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Are lightweight thermal mass systems effective?

User avatar
Armando Cobo | Posted in General Questions on

We have an on-going discussion about Thermal Mass Systems that is getting nowhere. Heavy-weight mass walls and floors are easy for all to understand, like concrete, rock, adobe, rammed earth, water, etc.; however the issues we have is with Light-weight mass like some ICFs and CIPs. How can an INSULATED product be an efficient thermal mass system at the same time? If the insulation is preventing the concrete core to get any heat/cold, how can that provide good thermal mass results. Also with products like AAC, if they are insulated CIPs, how can they be good thermal mass products?

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    As you probably know, thermal mass can't heat your house or cool your house. In order to take advantage of thermal mass, you need to live in a house with wide temperature fluctuations. If you aren't willing to accept these temperature fluctuations -- if you live in a house with a thermostat set to 72 degrees -- then thermal mass won't do anything for you.

    You are correct that the configuration of ICFs are all wrong if you want to take advantage of the termal mass of concrete. Ideally, the concrete wall would be insulated only on the exterior.

    Traditional thermal mass walls (like adobe walls) only make sense in a climate with wide diurnal swings in temperature, where the nighttime temperatures are regularly in the 50s or 60s, and daytime temperatures are in the 80s or higher. In these circumstances, the 12-hour time lag between when the sunlight warms the wall and when the wall cools off proves very useful.

  2. User avatar
    Armando Cobo | | #2

    Thanks Martin, those are exactly my points based on my experience with Trumbe & Adobe walls and concrete floors in the SW, and with a bead wall design (window system) that was not a Thermal wall, but more of an insulated window cavity.
    How would you respond to Light-weight wall and floor systems that make claims of good Thermal Mass properties, like AAC or Terralite Cement? As I understand if it is insulated and low density, it would have low thermal mass properties. Your thoughts?

  3. User avatar GBA Editor
    Martin Holladay | | #3

    I agree with you that most of the extravagant claims made for AAC -- including the claim that an AAC wall has a higher "effective R-value" than its actual R-value -- don't make any sense.

  4. User avatar
    Dana Dorsett | | #4

    The ICF industry has a history of marketing disinformation/exaggeration that has sometimes made it to FTC sanction levels of distortion, if not outright fraud. The mass effects are real enough, and even though the mass is isolated from the interior by insulation, it reduces the peak heating & cooling loads, sometimes substantially (even without huge diurnal temperature swings, though big swings make the benefits larger & more obvious), but there isn't sufficient mass in an ICF to trump R-value as the dominating performance factor. Concrete (or AAC) is a very EXPENSIVE way to add mass to a wall, but it is far more structural than some other mass-wall system.

    An ICF wall models moderately cleanly as an R-C-R T-filter in electronics- the "C" being the thermal mass, and the R being insulation. The exterior side R-C has a substantial time constant, so the variations in heat flux through the interior side R is fairly even over the course of any 24 hour period, shaving quite a bit off the peak load of the building as a whole, but the weekly average flux through the wall is pretty similar to what it is in a low-mass wall, except when the outdoor temperatures are crossing through the heating/cooling balance point. During those shoulder season conditions this averaging effect results in VERY substantial energy use savings by nulling out the loads almost entirely (even as an average.)

    While the mass in ICF is pretty good at averaging out diuranal loads, it in no way matches the performance (at the same insulation values) of truly high-mass R-C-R walls, or R-C walls that can also absorb and dampen temperature swings from solar gains or heat sources inside of conditioned spaces. AAC does a somewhat better job of managing solar gains than ICF, but like ICF, most real world examples in the US are relatively low-R systems compared to what high-performance home designs call for in colder climates. (I understand that in northern Europe/Scandinavia many high performance AAC homes use exterior EPS to bring that mass further inside the now much higher insulating value, to better utilize that mass.)

    Adobe and AAC have distributed R, distributed mass, and the electronic model analog would be similar to a lossy, low-inductance transmission line or a chained series of R-Cs, but the adobe version is usually higher mass, lower-R than AAC at any given wall thickness.

    Though many have tried, none of these systems can be really well-tuned to a 12 hour cycle for "typical" local weather conditions (even though the time constants sometimes align well under a particular set of circumstances.) They're more clearly thought of as filters than as tuned oscillators, and with truly massive walls it's possible to average out monthly or even seasonal loads with a minimal amount of additional R, but to achieve that you're measuring wall thicknesses in feet or yards, not inches.

  5. User avatar
    Armando Cobo | | #5

    Thanks Dana, those are really good thoughts. For years I tried to let my clients know that with today’s technology in wall and roof systems, I rather have an easy, cost effective wood framed construction with good insulation and outsulation than any other system; not that some other systems are bad.
    The few houses I’ve designed with Thermal Mass systems, I’ve explained to my clients the good. the bad and the ugly of those systems, and all expectations of perfect performance must me curved and adjusted with the seasons and temperature ranges. It is not that easy to control these homes, and I’ve seen several failures from homes not built properly and homeowners not knowing how to work with those systems.

  6. Greg Pittman | | #6

    Thermal mass is a concept in building design that describes how the mass of the building provides "inertia" against temperature fluctuations, sometimes known as the thermal flywheel effect.

  7. Jin Kazama | | #7

    Ok, are we on a green building website ?

    Thermal mass is REQUIRED to be able to effectivly use any SHG in any heating climate.
    Super insulated buildings provide the desired performance in terms of heat loss,
    but require a minimum of mass to use "passive" energy sources.

    ICF thermal mass is wrong only because everybody uses it the wrong way.
    There should be much more insulation on the exterior than there is in most "equal in-out" systems.

    Look at what Quad-Lock offers ... ( what i should've used for my house )
    When offsetting insulation to the exterior, the concrete wall mass are used more effectivly to store
    the interior gains , helping maintain an comfortable temperature with higher energy storage.

    Though it would be much more practical to be able to end up with a concrete only interior wall
    with all insulation at the exterior, but that requires different building approach and probably different related costs.

    I may be wrong, but logic tells me that the more "storage mass" ( concrete, steel, marble etc.. )
    you have in contact with interior temperature the more stable the temperature of the interior will be even with large energy content change.

    Adding 10KWh of heat to a room of air brings the temp up quite some,
    if the same room as 1 ton of steel in it to accumulate the energy, the air rises much less,
    but you have accumulated the same quantity. nah ?

  8. Bob Manninen | | #8

    Dana, I really appreciate the electrical circuit analogy (which is used frequently in engineering disciplines) to describe a conductive heat flow condition... hear hear! Too bad we can't use non-linear device analogies .... yet :-)

  9. User avatar
    Dana Dorsett | | #9

    Bob: Being an 'lectrical enginerd by profession myself, I'm used to applying electronic analogs for solving thermal delay & heat transfer issues, particularly in apps where precise temperature control of some components are essential to the function of the product or device (such as infra-red and other imaging sensors, controlling the center-spectrum of superluminescent diodes & laser diodes in fiber-optic gyroscopes, etc.)

    You don't need to run the hard-math on most building-science heat transfer issues- the temperature signals are noisy, R-value drifts both with temperature & humidity, but 0.01% precision (or even 1%) is never called for. The really blocky simple-math linear models almost always hits within the construction-fault error limits, and even a 5% error doesn't present a functional disaster (if it does, it's a truly crappy design!)

    Jin: While it's true that ICFs are a less effective use of thermal mass than other systems, and nearly useless from a passive solar point of view, the mass effects are still real, and the effects on energy use can be modeled/quantified fairly accuraetly even with 2-D modeling like DOE 2. AAC is slightly more useful from a passive solar point of view, but not much. Even naked concrete has a depth distributed R & mass, with a predictable R/C thermal delay, but it's short enough to be more useful for passive solar.

    BTW: Steel has a very short thermal delay, but the thermal mass of steel is pretty low compared to concrete or marble, making it a lousy heat storage medium, ton-for-ton. Concrete stores 50-100% more heat per degree-ton than steel depending on type, marble stores about 90% more. (Look up the comparative specific-heat numbers.)

    Soapstone stores a bit more than concrete, and ~110% more heat per ton-degree than cast iron (which is similar to steel), which is what makes soapstone a material of choice for high-mass fireplace wood burners or woodstoves. The high thermal mass and thermal lag of the stone evens-out the heat flux of into the room allowing these appliances to burn at a high rate for high- efficiency/low-polluting in bursts without overheating the space. Ceramics have a comparable specific heat to soapstone and are much easier to fabricate into complex shapes, making them another decent choice material for high thermal mass woodstoves. (I recently installed a soapstone woodstove that ran about 475lbs, but to get the same thermal mass with a cast iron stove would have run about 1000lbs. Placing it on 5" concrete slab-hearth with half-inch slate cladding (insulated with rock wool from below) added both mass-storage and radiating surface than a simple code-min approach would have been, but still much lower mass than mass-fireplace heaters like the Tulakivis.

    OK, enough thread-drift...

  10. Jin Kazama | | #10

    Dana, would have to come up with some sort of capillary heat conduction system in concrete ..

    I can tell you that my 2nd floor heats up to ~ +24c since end of FEB ( going up from 21-21.5c set point)
    and it takes quite some time to get back down to heating point ( if 0c outside, heating doesn't start before ~10pm )
    ICF, concrete slabs for all floors .. it does work

    The floors usually get hotter because they receive direct sunlight,
    walls are heated by air, and would required a much larger surface for proper heat exchange...

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