof: R50

Basement/crawlspace wall: R10 (continuous)
Slab edge (slab on grade): R8 (to 2' below grade)

Sub-slab: R4 (non-radiant), R12 (radiant heating)

Windows: U0.30, SHGC 0.3.

You may be able to get better thermal performance out of slab by utilizing the thermal mass of the subsoil. Rather than simply insulating the slab edge with a grade beam, going with deeper with an insulated footing, and skipping the sub-slab foam. This has some consequences for the radiant heating, and should be considered carefully before going that route.

Window placement and sizing need to be considered, but going big on the south wall for wintertime gains is going to overheat a higher-R house in mid-day. Limit or eliminate west facing glass to avoid late-in-the-day summertime gains, which can drive peak air conditioning loads skyward. North facing windows are great for shadow-free daylighting, and south facing glass should be designed with roof/porch overhangs that reduce the summertime gain, while still allowing some winter gains. Resist the temptation to increase the south window sizing, unless you've simulated the house very carefully with good design tools.

A house like that can be heated/cooled with high efficiency with a ductless (or ducted, if the ducts are all inside the insulation) mini-split, and the annual power (all uses) could be covered by rooftop PV. While heating with a radiant floor adds a bit of extra-cushy comfort when heating was needed, the higher performance house shortens the heating season by at least a couple of months, and the peak heating loads are low enough that the floor temps are barely above the room temp. In your climate you'll have days with cooling loads even in winter, most of which might be addressable with a night-time ventilation strategy (a strategy that may work through much of the summer).

For a little light reading- download a copy of BA-1005, and read at least the first chapter:

http://buildingscience.com/documents/bareports/ba-1005-building-america-high-r-value-high-performance-residential-buildings-all-climate-zones/view (note Table 2, p.10. You're in zone 3B.)

With ultra-light heating and cooling loads mini-splits are usually the best option, since they're cheap, modulate over a wide range, and can be easily sized correctly for the low loads. It's also worth thinking about using a heat pump water heater for the domestic hot water, which takes a small bit off the cooling load. In a higher-R house your annual cooling energy use will exceed the heating energy use in your climate, even though it's a heating dominated climate for older lower-R houses.

IIRC beginning in 2020 all new houses built in CA will have hit Net Zero Energy. Fortunately that's neither a big cost-adder or difficult to design in your climate, far easier than up in ski country in the middle & northern part of the state.

Note that thermal mass is relative to the BTU's needed. Ie, with a well insulated house with moderate outdoor temps, drywall alone offers significant hours of free heating/cooling. Adding more has diminishing returns. The effectiveness of internal thermal mass is also highly dependent on how much variation you can tolerate in the interior temperature - keep the interior right at 72F and there is practically no benefit to any amount of internal thermal mass. Nor is there any benefit to thermal mass when the outdoor temp is a steady high or low (excepting solar gains - that works at steady cold outdoor temps).

Thanks for all the information.

Just wondering though, in seismic prone areas such as SD, when using CMU for 9' walls or so, i assume rebar is required with all cavities filled with concrete. Is that right?

If so, depending on the site's restrictions, would 8" poured concrete walls with rebar be more economical?

The rules related to seismic resilience are all extremely local. Consult your local building department for what's required where you live.

ok thanks.

I had a project back in south carolina where the client built a 2x6 framed walls, OSB sheathing, 3" of staggered blue board sheathing all seems taped, then 1x4 strips. He wanted 2.5" of stucco for thermal mass. But it was on outside. The stucco sat on a brick ledge so the wall wasn't supporting the weight hanging over the slab. I have no pictures as it was 10 years ago. Might work out, he was happy I know.

I expect that his external thermal mass had very little or no beneficial effect.

I honestly couldn't tell you. I'm no expert on the matter. I understand what thermal mass is and why it's desired.

To the OP, maybe instead of trying to do something different, would ICF work for you instead? You'll have the thermal mass between 2 layers of insulation. You could probably stack these yourself and get an experienced concrete contractor who can brace the walls and pour it for you.

In some climates extra thermal mass on the exterior increases total energy use slightly. The greatest advantage is when the thermal mass is mostly but not completely inside the insulation layers. When 3/4 of the R is on the exterior and 1/4 on the interior the temperature of the thermal mass will swing over the course of the day, averaging out the load of the interior space, but lowering the peaks dramatically. If the interior of the house is locked in to an extremely narrow temperature band by the HVAC the interior thermal mass completely inside the insulation boundary of the house is nearly useless, but thermal mass part way into the insulation value can still swing in temperature.

In San Diego if the interior temps are allowed to swing a handful of degrees over the course of the day, the thermal mass of the first foot or so of soil below the slab is useful, since it has both thermal mass and some R value. It'll soak up some of the daytime peak gains, and deliver it back at night with reasonably appropriate time delays, limiting the temperature swings inside the house.

If you're in the right part of San Diego (relative to the coast versus inland), it's one of the few places where you could successfully design a comfortable home without any HVAC. The right solar orientation, window placement, insulation, and you're there. I like the OP's plan of CMU/concrete exterior, but with San Diego county's termite and fire issues why not steel framing for interior partitions?

I agree that if you look at just a standalone wall (or don't allow any internal temp variation), thermal mass somewhere in the middle (like ICF) is effective. But consider that you want this wall to make up for other heat gains/losses (air infiltration, windows, ceiling) and the interior side insulation restricts the needed heat transfer (convective and radiant). Same issue for "charging" the thermal mass. You can blow cool air through a house all night and not move many BTUs out of an ICF core.

http://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-thermal-mass

While I'm not convinced that it is cost effective (consider just adding more insulation), if I wanted even more passive thermal mass in a slab on grade house, I'd consider plastered CMU (dry stack if allowed, maybe pea gravel filled) interior walls. Both sides are then exposed for maximum heat transfer (ie, more effective than the same CMU used in an insulated external wall).

I'd consider a smart, active ventilation system to optimize outside air to thermal mass heat transfer. Open/close/open the windows gets old and relying on natural air movement limits BTU transfer. FWIW, I spent 3 months of this year in a very hot days, moderate nights, no AC, SCIP (concrete on both sides) house. Every degree counted!

While in use, radiant floor heating removes the thermal mass effectiveness of a slab.

I'm actually further inland in a valley. AC is a necessity, at least if future summers are going to be like the last. Thermal Mass would be a means to reduce energy use, not as key component in Passive Solar design.

Steel studs for the interior though certainly makes sense. Thanks.

My understanding is that if the insulation is on the inside of the wall, any effect of the thermal mass is prevented. And this certainly makes sense in cold climates where the sun is allowed to penetrate the internal thermal mass so that the heat will radiate back into the space.

I intend to stucco the inside of the walls as well, but imagine that would have little thermal mass influence.

The scientific models showed that ICF is really the perfect environment for thermal mass. The online scientist (Sailaway ?) that frequents here did over a year of temp studies and models in her ICF home (Zone 4 or 5). It showed that the ICF performed from R23 up to R70 depending on the time of the year.

When the insulation was put all on the outside wall, the R-Values dropped. The ICF sandwhich of 2.5" EPS x 6" concrete x 2.5" EPS showed to be the perfect environment for thermal mass and performance.

Having concrete mass without any insulation to the interior of the house is like living with and elephant. Some times he your friend, most times he stepping on your toes.

The "effective R value" of the wall itself is less important than its ability to transfer heat to/from the room as it gets too cold/warm. That's why fully internal concrete walls and slabs (zero beneficial R value) reduce energy bills.

ORNL looked at whole building energy performance and found: "Data presented in these tables show that the most effective wall assemblies are those in which thermal mass (concrete) remains in good contact with the interior of the building ... Wall configurations with a concrete wall core and insulation placed on both sides of the concrete have DBMS values that fall in the midrange".

So many conflicting opinions as to where the insulation goes with a thermal mass building (house) in a semi-arid environment as San Diego:

- on the inside of the exterior wall relegating the function of thermal lag to the outside of the home (but then what would the difference be in just having framed walls with high R-values blocking all heat energy from entering?)

- sandwiched between 2 layers of insulation as an ICF (again, what's the point? why not have that space filled with insulation OR does the thermal mass play a role in mediating temperature in an indirect way?

- on the outside so that ample thermal mass remains in contact with the internal environment of the home



Maybe the question then is: Does the Thermal Mass play a direct role in mediating temperature as in Passive Solar Design and therefore all TM belongs on the inside (insulation on the outside) OR an indirect role where there is some benefit as to using less energy but again, maybe not much more than tight, high R-value framed walls filled with insulation.

No conflicting opinions. Only people on this site who have forms to sell. I believe it was 2008 when I first trotted out these studies which are well known to Lbear. This is Canada's housing agency saying "...this study could not attribute any effective insulation value that is higher than the nominal insulation values of the polystyrene layers...." The test apartment building in E
waterloo Ont had sensors embedded in the concrete core of ICF walls. http://www.cmhc-schl.gc.ca/odpub/pdf/65863.pdf

Here is Gary Reysa at Builditsolar.com writing about an ORNL study I must have cited 20 times. http://www.builditsolar.com/Projects/SolarHomes/ICFBotLine.htm It shows almost no advantage of internal mass over ICF in Minneapolis vs a huge advantage in Phoenix. No mystery. The dynamic part of dbms refers to its ability to cancel out daily swings in temperature. It works best where is a huge swing between daily highs and daily lows. Dry climates are favorable as well because mass can do little about humidity. The daily range must swing above AND below what's considered comfortable. In Waterloo Ont, heat this time of year is flowing in only one direction -- out. Limiting the benefit to 24-hour cycles, it makes perfect sense that putting the mass behind 2.5 inches of EPS hurts performance rather than help it. In short, ignore Lbear.

The daily range must swing above AND below what's considered comfortable.

Based on personal experience (and theory), this can be OR. Say the comfort range is 65F to 75F. Internal thermal mass works even when it's 90F all day and only goes down to 70F at night. But this requires opening the windows and moving air through the house all night (trying to get the concrete close to 70F). Then closing the place up all day. Ie, active management of the internal thermal mass. With ICF in this scenario - go buy an air conditioner.

For the Waterloo study, note that their lack of results only applies to Winter: "Dec. 1, 2005 to Feb. 28, 2006".

Not surprisingly, the Waterloo (student housing??) project found exactly what they where measuring. They found the concrete did not provide insulation and that the foam did. They found that heat patterns did not reverse and who would expect it to when you have a constant heat source on the interior. This does not mean at all that there is not dbms value to ICF. They did not proof that ICF is not more effective then wood frame models. They did not proof anything other then ICF is a very air tight building model. They did not even compare total energy used. Without measuring how much energy was required to match exterior temperature swings, how can they say there is no benefit.

And still the Waterloo studies is not flawed like the ORNL study which is worse then totally useless. One house had a swimming pool for a basement while the other was dry. As I have mention many (probably not 20) times this study was a nightmare and Kosny etal have no business calling themselves scientist. Even worse they then used this flawed info to model the other cities. It's best to ignore ORNL studies.

As I live in an ICF house that was properly modelled for sizing heating equipment using R-50 for the ICF walls and I deal with the results every day, I have much more faith in the performance of my house then in any improperly modelled research paper. For more then five years now, my heating equipment has never run more then 75% of the time even when sustaining multi day patterns at close to and under design temperature of -30 C even though it is sized 5% under the recommended. Further more there are other ICF houses in southern Alberta that show the same results.

Any one can quote studies but few tend to look at them carefully. They just find the tidbits they want and then say,"see, this proves it" Crap!

Also, we don't have diurnal swings here. That is except for what Leo DiCiprio calls instant climate warming. We just call them chinooks>