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Is this thermal mass floor too thick?

joecoxetti | Posted in Green Building Techniques on

I am currently renovating a earth bermed home.  The home has a 10 inch concrete flat roof.  I plan to put a second floor on this (above grade) and use direct passive solar gain for heating.  The south wall is 32 feet long.  I have no choice about the thickness of the thermal mass here.  I realize I have plenty of mass but everything I am reading recommends keeping the thermal mass thickness to 4 – 6 inches.  Can I accommodate for the increased thickness by going simply going over the 7% rule?  I am in climate zone 1 (Canada)

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  1. Jon R | | #1

    The problem is getting the heat into and out of the passive thermal mass without excessive fluctuations in air temp. An issue at 4-6" and much more so at 10". It will work as is, but don't add more solar input.

  2. Expert Member
    Akos | | #2

    There is really no such thing as too much thermal mass. The only time thermal mass is problematic if you have heated floor as it would slow the response time of the system.

    In your case the 10" of concrete is about R3, the heat you put in there will disperse quite well. Usually the bigger issue is getting the surface to absorb enough heat, this means hard darker surface (concrete or tile). That ends up being the limiting factor to the amount of south facing windows.

    My place has about 10% south facing without a larger thermal mass, can get up to 27C in the winter which nice on those super cold but sunny days. You don't want to go too overboard with the windows, if it does get hot in the winter, you can always open some windows.

    Overall size the windows for the view you want without going too crazy on the surface area.

    1. AlexPoi | | #5

      I totally disagree. I'm an electrical engineer and thermal mass is the same as a capacitor in a circuit. The goal of thermal mass or a capacitor for that matter is to act as a filter: you want to filter out quick changes (high frequencies) but not slower changes (low frequencies). We call it a low pass filter. To put it more concretely, you want to flatten day to night temperature differences but account for day to day changes.

      If you have too much thermal mass, it won't be possible. Your house temperature will always lag behind the desired temperature unless you live in a very stable climate. Your house will probably overheat. The problem with thermal mass is that can you can't choose a precise value for it like you would pick a capacitor for a given circuit. So you always end up guessing and hoping it will work. Personally, I would not bother with thermal mass. I think there is more cons to it than benefits. If you want to store heat, design a solar water heating system where you will be able to design the entire circuit and make it work correctly. Or better, invest in pv panels.

      If you can't control it, it doesn't matter if it's free heat or not...

  3. GBA Editor
    Peter Yost | | #3

    Unless you live in a climate or area that has consistent diurnal temperature swings, and even then, it is really difficult to get the Btus moving in the right direction in the right rate for thermal comfort.

    If you have not, give this a read:


  4. Robert Opaluch | | #4

    Agree with Akos that the 10" depth of your concrete slab isn't the most critical factor here.

    Passive solar (and solar tempered) can work well (only if the building is designed to work well) but only in certain cold-winter climates (not overcast winters). What is your location? Do you know (or can you find out) the percent of sunlight and BTUs/SQFT of solar heat gain (insolation) for your location for January for south-facing windows/vertical surfaces? Need to perform detailed calculations of BTUs/hr solar heat gain to design the building well. Its not difficult, but is tedious (need to use spreadsheets). Without doing quantitative calculations, its unlikely you would guess the appropriate size of windows or design a well-functioning passive solar building. (Just like you need to perform wintertime heat loss calculations to design a building well.) I think this is why naysayers diss passive solar, they use poorly designed or poorly located buildings as examples. I can volunteer to help if you (or someone else on this Q&A) supply some critical data.

    The following article only applies to US locations specifically, since I couldn't find comparable data for elsewhere.

    1. joecoxetti | | #6

      Thank you for responding.
      I am at 44.20 degrees north and 77. 8871 degrees west Zone 1 climate. In jan heating degree days ave 857 with 21 out of 31 days are sunny. percent sun is 31. Hours of sunshine The windows face south. The concrete is already there. We plan to insulate walls to r 50 and ceiling r 100. Square feet of building 1280 main floor and 1280 basement.. main floor has the 10 inches of concrete. Basement has 4 inches concrete with hydronic heat and ceramic tile.

      1. Robert Opaluch | | #7

        I'm attempting to attach charts from, a great web site for climate information. You can specify your location for lots of useful weather and climate information. You noted that your percentage of sunlight in January is 31%, which is low (most of the potential solar gain is blocked by clouds). You also mention 21 of 31 days are sunny, so your specific location may not be what I referenced on WeatherSpark (see attached chart on cloud cover).

        Given the limited information about your location and building plans, here's some rough guesses about your situation. Assume you are located somewhere near Quinte West, ON, but the microclimate of your building site is more important. Assume that wintertime sunlight strikes your second floor south side from 9 or 10AM to 2 or 3PM during mid-winter (when the sun is low on the horizon). So no buildings, trees, etc blocking the sun midday during mid-winter.

        Your location seems geographically and climate-wise most similar to Buffalo, NY in the cities listed in the "Solar Gain" article (
        Therefore, it doesn't seem a good candidate for passive solar design (winter too overcast). Regardless, you have to locate your windows somewhere…and if the majority of the glazing area faces south, the solar gain on clear sunny days in January would be substantial (clear skies 12% of the month on average for Quinte West). The additional 10% of times which are mostly clear would work well too. The problem is that half the time, its overcast in January in Quinte West, with minimal solar heating gains.

        If you used double pane high SHGC glazing (e.g., Cardinal glass LoE-180) to maximize solar gain (but with less insulating value like U=0.25) on about 100 SQFT of south-facing window glass, you could capture more solar gain yet not experience excessive heat losses through your windows. It would be very helpful to minimize heat losses through windows by covering your windows with tight-fitting cellular shades, insulated shutters, window quilts, or something better than loose-fitting curtains at night (which is two-thirds of the time during winter).
        Assuming 1280 SQFT ceiling @ R-100, 32' north and south walls at R-50, and 1280/32=40' east and west walls at R-80…and assuming your cloudiness is similar to QuinteWest, I'm guessing your solar gains would provide about half of your space heating needs for January. On clear average temperature January days, solar gains might equal about all of your space heating needs. However, about 15% of January days are typically mostly cloudy, and almost half of typical January days are overcast, providing minimal solar heat gain.

        So locating most of your glazing facing south would be helpful if you can design your layout to accommodate most of your glazing area facing south. (It would be ideal if you have a south-facing view!). But a mini-split or other wintertime space heating system would be required for perhaps half your space heating needs, especially during overcast periods.

        For those who fail to appreciate passive solar, solar-tempered buildings, and the power of wintertime solar heat gains: If nothing else, lower utility bills, greater resiliency during a power outages, and lower space heating system costs would compensate for additional costs of more efficient windows and window coverings. People fail to pay attention to the costs of installing a central heating system, costs of regular maintenance, repairs and eventual replacement. Air-sealing, insulation, and free solar heat gains are a good deal financially, especially with new construction, when it often costs nothing additional for a better quality structure that eliminates a central heating system.

      2. Expert Member
        Akos | | #8

        One thing to watch for with passive solar is your heat gain during the shoulder season.

        With an highly insulated building, a bit of extra heat can significantly increase your cooling loads. During the build I ended up moving my windows out a couple of inches without increasing the overhang, that bit of extra thermal gain in the fall means running the AC for an extra month.

        To have sharper cutoff on the shading without decreasing winter sun, you would want the overhangs to be higher up from the windows. Generally going with wider instead of tall windows also help. A bit of adjustability is a bonus if you can include it (maybe some movable slats).

        Getting this kind of detail figured out would need simulation work, but it is worth it in the end. Going into it blind will can mean a lot of comfort issues.

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