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Thermal mass and insulation

AlbertoArriaga33 | Posted in Energy Efficiency and Durability on

Hello All,

I have a question regarding thermal mass for a home in a tropical climate (hot and humid year round). The exterior walls will be built using grouted CMU, which is pretty standard in the tropics. For simplicity, I was planning on having exposed masonry on the exterior and finishing it with a cementitious plaster and an exterior grade paint. On the interior, steel stud framing to provide a service cavity and also to fill the bays with R15 mineral wool. 

However, I have many doubts and still not sure if I quite understand how thermal mass works. I’ve read that typically you would want to keep the thermal mass inside in a hot climate, in order to keep it from absorbing solar energy, and subsequently heating up your house at night. I guess the question that pops into my head is whether the interior insulation is going to perform well. My guess is it would slow down the transfer of thermal energy from the walls to the interior space as it radiates throughout the night. Is the thermal performance of the assembly affected substantially by whether the insulation is on the exterior or interior? 

Any reading material on the matter would be greatly appreciated, thanks in advance.

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Replies

  1. Jon R | | #1

    What are nighttime temperatures? Are you going to run AC or do you need to passively cool the interior?

    1. AlbertoArriaga33 | | #2

      Nighttime is typically 78-79. The home will be fully conditioned, sensible load shouldn't be too high with proper roof insulation but latent load will still be high, hovering at about 70% RH outside.

      1. DCContrarian | | #4

        What is your desired temperature setpoint? If it's not around the average of the daily high and low you're not going to benefit from any sort of heat retention. Just insulate and seal the house as much as possible.

        1. AlbertoArriaga33 | | #5

          69-70 is ideal for the setpoint. Not near, average is around 86, with peak temps in the high 90s.

          1. DCContrarian | | #8

            Heat retention is not going to benefit you. You're going to be cooling all the time. Your best strategy is to make the building as tight and as shaded as you can stand it.

  2. DCContrarian | | #3

    "However, I have many doubts and still not sure if I quite understand how thermal mass works."

    "Thermal mass" is not a term used in science or engineering.

    1. Expert Member
      Malcolm Taylor | | #6

      I'm not sure that makes any difference.. Thermal mass in building design is understood to be the capacity of a high mass material to absorb and store and release energy as heat.

      1. DCContrarian | | #7

        And this is why I find the term so grating -- it's not the mass that gives it that capacity, it's the heat capacity. People act like simply being heavy makes something a good storage medium. And often the substances being suggested have rather low heat capacities.

        When I hear the term "thermal mass" I've become conditioned to expect that the ration of wishful thinking to engineering is going to be quite high.

        1. Expert Member
          Malcolm Taylor | | #10

          DC,

          That makes sense. I get it now.

        2. Andrew C | | #11

          DCContrarian - This is an excellent explanation. I know better, but I've fallen into the habit of using this term ("thermal mass"), and you are quite right, it tends to be mis-leading. Thanks for the reminder to be less casual when confusion could result.

        3. Tom May | | #18

          Thermal mass is an engineering thing...that's why there is the study of the fundamentals of mass and heat transfer. Something that is heavy or has weight is mass x gravity. Thus mass and weight are two separate animals. Density is mass / volume. You would need a volume of many times more of a substance that has a lower mass than a substance that has a higher mass to equal the same weight. If you build a wall out of wood and the same size wall out of brick the brick wall will have more mass and hold more heat.
          Another good example is momentum which is mass x velocity, not weight. Or why you weigh less on the moon or are weightless in space even though your mass doesn't change.

          1. DCContrarian | | #21

            "Mass" is a term from science and engineering. "Thermal mass" as far as I know is only used in alternative energy literature.

            "If you build a wall out of wood and the same size wall out of brick the brick wall will have more mass and hold more heat."

            This sort of thinking is why I rail against the use of "thermal mass." An object's heat capacity is not determined by its weight, or even it's mass! It's the product of the mass and the specific heat. Most of the substances that are proposed as good candidates for "thermal mass" actually have low specific heats, they just happen to be dense. Brick, for example, has half the specific heat of wood and a quarter of the specific heat of water. And often people talk about using concrete, which is a major emitter of greenhouse gases in its production.

          2. Tom May | | #26

            It's the product of the mass and the specific heat. ...What? That makes no sense. Specific heat is a property of a material, how can it be the mass x specific heat......get your facts straight. And saying "as far as i know' doesn't prove anything. Give us the facts.
            If you check your thermodynamics you will see that Cp is used in conjunction with density and this product is in the denominator when calculating a material's ability to absorb and diffuse heat. Thus the smaller the Cp the larger the heat gain. Gases and liquids tend to have a higher Cp.

    2. Tyler Keniston | | #27

      DC,
      The term 'thermal mass' may indeed be jargon of the building science world, but this:
      "it's not the mass that gives it that capacity, it's the heat capacity," and this: "An object's heat capacity is not determined by its weight, or even it's mass!"
      is not strictly correct (or is at least confusing). As you say, immediately following the latter quote, "It's the product of the mass and the specific heat."
      So it IS determined by the mass, just not ONLY the mass. Specific heat is the relevant intensive property, as you point out.

      'Thermal mass' is little more than synonym for 'heat capacity.' If you're suggesting to use that term instead, I'm not necessarily arguing with you (perhaps even agreeing).

      It's mostly that I want to be clear that mass IS of relevance. The dials we can tweak in design are the specific heat of the material, and the amount of it (mass). Since we are often restricted in tweaking volume to significant degrees, we are inevitably left tweaking density in order to increase mass (though we can include or exclude existing mass depending on where we put the insulation.)

      You are right that there is also the specific heat dial to tweak (not too much beats water here, and your point about environmental impact of concrete is a good one).

      Note, as example, that Hydrogen blows both concrete and water out of the water in its specific heat. Yet, we'd never fill a room with balloons of hydrogen as a solution for thermal storage because the heat capacity would still be tremendously low due to lack of mass. Or in other words, due to the low density. Perhaps volumetric heat capacity should be the true gauge (along with other things like environmental impact, etc).

      I kind of like the term thermal inertia too. It's not really any more precise, but perhaps doesn't suggest 'mass' is the only variable.

      1. Ethan Foley | | #30

        The Hindenburg makes me think of another reason we wouldn't fill a room with balloons of hydrogen... :D

        1. Tyler Keniston | | #31

          Good point, I suppose that would make the house too light...

          Maybe better to go with liquefied Ammonia, which has fairly good volumetric heat capacity (3.26 J-cm^3 - K). Or better still, water-rich animal tissue at around 3.7.
          Welcome to my high thermal-mass horror house.

          https://en.wikipedia.org/wiki/Table_of_specific_heat_capacities

          I didn't realize this was a thing until recently, but phase change materials (PCM's) have been investigated as a way to store significantly more energy per volume via latent heat. Paraffin seems to be a common one. Not sure of the practicality or desirability, but is intriguing.

          https://journals.sagepub.com/doi/10.1177/1687814017700828

  3. Jon R | | #9

    Interior thermal storage can still be useful when the average outdoor temperature is much higher than what you desire indoors - because you can change the thermal transfer rate (eg, with fans at night). But in your "too hot" case, don't worry about thermal mass.

  4. Andrew C | | #12

    If humidity is a bigger issue than temperature, you likely won't get very comfortable just using air-conditioning, as it won't run enough to address the latent load. Plan on a separate dehumidifier and focus on air-sealing.

    1. AlbertoArriaga33 | | #15

      We are designing a system with a Mitsubishi dealer. Their VRF units run longer and kind of "coast" at lower speeds. AC in our climate is required pretty much year round. We're definitely considering a dehum but hopefully the VRF unit can take care of all the latent load.

  5. AlbertoArriaga33 | | #14

    Thank you all. Just to clarify, my concern was not to utilize the "heat retention", as DCContrarian explains, to my advantage, but rather if it would be detrimental to have it in the exterior.

    1. DCContrarian | | #17

      Ideally the house would be so well sealed and insulated that the exterior has no impact on the interior.

  6. Roger Berry | | #16

    Alberto,

    If you are seeking to have interior temperatures near 70 in an 80F - 70% night environment, then you will need to be insulating and sealing very carefully as others have noted. The dew point chart, which I am not absolutely certain how to read, seems to indicate that your outside air will condense on surfaces that cool. I live in an arid environment, so I would strongly recommend you speak with people that are use to dealing with constant high humidity.

    Removing the latent load will be tricky and likely need to be done in stages by dropping your set point slowly over days. At that, you will need to have a very tight structure to keep exterior air intrusion from being so high you need constant dehumidification, which, unfortunately, would be dumping warm (but drier) air back into the living space.

    Your desire to make use of "thermal mass" is feasible if you isolate the interior masses from as much exterior heat gain as possible. A ground coupled slab with tiles and insulation around the perimeter at the CMU walls would be a good start. Interior walls of half thick CMU and plaster would be a next step. All this mass, once cooled to your set point would act like the food in a freezer or refrigerator. Opening and closing the door does allow warm air to come in, but the relative specific heats and masses means the air cools fast and the food mass warms negligibly. The risk for you is the very high humidity you face. Think of how frost build up occurs in freezers. Cooled thermal mass will act in the reverse manner of a giant stone fireplace up north. Oddly, you would benefit from an airlock style front door as much as an Alaskan.

    Overall, the advantage of cooled interior mass would lie in evenness of temperatures through the home and act as a buffer for the sudden heat of large groups of people or cooking. The A/C can then be sized for a smaller and (hopefully) controlled amount of heat gain. Due to the high humidity you face, finding the balance for needed fresh air and opening the door minimally will be tricky. Leaving a large patio door open could result in a lot of damp furniture and clammy floors. It would also require cycling the AC through the temp drops to effectively handle the moisture load. Drying clothes will be interesting. A dehumidfier type dryer might be a wise choice. Otherwise, you do not want to be running a standard dryer unless it is outside your controlled AC environs.

    Low gain windows with dual panes and maybe exterior shade trellises will help damp down direct heat gain. It will also help keep them from endlessly fogging. Single pane glass chilled to 70F could end up looking like an iced drink. Insulated, but high ceilings will help stratification and keep the coolest parts where you sit and relax.

    The exterior walls of CMU and plaster could be insulated and clad, but I suspect that method of construction would baffle the locals. Besides, the choice of CMU for walls may also reflect durability decisions learned over time. I would think termites and ants would be extra large in such a warm moist environment.

    1. AlbertoArriaga33 | | #25

      Roger,

      Ground coupled slab is standard in our area, and is most definitely a heat sink. My dog loves to lay down in the bare floor because of this. The ground is always below our indoor set point. And interesting observation in the last paragraph, it's true that the local builders have no idea about any insulation methods. Im having do to a ton of research and work closely with an engineer to get the assemblies right.

  7. Expert Member
    Armando Cobo | | #19

    Tried and true designs for over centuries in South America have wide overhang tile roofs over purlins to keep attic cool and shaded walls, since the sun is mostly straight up. White walls, ample hallways, courtyards in the interior, thick tile or brick floors to keep floor cooler, and lots of cross ventilation. Growing up in a super hot humid climate, and I don't recall any of our houses having AC.
    Here are some centuries old and more modern houses.

    1. DCContrarian | | #22

      Everything changes when you go to air conditioning. Building science in cooling-dominated climates is not nearly as well-understood as in heating-dominant climates. Joe Listurbek likes to point out that the US is the only major industrialized country where a significant number of people live in cooling-dominated climates.

    2. AlbertoArriaga33 | | #24

      Armando,

      I am aware of these designs but also having grown up in a hot humid climate I can tell you that it is unbearable to have no AC at least in our location. On a cloudy day we may get by with fans, but not on a sunny day. Cross ventilation is also a pain because not only does it bring pollutants in (I live in a fairly industrialized city) but it also brings in copious amounts of dust. We will however, use white colored exterior paint and exterior shades for windows on the east and west.

  8. Expert Member
    Dana Dorsett | | #20

    >"On the interior, steel stud framing to provide a service cavity and also to fill the bays with R15 mineral wool. "

    That's a nearly complete waste of mineral wool.

    Steel studs are HIGHLY thermally conductive, and will reduce the thermal performance of the insulation by about half. Using shallow steel FURRING to hold a continuous layer of stacked batts in place (no steel passing through the mineral wool layer) would allow the batts to provide full function.

    Whether R15 would make an appreciable difference over R7-ish in peak or average cooling loads in that assembly & climate is questionable though.

    To be sure "thermal mass" is indeed a term used in engineering on everything from jet engine component design and thermal management/conditioning of space probes & satellites- where component temperatures need precision control. It's not just used in reference to HVAC & houses, where many terms are often used imprecisely and the actual thermal mass is rarely (if ever) calculated. Nobody calculates or estimates the thermal mass or incident heat flux on a house the way they might need to for say, developing precision temperature controls for a laser diode module used in a satellite navigation system, or for understanding/controlling the thermal stresses on a turbine blade as the firing rate changes suddenly, applications where knowing the time delay inserted by the thermal mass of the components under time varying thermal input really matters.

    1. AlbertoArriaga33 | | #23

      Thanks for pointing out the fact about the steel studs. I had thought about that but honestly dismissed it because the steel studs are only about a millimeter thick going through the mineral wool. The studs would be pretty much sandwiched from both sides. You gave me the idea of using an inch of foam to provide a thermal break for the studs. But honestly, with the studs sandwiched, do you reckon it would reduce the thermal performance so drastically?

      1. Jon R | | #28

        I can tell you that a correct metal stud thermal bridging analysis needs a 2D or 3D analysis. What you put inside/outside the studs matters - a 1D parallel paths R value isn't correct.

  9. Tyler Keniston | | #29

    >"with the studs sandwiched"

    How do you mean? If you just mean there is insulation between the studs, that's not really a sandwich, in regards to the thermal path. If you add continuous insulation to 'break' the bridges from inside to out created by the studs, then you've accomplished something.

    This might help for understanding the heat flows and why a highly conductive material like steel doesn't need to be all that 'thick' to diminish performance fairly significantly:
    https://www.greenbuildingadvisor.com/article/the-fundamentals-of-series-and-parallel-heat-flow

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