GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Thermal mass questions

user-716970 | Posted in General Questions on

I am considering a well insulated concrete slab floor as the main thermal mass collector for a future super insulated home in a cold climate (9500 HDD). I also intend to use a very simple single zone radiant heat system for the floor. My research has indicated that a floor thicker than four inches is not only unnecessary but can be counter productive. I have also gathered that unshaded and uncovered mass in direct sunlight is at least twice as effective as surfaces that receive indirect sunshine in the same room and four times as effective as surfaces in remote areas of the home.

My questions are
1) Will adding a PV powered circulating pump, such as the “El Cid”, into the floor heat loop, help to increase the effective area of mass, as it would move heat from directly lit areas to the rest of the floor? Note that by using a PV powered system that pump controls are unnecessary and that pumping energy is free. Not quite passive but close.

2) For this to work well, would it be necessary to use a setback thermostat to lower the floor temp in order to capture more solar heat? I see problems with this approach on days when the sun doesn’t shine…

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    You're starting to encounter some of the complicated control issues that arise when a designer tries to marry a slab designed to act as thermal mass in a passive solar house with in-floor hydronic heat. These two aims aren't mutually exclusive, but at times they do conflict.

    For passive solar purposes, the ideal slab is well insulated but unheated. On the morning of a sunny day, the colder the slab, the better.

    In-floor hydronic heat has some disadvantages: (1) it's slow to respond and has a long lag time, and (2) if the thermostat calls for heat when it's cold at 3:00 a.m., the slab is likely to be warmer than ideal for collecting heat on a sunny day.

    One possible solution: deliver the heat a different way -- perhaps with a wall-mounted space heater or a furnace. Either of those systems will be much more responsive and will conflict less with your passive-solar heat storage goals.

  2. user-869687 | | #2

    Any attempt to cool the slab prior to the sun's arrival would be pointless unless you could effectively store any heat that got flushed out, and then reuse it later. That would presumably mean multiple insulated storage tanks, which could certainly get complicated. As you say a solar powered circulator would distribute heat to shaded areas of the floor, and that would be cooling the sunny areas.

  3. Riversong | | #3


    I wrote an article that discussed the successful marriage of a solar mass floor and radiant slab heat for the Apr/May 2010 issue of Home Power magazine: Designing a Passive Solar Slab.

    I don't believe there is any incompatibility between the two. But I would not try to incorporate an inline DC circulator into the system, since it will run long before the solar heat penetrates to the depth of the tubing and may shut off when the heat is at depth. Heat flux through concrete is about 1" per hour.

    Ideally, radiant tubing is placed 2" below the surface of a 4" thick slab, but rarely is this the case. Because the tubing is typically attached either to the sublsab insulation board or to the steel reinforcement, it ends up near the bottom (which also puts it out of harm's way of control cuts or fasteners).

    I would definitely not use a setback thermostat with a high-mass radiant floor - they ARE incompatible because of the high thermal inertia and long lag time.

    The reason thermal mass is so much more efficient in direct-gain than indirect gain is because it is warmed NOT by conduction from the air but by direct radiant transfer between the sun and the floor. A floor at or near room temperature will not pick up much heat from slightly warmer air (indirect-gain), but is fantastically cold compared to the surface temperature of the sun and will pick up considerable heat from radiant energy - even if the heating system is on.

    And, because the heat transfer from the radiant fluid to the concrete is directly proportionate to the delta-T between the two, the radiant heating system won't transfer much if any heat to a sun-warmed floor (once the heat penetrates), but will transfer heat to the colder parts of the slab - thus effectively redistributing heat to the non-direct-gain areas.

  4. user-716970 | | #4

    Would it help to face the PV panel that runs the DC pump in a SW or WSW direction? That way it would come on later and perhaps run a little longer....more in sync with the isolation reaching to the depth of the pipes?

  5. user-716970 | | #5

    You wrote "On the morning of a sunny day, the colder the slab, the better." but on the morning of a cloudy day, the colder the slab, the more uncomfortable the space...

    I don't believe that in a super insulated home, that radiant floors would be slow to respond or that there would be a significant lag time. I think that is more of a problem in homes that are losing heat too quickly. It is likely that fluid temps could be maintained at 80 degrees or less, which might help to overcome the problems you describe.

  6. user-716970 | | #6

    Sorry... I meant "insolation" in Post #4

  7. Riversong | | #7


    I was assuming that you will have a backup heating unit to supply the radiant heat, since you're not going to get sufficient passive solar gain to heat a house in a 9500 DD climate, and my answer was based on that assumption.

    Facing the PV panel toward the west, assuming good solar access in that direction, would certainly make the solar redistribution more effective - as long as the slab at tubing depth could warm to a higher temperature than the fluid, which may not be the case (since the fluid temperature will probably be in the 85°-90° range).

    And the DC pump would have to be compatible with the fluid dynamics of the rest of the radiant system (gmp at tubing loop head loss) and not restrict flow when it's not activated. You might want to consult a highly qualified radiant heat designer or a mechanical engineer.

  8. Riversong | | #8

    I meant "insolation"

    Such insolance!

  9. user-716970 | | #9

    Yes, the radiant floor will be heated by the domestic hot water system. The passive solar will be to help reduce the heat load. I am thinking that the DC pump could easily be run parallel to the main AC circulating pump, each with check valves to prevent backflow. Simple yet elegant? Or maybe insolant??

  10. GBA Editor
    Martin Holladay | | #10

    I'll try to answer your question: "I don't believe that in a super insulated home, that radiant floors would be slow to respond or that there would be a significant lag time. I think that is more of a problem in homes that are losing heat too quickly. It is likely that fluid temps could be maintained at 80 degrees or less, which might help to overcome the problems you describe."

    Here's the thing: thermal mass inside a passive solar home is only useful if residents are willing to accept a wide range of indoor temperatures, with some hours being cool and some hours being hot.

    If a home has a thermostat that always maintains the interior at 72 degrees F, so that the furnace kicks on when the house gets cold and the air conditioner turns on when the interior gets hot, what's the point of the thermal mass? In order to take advantage of the "free heat" of the sun, you need to store it when there's lots of it available. That means, on a sunny afternoon, you need to let the house creep up to 80 degrees F, so that your concrete gets warm. Then, later, the concrete can give off some of that heat when the house cools off.

    Conversely, in order to be ready to store heat anticipated on a sunny day, the concrete (ideally) should be below room temperature at the start of the sunny day, so that it can absorb heat that will soon be available.

    In the past, such temperature swings were accepted as a normal part of life by most people. In some parts of Arizona, New Mexico, and Colorado, there are many months of the year when the temperatures cool off below 72 degrees F every night, and rise above 72 degrees F every day. These climates are well suited to uninsulated walls with a lot of thermal mass (think adobe).

    Now, let's talk about what happens when you embed hydronic tubing in a floor slab. You get a heating system that is much slower to respond than a furnace. If you have a weekend vacation home, and you leave the thermostat at 48 degrees F when you are away, it will take many hours to bring the home up to 72 degrees when you arrive on Friday night. A house with a furnace would warm up faster.

    But a house with in-floor hydronic heat will cool off more slowly if the heat is turned off. The heating system has a built-in flywheel.

    You can see how these two elements of the house are engaged in a push-pull conflict. For passive solar heat, you want a house that fluctuates between 64 degrees F at night an 80 degrees F during the day. That way, your slab will be able to absorb and store solar heat most efficiently. But the in-floor hydronic tubing tends to keep the slab at a warm, stable temperature, and keeps the house close to the thermostat setting.

  11. Riversong | | #11


    No wonder you panned passive solar in your blog "Solar Versus Superinsulation: A 30-Year-Old Debate" - you're talking solar like it's still 1980.

    The reason that the early passive solar homes experienced wide diurnal temperature fluctuations was that they had too much glazing, no overhangs, and too little or too much or inappropriate thermal mass.

    A properly-designed passive solar home, with between 7% and 12% of floor area equivalent in south glazing, overhangs engineered for the latitude and window height, and direct-gain thermal mass in the correct ratio to the solar glazing and of the correct thickness will maintain uniform temperatures AND receive, store, and release free solar heat on a diurnal cycle.

    It is only indirect-gain thermal mass that has to be heated by overheated air, which is why indirect mass is so much less effective than direct-gain mass. Direct-gain mass does NOT require overheated air in order to absorb solar heat - it is directly heated by radiant transfer from the sun to the mass. It has to be in the sun's path (and not shaded by furniture or rugs), of low specular reflectivity, of medium-to-dark hue (reds, browns, greens are best) for good solar absorptivity, with good thermal diffusivity and effusivity, and ideally 4" thick for a floor or 8" for a wall for an effective diurnal heat capacity.

    With a properly-engineered and designed passive solar house, there is no inherent incompatibility between a solar thermal mass floor and radiant high-mass in-floor heat. In fact, I think they make excellent partners, since (as I described earlier) the radiant tubing effectively redistributes solar direct-gain heat to the non-solar parts of the floor.

  12. user-716970 | | #12

    Is there any way to see a copy of your article in Home Power magazine without buying an annual subscription? Yeah, I'm a cheap b&%#ard...

  13. Riversong | | #13


    Send me your email address and a bottle of vodka ;-) and I'll be glad to send a pdf.

    HouseWright (at) Ponds-Edge (dot) net

  14. user-716970 | | #14

    What are your guidelines to shading of south facing glazing? Totally shaded on June 21 and totally unshaded on December 21 ??

  15. Jesse Thompson | | #15

    It's only $10 / yr online. I hope you're not sending Robert vodka cheaper than that...

  16. user-716970 | | #16

    I e-mailed the vodka...hope it gets there.

  17. Riversong | | #17


    Yes, I design my overhang width, header depth below overhang, and sill depth below overhang to get full sun at noontime winter solstice and full shade noontime summer solstice.

    The formulas are then simple:
    Solar incidence on a vertical surface is
    90° - Lat + 23.5° at summer solstice noon
    90° - Lat - 23.5° at winter solstice noon

    OH = sill depth / tan(SS)
    header depth = OH x tan(WS)

    OH = overhang projection beyond window
    sill depth = distance from bottom of glazing to overhang
    header depth = distance of top of glazing to overhang
    SS = summer solstice noontime sun angle
    WS = winter solstice noontime sun angle

    Others argue that it's better to have less shade for the "shoulder" seasons, but I'm more concerned about overheating in summer than about a slight reduction in passive solar potential. Even with full shade at noontime midsummer, there's still south sun intrusion before and after noon - and, of course, through east and west windows. But, with these formulas, the south sun intrusion is very limited.

    I've seen super-insulated homes here in north-central Vermont in which homeowner's have had to install window air-conditioners for summer comfort. That's absurd in this climate. The most that should be required would be lowE shades on east and west windows.

  18. Riversong | | #18

    Oh, and I didn't get the vodka. I was hoping you'd at least email a picture so I can salivate over it.

  19. user-716970 | | #19

    Sorry you didn't get the vodka...darn cyber is a link to tide you over til you can pick your real bottle in person.

    Google Sketchup has a nice feature where you can enter your latitude, the date, and the time of day and then you "turn on the sun". Really cool to watch the shadows move across the model...

  20. Riversong | | #20

    Geez, they wanted to card me to get into that site. Can't they tell from my slow-moving electrons that I'm over 21?

  21. user-716970 | | #21

    They probably wanted to card you because they could see your baby face...

  22. DoctorBeer | | #22

    I know this is a little bit of an old thread but it is pertinent to the design for the home I'm about to build. Basically I'm trying to figure out how to calculate the proper amount of thermal mass to put in the flooring of the main level and haven't been able to find a good formula to use for doing this calculation based on the actual mass (by weight in this case) wrt south facing window area.

    Riversong says:
    "A properly-designed passive solar home, with between 7% and 12% of floor area equivalent in south glazing, overhangs engineered for the latitude and window height, and direct-gain thermal mass in the correct ratio to the solar glazing and of the correct thickness will maintain uniform temperatures AND receive, store, and release free solar heat on a diurnal cycle. It is only indirect-gain thermal mass that has to be heated by overheated air"

    What formula does one use to properly size the thermal mass?

    My house will be a direct gain system in that the windows will allow light right into most of the living area. It will have 80sf of south facing windows in a south facing walk out basement wall 48ft in length (a 30ft west wall is also walkout, north & east are below grade) which will allow light onto a 4in thick basement slab built over R-10 insulation.

    The upper stories are essentially a standard pitch roof cape with the south wall having a 44ft dormer in it to create a cathedral ceiling area that occupies 1/2 the space of the 1st floor. The other 1/2 of the first floor has a 2nd floor over it. The cathedral ceiling area on the 1st floor (where the thermal mass floor will be) is 824ft +/- that should get mostly illuminated by 120sf of south facing window.

    There is also a TV room of 256sf on the main floor with 20sf of south facing window and a 2nd floor bedroom of 230sf with 20sf of south facing windows.

    I'm planning on putting a cement floor with no radiant sub floor in it at least in the cathedral ceiling area of the house's 1st floor. The reason I don't want to do radiant subfloor is because this will be a vacation home for quite some time so the temperature will be getting set back and if we have radiant subfloor heat it will take a while for the home to heat up when we go up there in winter.

    My research shows cement weighs around 100lb per cubic ft and can hold about .21BTU per lb per F degree. I've seen the amount of solar energy radiating the earth listedas 500BTU/hr/sf but in VT in the winter with a low sun angle I'm assuming only 100BTU/hr/sf. Also assuming 4-6 hrs of useable sunlight for passive heating in the winter that comes out to 400-600 BTU/hr per sf of window per day. With 120sf of window in the cathedral ceiling area that translates into 48,000 to 72,000BTU per day . It can also be thought of as 12,000 BTU/hr.

    To absorb 12,000 BTU/hr with cement that has a thermal capacity of .21BTU/lb/F would require something like 53,333lb of cement for a 1F/hr temperature rise. For cement at 100lb/ cubic foot that means each inch of cement weighs 8.33lb/sf. To absorb that much heat at a 1F/hr rise with a 1in floor would require about 6400sf. For my room which is about 824sf that's 7.75in thickness for a 1F/hr rise.

    If the floor can heat up faster than 1F/hr though then it doesn't need to be as thick. A 2F rise per hour would require a 3.88 thick floor, and a 4F rise per hour a 2in thick floor.

    Assuming I've done all these calculations correctly one thing I don't know is what type of absorption rate I should design for and/or realistically expect. Without knowing that I don't know whether to make the floor 1in thick or 4in thick.

    Can someone possibly please sanity check my assumptions, calculations, etc and provide me some insight about what type of rate of absorption and re-radiation I should be planning around. This is one of those things that you need to get right the first time since tearing out or adding to several inches of cement slab can be a big problem once things get built.

  23. GBA Editor
    Martin Holladay | | #23

    I don't have time to check your calculations, but here are two rules of thumb -- for what they are worth:

    Green Building Guidelines from SBIC: "The rule of thumb is that the thermal mass should be about six times the area of the direct-gain, south-facing glass. ... For most thermal mass materials, their energy effectiveness increases up to a thickness of about 4 inches. Mass thicker than 4 inches typically does not absorb and release heat quickly enough to be effective and worth the additional investment."

    The Green Studio Handbook by Alison Kwok and Walter Grondzik: "A general rule is to provide a concrete mass of 4-6 inches thickness that is about 3 times the area of the solar glazing. This assumes the mass is directly irradiated by solar radiation. A ratio of 6:1 is generally recommended for mass that receives only reflected radiation."

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |