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Optimizing R-Value of Sub-Slab Insulation

maxwell_mcgee | Posted in Energy Efficiency and Durability on

Planning out the envelope of a new house in Southern Ontario (CZ 5/6).

Builder is recommending R10 under slab insulation (most likely EPS).

Wondering whether there is merit to pushing this to R20 or R30 even.  I understand it’ll cost marginally more, but realizing this is my one and only shot to add insulation in that location, would it be worth it to spec a higher level of insulation down there?

Any downsides beyond cost or other unforeseen consequences to take into account?

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  1. Expert Member


    Like any inversely related quantity, every additional R value matters less and less. Adding R1 to R10 to get R11, is not nearly as impactful as adding R1 to R9 to get to R10, and so on.. So while it's pretty easy to toss another layer of insulation in there, the benefit quickly decreases as you realize the temperature difference between the inside of the house, and the ground isn't all that large. This is especially true when the perimeter of the slab is insulated.

    The only real down sides are that you could put that insulation in a different place, where it would have more impact, such as the roof or walls, at the same price. For a fixed budget, you'll likely be better off, energy and performance wise, to instead put the money you would have spent on an additional R20 ( $2,000?) into better windows. They're basically holes in the wall when it comes to energy flow, compared to other assemblies, and it makes more sense to make those as good as possible.

    If money isn't a concern, toss it in, there's really no downside.

  2. Expert Member
    Michael Maines | | #2

    Cost and up-front carbon emissions are the main concerns. It's also harder to place plumbing penetrations accurately the thicker the sub-slab insulation, but that's usually easy enough to deal with. The official Pretty Good House recommendation (based on Dr. Joe Lstiburek's recommendations) is R-10 under the slab, but I usually push that up to R-20-ish since it will be there forever. Beyond that, the returns diminish rapidly.

  3. Expert Member
    BILL WICHERS | | #3

    The main downside is cost, and limited value compared to insulating something like an above grade wall. The reason under slab insulation is of "limited value" here (comparatively) is because the average temperature below the frost line is around 18*C or so, which isn't super cold. You get the most benefit from insulation at higher thermal differentials, so insulating a wall with below-freezing temperatures outside will be of more value in terms of reducing energy loss compared with insulating the slab which has more moderate "cold side" temperatures.

    I probably wouldn't go much above R15 under the slab, unless maybe if you were putting in radiant heat. Radiant heat would increase the average temperature of the slab, which would increase the average thermal differential with respect to the ground, which in turn would improve the usefulness of additional insulation under the slab. Assuming a normal slab though, it isn't of much value to put really high R values underneath -- you'd be better off spending the money increasing the R value of your walls (or roof) in terms of bang for the buck.


  4. Expert Member
    Deleted | | #4


  5. maxwell_mcgee | | #5

    Thanks all. This all makes sense to me.

    When costs are fixed, those dollars can potentially be better spent. But otherwise there's no downside (other than embodied carbon depending on what I use down there).

    (In my mind, when I upgrade the enclosure, I'm stealing money from our kitchen finishes and our furniture budget, not from other parts of the enclosure.

    My wife on the other hand may feel differently....)

    1. mr_reference_Hugh | | #7

      Also remember that footings are often a nominal 8 inches a little bit north of you. That means that you have about 7 inches of space in the footings.

      In that 7 inches you will need to fit a few things

      On top of the soil:
      possibly horizontal plumbing, possibly a perimeter drain around the inside of the footing, possibly a radon collection pipe. At this same layer you will need some amount of clear gravel to allow for drainage (at least in our area north of you).

      Above the gravel:
      above the gravel you will be able to fill in with insulation to the top of the footing. You could fill with insulation above your footing and at least one layer would allow you to isolate your footing from the slab. If you built up insulation above the footing, now you are taking away height from the walls/finished ceiling. You may have to take measures to increase the wall height. If you were planning for 8' concrete forms, you will pay extra for the 9' forms (quite common in this era). This is more concrete and concrete is not cheap. If you chew up too much space with insulation, you may want to add a sill plate or 2. So any extra costs are actually adding to the cost to get additional insulation. If you have height limits where you are building, consider whether this would ever become a factor.

      1. maxwell_mcgee | | #8

        Very helpful! A lot of points I hadn't even thought about. Thank you.

        1. Expert Member
          MALCOLM TAYLOR | | #10


          You should never pour your slab directly on the footings and good practice is to include several inches of fill between them and the foam. This leaves a drainage path for any moisture that makes its way in through the joint between the footings and stem-walls, and also minimizes the chances of the slab cracking due to differential settlement.

  6. DennisWood | | #6

    You are in a considerably warmer climate than we are, however I’d be doing R20 for sure (if slab on grade) and skip radiant because it creates a lot of added expense, complexity and materials mechanically when you also need AC and fresh air ducting. Been there, done that. Our frost depth here can exceed 5 feet, so if a slab is on grade, some degree of accelerated heat loss will occur around the periphery..where more insulation makes sense. NRC has quite a bit of information on ground temps over winter across Canada…and the large impact snow cover has on these temps. Undisturbed snow (lots of it) is a good thing if heat loss via your foundation is a concern :-)

    Are you slab on grade, or building a basement?

    There are good guides already out there for building using a shallow frost protected foundation ..a good start if you’re looking for R values and techniques to prevent heat loss. This one is an interesting read:

    1. maxwell_mcgee | | #9

      I'm mostly building a basement for ~70% of the footprint, though part of the house (the attached garage) is at a lower elevation (basically like a split level) and that part will likely be slab on grade.

      I'm in an urban location, so won't have the benefit of lots of snow cover.

      Will review the document you linked to. Thank you.

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