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Heat Load Calc for Finished Basement with Uninsulated Slab

BryanTroll | Posted in General Questions on

I’m having trouble determining an accurate heat load calc in my finished basement and hoping some can help here. Using Heat Load Pro software demo but open to others if recommended. Basement specs: 1600 square feet, no insulation or vapor barrier under slab 7.5’ ceiling height Basement walls have 2” foam insulation around the perimeter going 2’ below grade Slab is 4” thick, walls are 9” thick Located in Evergreen, CO which is climate zone 5 and a design temp of 0 F. I am using the Heat Load Pro demo program and it is asking for exposed floor and R value of the basement. If I put in R-1 for the floor the results come to 107k BTU heat loss. R-5 in the slab calculates 24k BTU heat loss. Huge difference — want to get my numbers correct here. Can anyone help? Also, are the numbers run differently for basement walls and ceilings being they they are below grade? I’d have to figure that would be different but haven’t been able to confirm. House was built in 1982, originally had hydronic radiant heating poured into basement floor and main level floor. I plan to frame basement walls and add R15 to them. I was hoping to avoid adding insulation to the slab to avoid loss of ceiling height and due to some complexity of install, but if there’s a strong case for it I want to do the smart thing. Thanks for the help! Bryan

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  1. Expert Member
    Michael Maines | | #1

    It's very difficult to model conditioned basements accurately. Heat loss to the ground varies depending on soil type and moisture levels, and the ground temperature varies with depth and time. Heat loss calculations are dependent on the temperature differential between interior and exterior, and the specific heat capacity of the exterior.

    I'm not surprised that there is a large difference between R-1 and R-5 under the slab; doubling the R-value cuts heat loss in half, so increasing it 5X has a big effect. But energy modeling programs are likely assuming that the ground is an infinite heat sink, when in reality the soil under the slab will be warmer than the surrounding soil (in a cold climate).

    Does your program offer the option of an uninsulated slab? Concrete is R-0.08 per inch, effectively zero, so even calling it R-1 will skew your results.

    Is your basement entirely below grade except for 6-12" above grade, or is more of it exposed? Foundation walls exposed to air will perform like above-grade walls; foundation walls against earth will lose heat more slowly. For approximating walk-out basements in BEopt, the program I use, I model it as both fully below grade and fully above grade and average the results. Crude, but close enough for my purposes.

    My guess is that your actual heat load is somewhere between the numbers your program gave you. Adding even R-5 to the slab will make a significant difference in your heat load.

  2. onslow | | #2


    Area x deltaT divided by R, so 1600 x 15 over one IF assuming the underslab ground temp is closer to 55F. If you like to use U values, q =(UxA) x delta T. For R1, U =1. Either way, 24k BTU.
    The program seems to be assuming a delta T about 68F. Normal interior to 0F design temp.

    For a reality check, I am in western Colorado with a full basement clad on the exterior with 3" of reclaimed XPS down to the footing tops. My exterior above grade exposure ranges from 6" to 48". I have checked the bare wall temps near the footing height to the sill plates. The tops are rather warm (65F) due to receiving downward heating from the first floor. We keep temps there at 70F. The lowest part of the wall reads 58F almost year round. I manage to keep my basement shop at 67-68F without much effort.

    The slab sits on 3" of reclaimed XPS ontop of a washed rock bed ranging from 6" to 28" thanks to the underlying bedrock slope. The slab stays a steady 59F. The edge insulation apparently helps. I am inferring that the bedrock and likely much of the soil is a bit cooler.

    For your purposes it will be important to realize that 2" down 2' below grade leaves a lot of direct contact with soil of varying temperature and moisture levels over the year. My walls are fully encapsulated with Grace Bituthene over the foam so my walls do not contact wet soil if indeed there is any. My area is high arid CZ6.

    The heat capacity of dry soil is much less than wet soil so if your walls below the foam are in contact with wet soil your losses will be greater. You also may face a hidden moisture issue with both floor slab and walls that need to be addressed before you finish the basement.

    Do you have any idea if the slab is decoupled from ground water with a bed of washed stone? If yes, it doesn't mean there is no water under your slab, but it does mean some form of drainage was put in. Sump? Daylight exit of footing drains?

    One quick test to do on the slab and the walls is to tape a 18-24" square of clear (thick) plastic to them and wait a week. If the walls or slab are transpiring a significant amount of moisture, the plastic will show the condensate. It may not be a great amount, but before you seal off air exchange with either it will be best to know how moist your concrete really is.

    The losses through the basement walls are quite likely as high or higher than your slab, so insulating them will make a big difference. You just don't want to create a soggy mold farm behind your walls either from the concrete transpiring moisture inward or room air leaks transporting moisture behind the new walls to now even colder concrete.

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