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Thermal mass interior walls useful or not in passive solar house?

DoctorBeer | Posted in Green Building Techniques on

In the house we’re just about to build at times of the year with low sun angle because the south wall is 16ft high and has 120sf of windows & doors the light coming in is sure to strike the back walls.

I’m putting in a concrete thermal mass floor but was thinking of some approaches to add mass into the walls that will get directly lit.

One is to stack bricks in the framing before finishing the walls. haven’t done calculations on their mass yet.

The other is to take 4in diameter pvc pipes and seal them after filling them with water. a 4in pipe holds 5.4lbs per foot so an eight foot tube in a wall yields 43lbs. So every linear foot along a 4in deep 8ft high wall could hold 129lbs. A few feet of wall can add a bit of thermal mass collection capability which can be located between rooms..

Is this an idea with any merit and worth possibly considering as a way to hold and re-radiate passively collected heat?


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  1. GBA Editor
    Martin Holladay | | #1

    I think the water-filled PVC pipes are a bad idea, for one reason: potential leaks. (Thermal expansion and contraction will pressurize the containers and stress the joints.)

    When it comes to thermal mass, the best approach is to keep it simple. For more thermal mass on walls, many builders use double 5/8" drywall.

  2. dankolbert | | #2

    Hey Martin - what's your take on paraffin drywall -

  3. GBA Editor
    Martin Holladay | | #3

    1. It's expensive.

    2. Like all thermal mass schemes, it has its limitations. Getting an advantage from thermal mass requires homeowners who don't mind temperature swings. If you have HVAC equipment operated by a wall-mounted thermostat that keeps your home at 72°F, you'll never see any benefit to thermal mass -- whether or not the drywall has phase-change capsules.

    To get the thermal mass benefit, you need to fool your thermostat, use temperature set-backs, or live without central heat and air conditioning.

  4. gusfhb | | #4

    I would think that thermal mass directly exposed to sun from south facing glass would help dampen daytime temperature spikes.

    I would think all houses with any significant amount of heat collecting windows would suffer from temperature swings, most people don't mind excursions to 72 when the thermostat is set to 68.

  5. GBA Editor
    Martin Holladay | | #5

    You're right; materials will warm up and store heat when sunlight strikes them directly.

    Here's a more accurate statement of what I meant to say: if your furnace prevents your home from cooling at night, and your air conditioner prevents your home from overheating during the day, you'll get far less benefit from thermal mass than if you allow greater indoor temperature swings.

  6. gusfhb | | #6

    I wonder if skimcoat plaster is a significant enough amount of added mass to make a difference. In my area it is the same cost as drywall, and is a more durable finish. Drywall guys all understood that I wanted an edge bead wherever wall met wood, while the plasterers needed to be bribed, something that may come into play in your house.

    Gotta warn about the huge, huge[did I mention huge] amount of water it puts in the building tho.

    It is almost dustless and over completely in 2 days

    I still like the big brick chimney across from the south facing windows. 21st century version doesn't go thru the roof and only has an airtight woodstove.

  7. user-723121 | | #7

    My experience tells me a conventional built house will store about 6 Btu's per sf per degree F. This would be standard 1/2" drywall, typical interior walls and furnishings.

  8. vCWJKZSiuP | | #8

    The problem with any passive heat storage is the rate of heat transfer.Exposing one surface to a radiant source of heat is very likely to lead to overheating the space as the wall can't absorb the heat as quickly as it builds.There is some gain,to be sure(The soapstone sink in my kitchen has a cozy warm feel after a day in the sun) but I think adding serious mass to your wall would not help store the limited heat available. Martin's suggestion of another layer of sheet rock,or some applied material like plaster,seems more cost effective.

  9. user-723121 | | #9

    I agree with Brian, even with standard construction I believe the heat exchange from air to thermal mass takes about 8 hours to reach equilibrium. On a cold, cloudy and windless (steady state outdoor temp.) day I monitored furnace run times from the morning setback temperature 62F to the setpoint temp. of 70F. It took approximately 8 hours for the furnace to reach it's minimum hourly run time.

  10. user-788447 | | #10

    Have you taken a look at Ed Mazria's 1979 book "The Passive Solar Energy Book"?
    Don't have it in front of me but in the book there are descriptions of the performance of several mocked-up structures. You will find the results of these helpful to determine the amount of thermal mass you should shoot for. If I remember correctly 4" thick thermal mass material is ideal and you want 9x the surface area of your collector windows.
    I would be curious to know if there are any more recent books that have studied the use of solar mass in home beyond what Ed Mazria's book details.

    I agree with Martin don't try to employ water. I've heard of many an example where this technique has ended up leaking at all scales of installation.

  11. user-723121 | | #11


    Google "trombe wall". There are quite a few entries that may help you with your calculations and decision.

  12. DoctorBeer | | #12

    I'm aware of trombe walls but they're not really applicable here. I'm already planning on doing a thermal mass floor of about 3in plus the VT slate tile on top. I'd actually posted a different thread about trying to determine the mass (by weight) that this floor needed to be in a more exact manner than the fuzzy guidelines I'd seen. I had to make several assumptions and in the end all I could really determine was that something in the 2-4 inch range should work. Anything thicker makes the floor structure needed to support it increasingly expensive and it isn't clear the extra mass adds more benefit. The issue, as pointed out here, is to have enough mass to absorb the incoming heat such that the air in the house doesn't end up over heating during the day but not so much that the mass doesn't heat up enough or isn't able to re-radiate the heat absorbed at night.

    The reason I was trying to see about whether adding mass to the rear walls of the room might work is that, at least in winter, the low sun angle will cause a few hours of daylight to strike the wall in addition to the floor. So I thought that having added mass there might help.

    It sounds like from reading this that unless the mass is actually an integral part of the wall's surface (i.e. like the drywall suggested but which we won't be using in that location for aesthetic reasons) it's not likely to absorb much heat or re-radiate it well.

    At this point it sounds from these discussions like I should stick with just the thermal mass floor and not really worry about trying to integrate these rear walls into the overall thermal mass system in the home.



  13. user-651098 | | #13

    Hello Jay,
    The South wall of my 100 year old house faces almost due Solar South. The top of the South facing window is 78 inches off the floor. At midday, from Dec 15th to Jan 15th, the sun projects about 15 feet into the room. I'm just west of Ottawa, Canada so this track reflects my latitude. Where are you?

    How high is the top of your highest window from the floor? What is the distance between the back wall and the South wall? From 1 Nov., to 1 March, what is the track of the top of the sunbeam on the back wall?
    The best use of thermal mass is through direct solar exposure. If you installed a veneer wainscott of red brick or dark stone on the back wall, this could act as a direct gain solar mass. Or, as Martin suggested, install double 5/8" drywall and paint it a dark colour. Or install a veneer of dark tile.

    120 SF of South glazing can result in substantial heat loss. What windows are you considering?
    Will you install overhangs to reduce summer overheating? How will you cool the house in the summer months?

    My reading indicates that trombe walls have a neutral net gain. -Jim

  14. DoctorBeer | | #14

    Hi Jim,

    the room is 19ft from the south wall to the north wall. given the highest sun angle of 22 degrees on Dec 21st only the light coming through the windows up high on the 2nd floor wall of the cathedral ceiling area will strike the back wall at that time. Obviously on that day and other winter days the early and later sun will also be low enough to strike the back wall but I'll use this as a reference point.

    This sun angle means the light through this window strikes the wall from about 3.5ft off the ground to about 7ft off the ground.

    The windows are Inline 325 Series fixed up top and 800 series sliders down low on this wall. I chose these windows due to the ER ratings on them which, if I understand correctly, is a rating system that takes into account both U values and SHGC to determine the net gain of a window. The 325s have an ER of 36 and the 800s an ER of 31. So I believe they're good windows for this use.

    Overhangs are a little problematic with the SIPs. Without having rafter tails penetrate the walls I can really only go about 12-18inches out. I'm considering roll down awnings.

    On top of the house's ridge peak in the center of the house I am building an 8ft by 8ft cupola with 4 good sized sliding windows in it. It will be accessed from a trap door on the balcony of the cathedral ceiling area. This will allow for a chimney effect in the summer which can be boosted through the use of ceiling fans in the cathedral area and box fans in the cupola windows.

    I've run thermal modeling on this design with REM/Design and results indicate total heating usage of about 43M BTU/yr and peak (at -6F) usage of 26.5K BTU/hr. I'm not sure how good a job REM/Design does of accounting for the passive solar gain. Unless it over estimates it by a bit the house should have pretty good performance.



  15. user-723121 | | #15

    REM Design is not super as far as breaking down results. I have found it to be accurate compared to Energy 10 in the prediction of energy use but I like to see more detail in how the heating/cooling load is assigned. The more detailed the input fed into the energy software, the more accurate the output should be. There are some very sophisticated simulation tools out there by DOE, but they lack an interface to make them friendly to the average green enthusiast.

  16. DoctorBeer | | #16

    Thanks Doug, I did see the DOE tools but as you say the lack of interface makes them cumbersome. I even took a look at the actual spec itself since having a background as a programmer I thought I might be able to actually write up my house's design but it's pretty impenetrable. It seems like it's mostly designed for commercial construction and from what I could tell there are 3rd party interfaces but they're still pretty new and they looked a little pricey.

    So for now REM/Design will have to do...

  17. jklingel | | #17

    Don't people sometimes employ large, dark-earth "gardens" and rock structures for mass as well? Those would be cheap and easy to remove if they don't work properly, at least.

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