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Thermal Mass

GBA Editor | Posted in Green Building Techniques on

I am contemplating building a house in a heating climate (NY state, 4800 HDD), and would like to increase the thermal mass at low cost while minimizing interior design constraints and space limitations. Adding thermal mass material to the inside of interior drywall walls would be appealing. Practically speaking, can one put wallboard scraps, broken brick or cement block pieces, or even local small stones into the interior space between the two wallboards of interior walls? Do any important constraints exist, e.g. is wallboard unable to handle the additional weight stress, will the interior walls bow out and/or split open, do any of these materials attract surface condensation and become a mold medium, etc.?
If practical, this low tech idea could add thermal mass while lowering the amount of construction waste.

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Replies

  1. Riversong | | #1

    First question: why do you want to increase the thermal mass?

    If it's for passive solar storage, then loose mass that's not in direct sun or at least directly coupled to the interior air will do little good (and you may very well get bowed drywall).

    If it's just to dampen the diurnal thermal cycling from warm days to cold nights, then the mass also needs to be closely-coupled to the interior environment.

    If it's mostly to get rid of construction debris, you might be better off using masonry scrap for landscape fill or a driveway base, and clean drywall (not firecode or moisture-resistant) can be tilled into your garden as a soil ammendment.

    What's your goal?

  2. JJ | | #2

    Many thanks, Robert. Goal=help dampen/temper the diurnal temperature swings.

  3. Rick | | #3

    Robert's first and second paragraphs are two different fancy ways of saying the same thing. Thermal mass (a substance with high heat capacity, such as rock or water) contained within the house will store available heat and then release it again as temperatures drop. When that stored heat is released, yes, it will have the tendency to even out the day and night temperatures (the diurnal stuff). Either way, it is just stored heat that is being released when it is needed (passive solar storage).

    It is true that their will be MORE heat stored in mass that is in direct sunshine, just as would be intuitively expected. Like Robert said, though, there has to be a heat transfer path to be most effective. If you throw rocks into the wall, you will not only likely have bulging/breaking problems, but any heat has to get through the paper and gypsum (good heat capacity) and then past the inevitable insulating air pockets in order to be stored (or released) by the rocks. All that stuff in the wall would still end up being thermal mass, but probably not as effective as thermal mass that is more directly connected to the rooms and a heat source (such as the sun). You might be better off making a collage--cement all that high heat capacity scap to a sun-drenched wall and get extra credit for art work.

  4. Riversong | | #4

    JJ,

    I'm still not clear about your objectives and how you expect the thermal mass to integrate into your broader design program.

    Question: is this going to be a passive solar house and, if so, what south glass area to floor area ratio are you considering? You're building in a rather moderate (or mixed) climate zone and, if you have good daily insolation potential and a well-insulated and tight envelope, then you could easily overheat on a sunny day if you use more than 7% of floor area in south glazing, in which case you'll need direct-gain thermal mass (in direct sunlight) to buffer the temperature swings. Ideal solar mass is 4" thick masonry or concrete or stone, because that depth allows several hours of absorption and several hours of emittance to warm the early evening. But thinner mass would help. Solar mass that is in direct sunlight is 3 to 4 times as effective as indirect-gain mass (in other words, 3-4 times as much surface area is required for indirect mass).

    If this is not a passive solar house, then adding mass is unlikely to make a noticeable difference. Heavy thermal mass is most effective in climates which experience significant temperature swings between day and night, like high desert areas, and is more effective in warm than in cold climates. Excessive mass in a home increases the thermal inertia and makes it difficult to heat back up if allowed to cool (and vice versa).

  5. Brent_Eubanks | | #5

    First of all, let me say that I agree with Robert's statement above, about the questionable usefulness of this strategy in your climate.

    Secondly, the need to have the mass thermally coupled to the space is critical. Drywall is actually a semi-decent insulator (at least relative to low delta-Ts, as you would see in this context) so it's unlikely that the mass in the wall would ever have much benefit. This is doubly true if it consists of oddly-shaped broken masonry pieces, which are mostly surrounded by air and will not conduct heat between themselves, or between the mass and the drywall.

    Drywall can used as a thermal mass material, but only to a fairly limited degree. It has a density of 50 lbs/ft^3 and an R-value of nearly 1 per inch. This means that it can only hold so much heat/coolth, and you don't get a lot of benefit from multiple layers because the R-value is high (again, relative to context) so it will not tend to conduct heat to the deeper mass layers.

    If you still think this is a good idea for your situation, I would recommend using a heavy-weight plaster over drywall. A cementious plaster can have a density of ~100 lbs/ft^3, and an R-value of 0.3 to 0.6 per inch. Both of these statistics make it a much better space-facing thermal mass material than straight drywall.

  6. Riversong | | #6

    While I would agree with Brent about the relative advantage of thick plaster over gypsum drywall as thermal mass, I disagree about the value of additional drywall as either direct-gain or indirect-gain mass.

    Drywall is, essentially, rock (gypsum) and moisture with a thin paper facing - double the density of softwood and 40% more conductive. While it doesn't compare to much denser masonry or cementitious materials as thermal mass, it is an inexpensive and less intrusive way to add mass to a home's interior.

    Applying two layers of ½" drywall to walls that are directly swept by the sun, particularly if they are painted with medium tints of warm colors, is a cost-effective way of adding solar thermal mass.

    A typical 2,000 SF home may have 9 tons of drywall. So doubling even part of that will add considerable mass that is well-coupled with the interior environment.

  7. Will Goodwin | | #7

    Is it possible to do much to temper swings in temperature from a conventional woodstove (not a masonry stove) using thermal mass around the stove?

  8. Riversong | | #8

    Absolutely. That's the function of the slabs of soapstone that are either itegral with, or available as an accessory on, many quality wood stoves. Soapstone is ideal for this application, as it is not vulnerable to temperature extremes and has double the heat capacity per pound of cast iron or steel. A thick slab of soapstone on top of the stove can be used as a cooking surface, and food can be grilled directly on the stone.

    Building a high-mass surround is somewhat less effective than applying the mass directly to the stove, but will help buffer temperature swings, allow a quick & hot (clean) fire to charge the mass, and offer hours of gentle radiant heat. Ideally, the thermal mass would be next to each of the radiant surfaces of the woodstove: under, beside, behind, with some on top as well - and as close as possible. The mass should be at least 4" thick - 8" would be optimal if the structure of the house can support the weight.

  9. mapnerd | | #9

    If overall value (cost + effectiveness) is most important, do you think double sheets of 1/2" drywall can provide better thermal mass for walls in direct sunlight for at least a few hours per day than plaster or masonry walls?

  10. Robert Riversong | | #10

    Gypsum has twice the specific heat by weight of concrete or brick and 2 to 3 times that of stone, so in spite of its lesser density (~50 pcf), has nearly the same heat storage capacity as masonry per cubic foot (except for very dense stone such as granite, marble or soapstone).

    I would think that increasing the depth of drywall is far more cost-effective than plaster or masonry, even if thicked to 2" in direct-gain areas.

    The key to any direct-gain thermal mass is color and texture. A matt finish and medium colors (earth tones) will maximize the amount of solar energy absorbed. Using more polished flooring will help reflect insolation to the walls. Additionally, in northern climates, the low winter sun angle offers a more direct angle of incidence to vertical walls than to floors.

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