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Community and Q&A

R-value vs. thermal mass

David Epley | Posted in GBA Pro Help on

Do you have any recommendations on building science articles (at GBA or another source) that get into the weeds on the differences between thermal mass and R-value. I understand the two separately, but stumble when it comes to comparing the performance of each to each other. Is there a metric that can be used? For example, comparing the energy performance/consumption of a triple brick Pittsburgh home with at stick-built, BATT insulated home of comparable size. Does the thermal mass of the triple brick home make it perform in a comparable way to an insulated home? Is there any benefit or advantages of t-mass in this situation since Pittsburgh is cold-climate?

Thanks for the help!


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  1. Stuart Fix | | #1

    Hi Dave,

    Thermal Mass has the effect of storing heat while insulation has reduces the movement of heat across its path. Thermal mass can store the sun's energy during the day and release it during the night. Enough thermal mass (especially in the envelope as opposed to interior floors) can cause the indoor temperature to be steady at the average of the daytime & nightime temperatures.

    If you live in a moderate climate like New Mexico, where the average diurnal temperature is comfortable, then you'll be quite comfortable in an uninsulated thermally massive building. If you live in a cold climate, where the average 24hr temperature is still cold, then your uninsulated thermally massive building will be cold.

    In short, there is no replacement for thermal insulation in a cold climate. Adding thermal mass to a well insulated building will slightly reduce your heating bills, more comfortably accept solar gains to the home, and will reduce your summer cooling bill (when the averaging of night & day temperatures is comfortable).

    Hope that makes sense. Here's a paper that discusses this in more detail, dealing with the feasibility of rammed earth construction in cold climate Canada.

  2. GBA Editor
    Martin Holladay | | #2

    Stuart's explanation is excellent.

    Let's say you live in a remote desert town and you have to buy drinking water for $10 a gallon. Thermal mass is like owning a really big, empty cistern. It can store drinking water, but it's empty. You still have to pay the water merchant to fill it up.

  3. Aj Builder, Upstate NY Zone 6a | | #3

    Stuart, great explanation. I hereby recommend GBA Advisor status.

  4. James Morgan | | #4

    I love Martin's cistern metaphor. I'm going to use it next time I have to talk to a client - or a builder - about what thermal mass can and cannot do for building's thermal performance.

  5. Richard Patterman | | #5

    Too expand on the metaphor: Most days more free water is delivered than you can possibly use.
    If you don't find a way to store it, you will have to pay cash for night time delivery.

  6. Garth Sproule 7B | | #6

    To expand the metaphor a bit more: There is a limit to how much the cistern can hold. And the cistern leaks bad enough that there will not be any water left in the morning. You must refill it daily to make good use of it. ;)

  7. GBA Editor
    Martin Holladay | | #7

    Any house with south facing windows warms up on sunny days. That's a given. In some climates, though, there isn't much sun during the winter, and in almost all areas of the northern US, you won't be able to gather enough heat through your south-facing windows to get through the winter.

    So your "free heat" hypothesis depends on expensive equipment -- usually solar thermal collectors, pumps, and big water tanks. That will work, of course -- but you're still paying to "fill the cistern." In most cases, homes designed to get all their space heat from solar thermal energy alone pay A LOT more for their heat than people who burn some natural gas.

  8. James Morgan | | #8

    I think this discussion may have strayed away from the OP's question, I may have contributed to this by commenting on the cistern metaphor. I believe that David with his question about a triple brick wall was asking not about thermal mass storage within the building (the cistern) but the phenomenon known as capacitive insulation in a mass wall, a rather different issue. R-values are for resistive insulation, calculated and measured in steady-state conditions where heat flow is consistently in one direction, from the warm to the cold side of an assembly. Capacitive insulation effects result from non-steady state condition where the heat flow reverses on a regular diurnal cycle. This can only be useful where exterior temperatures cycle significantly above and below desired interior comfort conditions e.g. in hot desert climates - heat starts to move slowly through the wall during the day but changes direction to head out again when the exterior cools dramatically at night. This is why traditional building cultures in those climates frequently make use of extremely thick mud or masonry walls and roofs. Capacitive insulation has virtually no effect in steady-state heat flow, which is when temperatures are relatively constant for an extended period of time on each side of a material. This explains why, despite misleading claims from certain manufacturers of insulating masonry materials, capacitive insulation is not a significant factor in thermal insulation performance in most parts of the US. In Pittsburgh for sure you should count only the resistive insulation value of the triple brick wall, which would be about R2.

  9. John Brooks | | #9

    Perhaps we don't have to choose between R-Value and thermal mass......

    Last summer I challenged a statement that Katrin Klingenberg made about the merits of High R-Value in Hot Climates
    I received this very interesting E-mail from Thorsten Chlupp:

    "John ... seen your comments on insulation in cooling climates at GBA and I did not want to get into any debate again but sent you this info for supporting your analysis.
    Can insulation work against you in a Cooling Climate...not really...
    One of the key principles I was after with the study of the SunRise home wall system was one of course the diffusion open assembly which allows for drying in any direction and also the phenomena of lambda values and decrement delay in medium weight wall assemblies in higher R-Values. Little scientific research has been done - or at least to my knowledge and it is something not accounted for in our current outlook of wall assemblies. We generally simply look at the steady-state R-Value of the wall and forget about the mass-enhanced R-value or effective R-value of an assembly. ORNL has done some great studies and work on this in 1998 but these studies did not look into higher values in the 70-100 range.
    But here is the crux - it is IMO something very beneficial for both very cold and very warm climates and will help us to lower our respectively loads. Take a look at this logged temp chart from one of my south facing walls:
    What gibberish you say? Exactly. This is what is happening in a high R-value dense packed cellulose wall. You get such a time delay on the actual time it takes for thermal energy to traverse the insulation layer that the exterior temperature changes with day and night temperatures and reverse the heat flow direction depending on the only governing factor thermal energy cares about: warm to cold. Nothing else matters...
    Meaning what? Thermal energy which stay's within the assembly is not lost until it has traversed the threshold of your insulation layer. Heating - or cooling load you don't have to make up. Or heat traveling to the inside conditioned space might not reach it before the exterior face cools enough to reverse the flow.
    So for a hot climate like yours the decrement delay slows heat flowing into a building during the heat of the day with an effective time delay from 7 to 13 hours (depending on the insulation thickness). So the time it takes for heat generated by the sun to transfer from the outside to the inside of the building envelope and affect the internal conditions is extending to your natural time of cooling - at night. Dense packed cellulose offers a high decrement ‘factor’ and has a low lambda (thermal conductivity or k-value) value, high specific heat capacity and high density - and is ideal for your use. Besides it is inexpensive and carries a very low embodied energy. Foam as a light weight insulation material has a comparable small decrement delay of 3- hours.
    Anyways, hope this was useful. I am trying hard to finish up my report on the new ARCTIC wall system which goes into detail about the physics of the assembly and its benefits. All my data is very supportive and I switched all our construction to the new system. No more REMOTE walls ...."

  10. James Morgan | | #10

    Interesting perspective, John, however the effect does depend on the particular *kind* of hot climate and it is important to be specific. The heat flow reversal and the consequent decrement factor only take effect if the nighttime exterior ambient is significantly lower than the maintained interior temperature, a common characteristic of hot dry desert areas but not of warm humid climates. If I remember correctly the discussion to which you refer centered on a project in New Orleans: where the nighttime temperature may stay in the high seventies and above the heat flow reversal will never occur and the effect is null. This is not to say that in such locations wall density would not be a useful factor in wall performance in certain conditions and at certain times of the year, but it would be a mistake to count it as a significant factor during average summer design conditions.

  11. John Brooks | | #11

    James, I am for sure not arguing in favor of thermal mass/decrement delay in leiu of R-value.
    I want both
    I once had similar thoughts as you (that decrement delay would have almost no benefit unless night temperature dipped below the desired interior temperature)
    Attached are 2 snippits from a JLC forum that have caused me to rethink.
    The argument that peak load will be reduced makes sense.

  12. Richard Patterman | | #12


    I forgot to qualify my comment, I live and build on the Colorado Front Range and am a big believer in low tech Passive Solar, so no "expensive equipment" needed to harvest the "free energy".

    Passive solar, moderate thermal mass, super tight insulation and being comfortable from 65 to 75 degrees, can "almost completely eliminate heating bills and expensive heating & cooling equipment.

  13. GBA Editor
    Martin Holladay | | #13

    So how much fuel does your house use, on an annual basis, for space heat?

  14. David Epley | | #14

    Thanks for the responses. Does anyone care to comment on the "effective insulation" value put forth by industry reps for products like ICF's? It seems that James' explanation on capacitive v. resistive stands true here as well, and in a cold or hot dominate climate without a diurnal swing an ICF wall does not offer an benefits outside of structural / air barrier.

    Another thought, everyone here I'm assuming has swam in a lake. You go deep enough, it's cold - really cold. Now granted I'm simplifying this a bit, but water (ie. transparent thermal mass) at a certain depth becomes cold and the sun's energy heating the water reaches equilibrium at some point. With this logic a thick enough wall could keep a house cool in a miserably hot climate without any system involved (I'm not saying that cool = comfort). It would just have to be seriously thick. Think of a cave for instance. Has anyone seen any studies out there or have any insight into how thick a wall would have to be to create this effect?

  15. James Morgan | | #15

    how thick a wall would have to be to create this effect?

    Without the major diurnal hot/cold cycle you're back to steady state conditions in which R-value is the only significant player. 12" of brick is about R2, for good thermal performance you'll want at least R-20, which with no other insulation material requires a masonry wall about 10 feet thick. John B's references suggest that less extreme variations in outside temps while not reversing the heat flow can offset the the peak load in a beneficial direction, fair point, so you could probably factor that R-20 up somewhat in actual performance. But not by much.

  16. Bruce John Stracke | | #16

    According to Baruch Givoni. Passive Low Energy Cooling of Buildings (Kindle Locations 123-124). Kindle Edition. "In buildings that are well protected from solar radiation and that have high insulation of the envelope and high thermal mass, the indoor daytime temperature, in the absence of ventilation, could be well below the outdoor level."

    You need both and in a mixed or hot climate a radiant barrier, too. It is in finding the right balance, of these elements for the particular climate we build in, that is the magic.

  17. Kevin O'meara | | #17

    Hello Dave
    The short answer is R trumps M. This has been an area of increasing interest for building scientists, especially for Phase Change Materials, PCM's, which have 90+ times the capacity to absorb/release heat as concrete. Here is a nice reference summarizing the current state of what we think we know Like everything else in life the answer is depends! Kevin

  18. Richard Patterman | | #18

    Just added it up for the last 12 months:

    May thru October: Total: 51 therms / $31.02
    Per Month: 8.5 therms / $5.17

    Nov thru April: Total 196 therms / $136.53
    Per Month: 32.66 therms / $22.75

    Assuming DHW @ 8.5 therms & $5.17/month, my space heating was 13.8 therms & $17.58/month
    Would have been much lower but my girlfriend insists on 70 deg.
    This is for 2,100 sf with heavy thermal mass and does NOT include $11/month in meter fee.
    So I paid $168 for the gas I used and $132 for the privilege of being hooked up to the gasline.
    Which is why my next home will be all electric.

  19. GBA Editor
    Martin Holladay | | #19

    Congratulations! Those are great numbers.

  20. Elizabeth Kormos | | #20


    Those are super numbers! Could you share how you built your house (basement, wall and roof assemblies, etc.) and what it cost? I am planning to build a similar size house in upstate NY.

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