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Calculating Heat Loss for Below-Grade Foundation

conwaynh85 | Posted in Energy Efficiency and Durability on

I am attempting to figure out what factor to use when calculating heat loss for below grade foundations.  My area has 7,632 Degree days (Fryeburg Maine).

If I were to assume the ground was 45 degrees all year.  This would give me 20 degrees below the 65 every day of the year, which multiplied by 365 would give me 7300 degree days.  But some of that cooling is helping me in summer, so I shouldn’t count it……ahhhh.  So many factors, I’m sure a smarter person than me knows what numbers to use for foundations.  And yes, I know software does it, but I like to understand the math and not just let AI tell me what to do haha.

What number would I use?  Thanks in advance!!!!

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  1. onslow | | #1


    While heating degree days are a somewhat useful measure of seasonal conditions you still need to design for the local code temperature and/or minimum requirements for a basement if the 2018 IECC code is in effect for you. I would guess local design temp is zero or minus something given where you are. You can just go the prescriptive route and put your R15 on either side of the foundation walls and call it a day. However, it is still helpful to know what your losses are when sizing heating equipment.

    I would suggest it is reasonable to take half of the wall area as being exposed to the median temperature point between the code design temp and 32F. Use that to calculate a loss rate for half the wall area. For the other half of wall area, I believe a case could be made for a milder temperature gradient, one that reflects the soil temperature at a depth below frost line, but likely still close to 32F. More on why later.

    For the slab - the entire area against a soil temp that reflects the annual median temperature for your region. In your case, I think that does fall at about 40-45F. I would recommend minimum R15 under the slab as well. More important, make sure the foam under the slab is isolated from wet soil conditions with washed rock. Water has enormous heat capacity so the drier the better.

    Trying to relate heating degree days to the rest of your house structure is actually just as problematic. One can detail wall and window inputs and losses with great detail. Just the same, knowing that north, south, east and west walls will shift in their gains and losses per hour, per day, per season is still secondary to meeting code requirements first. Fancy programs may provide insights to overall energy demands, but you still need to show that you have the heating capacity to satisfy a static condition embedded in your code. The size of your heating equipment is stuck with that parameter.

    How you insulate the foundation, whether crawl space or full basement needs to be carefully considered for condensation reasons as much as heat loss rates. The conductive nature of concrete works to make more than the exposed area cold as the coldest part. Those 6-10" between grade and siding may be looked at as either heat wicks or cold conduits - your choice. This "feature" also means that the colder soil temperatures will work to keep more of your wall cool during the summer. The more your grade falls from sill plate level, the more pronounced the cold wall issue becomes. Stepping framed walls down with grade is one work around that is helpful.

    As you have discovered ground coupling does not always yield expected results when calculating heat loads due to its 24/7 nature. Earth bermed houses face similarly tricky heat load considerations and must take into account the loss of warmish weather, which normally reduces heat loss during shoulder seasons. If cooling loads are the major concern, ground coupling can be an effective strategy. However, it is often a more costly path to take than insulating and designing to reduce heat gain by other means. Burying yourself in the ground also means you need to be much more precise with water control. I am deliberately ignoring window gain/loss for the moment.

    Soil temperature as a truly steady state condition will only exist at depths well below your average basement floor and certainly well below a crawl space slab. The frost depth for your area appears to be 48" for placement of footings. This would mean for a significant chunk of the year your foundation could be "chillin" at 32F or lower. Frost leaves the depths last, especially with wet soil conditions. If you have a garden you probably know this already. Otherwise, consider permafrost.

    I am busy working on a similar heat load analysis for a current build and I have recently purchased an IR thermometer. I will be checking my basement walls tomorrow to see if what I have to say next backs up my beliefs in the choices I made in my current home. So more TL/DR in a day or so.

    1. conwaynh85 | | #2

      Thanks so much for your input and time. To be clear, I have built two homes recently with insulated basements and agree with all of the reasons you stated, heating, moisture etc.

      The part I laughed at was when you mentioned the building code. No one around here has adopted it or enforces is....its very frustrating. So when I try to convince other people to insulate the basements and how to do it well, the only thing they respond to is $. That brings me to trying to do load calculations on concrete.

      Ive been dabling with reducing by factors etc. Then I thought about going through each degree day for a few years and only counting below 65, but above 45, this would cap the extreme cold exterior temp from swaying the number of heating degrees. Then I would add the 20hdd to a fixed amount of days at a fixed ext temp (somewhere between 65-70). Thats my best guess, but I can only assume someone else has done this already.

      Let me know how your IR testing goes, that sounds interesting! What are you calculating for a concrete Rvalue? Ive heard .5 and 1 per inch.


  2. onslow | | #3


    Well, I made temperature checks on several areas of my foundation and floor slabs with the IR temp reader and found that pretty much all the walls read the same regardless of soil depth and sun orientation. Even the wall sections that are against the garage's interior fill differed by only a few degrees. The coldest readings came right at the double door on the down grade side where all the north side snow falls.

    The highest temp was 61F right at the sill plate, the lowest was 46F on the slab edge in contact with the metal door threshold. The middles of wall were 55F and the slab to wall junctions came in at 50F. This is somewhat surprising given the slab edge is almost three feet above the footing on my north wall, yet the slab almost sits on the footing on the south side and is only 6 feet below grade. Apparently having the R15 (reclaimed) XPS encapsulating the foundation has greatly moderated the wide variation of depth and above grade exposure. Most lowest grade exposure is 6" the greatest about 3 feet (other than the door opening).

    The basement is shop space and runs about 65F. The heat is supplied by the 1st floor ambient radiating downward and one 1500W cove heater. While humidity is a bit higher than upstairs, I do not worry about condensation affecting my plywood sheets or my far too numerous cardboard storage boxes. I may have significant advantage in this matter, as we only get 11" of precipitation each year. Based on past visits to relatives near Franconia, local humidity during the summer is high for you. And you get four times as much annual precipitation as I do, so you may find that insulation on the outside of the foundation may not work as well for you as it does for me. I can forsee that 55 degree walls in a higher humidity environ could result in an unacceptable level of damp. You might need to have interior sealed foam on the inside of the walls to avoid that. An ICF foundation could be very useful for you.

    As far as your heat calculations go, I guess you could use the 20 Delta T for half a year and maybe a 10 Deltal T for a quarter year. The remaining quarter year of cool basement might be functionally useful for reducing AC demand, but you still face the issue of high humidity if you can't get the AC to reduce the load enough. Picking up air from the basement could also be problematic since the thermostat upstairs might be satisfied sooner with cool air being mixed in from the basement. The AC would spend less time stripping moisture out of the air stream because it wouldn't run as long. If the volume of air inside the house is not constantly picking up outside air, there should be some point at which it all balances. Unfortunately, I know from real time experience that a spouse who insists on opening the windows for fresh air at night will undo all the drying achieved and the basement will become more humid by virtue of being cooler.

    Since you don't seem to have a code enforcer breathing down your neck, you have the option of deciding just how thin you want to cut it when sizing the heating and cooling. It still boils down to settling on a minimum design temp and calculating for that. The upstairs occupants (if not yourself) will set the thermostat based on that level not the basement. Balancing the heat if forced air may prove more manageable than trying to do it with heat pumps and cassette heads. I am not well versed with those.

    Not sure if any of this is really helping you, so I am not sure how to address the basement beyond suggesting where to put the insulation. I would and will be going with exterior or ICF if the numbers come in close enough to match the labor issues involved with doing exterior insulation on a "normal" foundation. An 8" wall should give R4. Run your numbers to see if you like the results better if insulated from the outside. Best wishes on your project.

  3. Expert Member
    BILL WICHERS | | #4

    If you have a constant set of parameters like "45 degree constant ground temperature" and "65 degrees indoors", then degree days don't really matter since you have constant conditions continually. All you need to do is calculate the BTU loss using the thermal differential, the area of the interface area (fancy way to say the "wall", or whatever is dividing the 65 degree side from the 45 degree side). The R value of that interface will slow down the energy loss.

    You don't need anything fancy to make a calculation like this. Things get fancy with degree days because weather conditions change with the season, and temperatures in any given season don't stay constant throughout the day. When you have constant conditions that don't change, everything gets much easier and you can use simple calculations from the theoretical world, which you can find in the definitions for R value. If you work things through with the basic calculations, you'll get a result in BTU/hr for heat flow through the interface/wall, which you can then use to figure out what you need to do to maintain a constant temperature. If you want to keep a constant temperature, you need to put in the same number of BTUs that you are loosing through the "wall". If you put in more BTUs than you're losing, you'll increase the temperature in the room (which will also increase the heat loss, since you'll increase the temperature differential across the interface). If you put in less BTUs than you're losing, you'll see the temperature inside drop.


    1. conwaynh85 | | #5


      Thank you for your time and knowledge. I understand what your saying and the math is straight forward for the 20f offset. The snap shot of an hour or day is straight forward. My question would be how to expand that to an entire year.

      Im not going to calculate any heat loss in the summer when it helps me, even though the concrete wall is still removing heat. Having said that, It seems I should only count days that im actually heating. This is what brought me back to counting HDD (for the whole year), then deducting anything over the 20f difference.

      Zephyr7, does that make sense?

      Thank you!

    2. conwaynh85 | | #6

      I went back through the last two years of data at for Fryeburg Maine. As a result, I came up with an average of 7,497.05 HDD.
      I also counted 347 days per year which required heating, which sounded high, but I looked back through average historical data and sadly realized how silly cold it is here on avg.

      347 total heat days at a constant 20f difference (65-45) = 6940 HDD
      6940/7497= %92.6

      This seems to imply that on an annual basis, the below grade portion of a concrete wall at a constant 45f, would have %92.6 of the thermal conduction of the above grade portion. So I should be able to use this as a proportion to calculating annual heat loss and not just an hourly or daily snap shot.....Right?

  4. dickrussell | | #7

    As to the R value of poured concrete, I always used 0.08 (English units), for typical foundations with nominal density of 145 lb/cuft. A quick search on "R value of concrete" turns up this reference:, which gives R values for concrete at different densities, perhaps a result of extent of aeration. Anyway, I wouldn't use anything close to 1, as that approaches the R value of wood.

  5. user-1072251 | | #8

    I’m not smart enough to figure all that out, so we’ll use 6” of EPS (r24) under slabs in basements or slab-on-grade, installed over washed stone,with a heavy, taped poly on top and around the sides of the slab, so it’s completeLy isolated. If space is an issue with a remodel, we’ll drop down to 2” if necessary. EPS is cheap, so cost isn’t an issue. I can definitely say that 6” is more effective than 2”. You can also eliminate an inch of concrete, or eliminate it all together.

    1. AdamT | | #9

      Do you put the poly on top of the washed stone and under the EPS or does the poly go between the EPS and the concrete slab?

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