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Community and Q&A

Choosing and balancing R-values

climb_on | Posted in Energy Efficiency and Durability on

We are in Minnesota (Zone 6a). The home is an “L” shaped, 2200 sq/ft single level, slab on grade with a 28’x40′ attached garage making the “L”

I’m looking to nail down the R-values of the wall, ceiling and slab. If we do a 10″ double stud wall with dense pack cellulose (R30) and 4″ of EPS (R20) for the FPSF…what should we aim for in the ceiling? Is there a formula or rule of thumb to follow to balance these?

We are thinking 8/12 pitch gable roof, 1′ energy heel, and scissor trusses (3/12 interior pitch) over the 26 x 34 living room/kitchen/dining room area.

The reason for leaning toward a 10″ vs 12″ wall is it simply seems like a reasonable balance between cost, interior sq/ft penalty, and window view obstruction. Since I don’t yet know the actual cost difference, I no idea if this is valid or not.

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  1. climb_on | | #1

    Also, we will be heating the garage space to about 40-45 degrees F in the winter. Should I be reducing R-values in the insulation in the garage area or keep same as the rest of the home?

  2. ethan_TFGStudio | | #2

    Once you are building double stud... how much of a cost is is really to make the wall 2" wider?

  3. GBA Editor
    Martin Holladay | | #3

    It sounds like you are aiming for R-values that are a little bit better than code minimum requirements.

    If you are installing cellulose insulation above your ceilings, it doesn't cost much to add a little more cellulose. Code requirements in your area are for a minimum of R-49 of ceiling insulation, so people in your position would usually bump that up to R-60. As I said, that won't add much to your cost.

    -- Martin Holladay

  4. STEPHEN SHEEHY | | #4

    I'd make the raised heel 18" to allow 16" of cellulose under the roof at the eaves. My double stud walls are 12 1/2" thick, with the windows in the middle of the walls. There's no issue with obstructed views. A nice plus is wide interior sills which are nice for plants and cats.

    If you really need an attached garage, I'd make the wall between the house and garage the same as the other exterior walls, i.e double stud and well air sealed. I wouldn't bother with the double stud wall with the rest of the garage, nor would I spring for super windows.

    On our house, the garage is 8' from the house, with a covered porch between the two. We used triple pane windows in the house and double pane, but otherwise identical, in the garage. We set the garage windows to the inside of the 2x6 walls, so from the outside, the window openings match the house.

  5. user-2310254 | | #5


    Do you have a plan for insulating the slab? (If not, see this article for more information:

  6. climb_on | | #6

    Martin, I was thinking 4" of extra insulation, with a thermal break, in the walls was more than a "little bit better" than code minimum...I am certainly am trying to go more than a little more than minimum. Are 12" walls the standard in northern regions for a double stud wall?

    Ethan, I don't really know - haven't done it before. Are you suggesting that it wouldn't add much more cost?

    I had read in this article that "Many builders use two parallel 2x4 walls with a 5-inch gap between them to create a 12-inch-thick wall. Of course, the wall can be thicker or thinner as circumstances dictate." Do they really mean, as climate zone dictates?

    Then this article says PassiveHaus suggests " envelope ratio of 5:10:20:40:60 should be considered a minimum in the northern tier of the United States. In other words, windows at R-5, subslab at R-10, basement wall or slab perimeter at R-20, walls at R-40, and ceiling or roof at R-60. So this tells me my under slab would be over insulated at R-20, as would be the walls at R30.

    Head is spinning now. Ugg.

  7. GBA Editor
    Martin Holladay | | #7

    My phrase, "a little bit better than minimum code requirements," was (like the phrase "pretty good house") and example of New England understatement. I guess such phrases don't always travel well.

    The note that "the wall can be thicker or thinner as circumstances dictate" refers to a variety of circumstances: climate, certainly, but also budget, lot-line limitations, and a desire (in some cases) not to lose too many square feet to insulation. Circumstances vary from project to project.

    In your last paragraph, you mentioned that your head was spinning after reading a sentence about Passivhaus projects. I don't know why this sentence made your head spin. Thermal envelopes on Passivhaus projects have high R-values, but no one is compelling you to meet the Passivhaus standard if you don't want to.

    -- Martin Holladay

  8. Expert Member
    Dana Dorsett | | #8

    A reasonable set of "whole assembly R" performance targets can be found in Table 2 of this document:

    With those numbers it's pretty easy to hit Net Zero Energy with a PV array that fits on the roof.

    A 10" double-studwall cellulose will come in at about R32 whole-wall, if constructed similarly to the 9.5" wall #4 modeled in this document:

    That's probably "good enough" for Net Zero Energy, given than PV efficiency has increase about 25% (and installed cost fallen by more than 60%) since the BA-1005 document was drafted. (Cold climate ductless heat pump efficiency has improved by more than 15% since then too.)

    The R60 compact roof would take 18" I-joist rafters to pull off, but an R75 vented attic with 22" energy heel trusses and loose fill cellulose would be a lot cheaper to build, even when resorting to OSB on the underside of the truss chords to support the weight.

  9. climb_on | | #9

    Martin thank you for the clarification. My head was spinning just wrapping my head around the various resources, suggestions and recommendations for r-values - not any comment you made.

    Dana, those are very helpful documents - thank you. Why do these recommendations, suggest double the insulation at the slab perimeter than the sub-slab r-value? Exposure to wider temperature extremes? I would tend to still go r-20 sub slab since I have to raise the grade on my building site and pay for fill, I might as well fill some of it with insulation instead of sand or rock.

    I do notice on this article the slab detail seems to show 6" of sub slab insulation vs 4" on the edge, so I am curious about that as well.

  10. Expert Member
    Dana Dorsett | | #10

    The slab edge is subject to far lower average seasonal temperatures than at the center of the slab, since there is less insulating soil (and thermal mass) between the slab edge and the great outdoors than under the full field of the slab. The deep subsoil temperatures are pretty close to the 10 or 25 year annual temperature averages, the temperature of the soil in the middle of the slab barely moves over a season or even a decade, whereas at the slab edge it can move several 10s of degrees over 10 weeks.

    Insulation makes for fairly expensive fill compared to sand or stone, but if it's in the budget, go for it. Reclaimed roofing EPS can be pretty cheap, and appropriate for this application. (Don't use polyisocyanurate under slabs, since can wick and retain moisture, whereas EPS will not.)

    The referenced slab detail isn't entirely clear how much EPS is under the slab. It indicates an unspecified amount of sub-slab EPS on a poly vapor barrier, with a minimum 4" of washed aggregate, on an unspecified amount of non frost-susceptible fill. Even though the crosshatching is the same as used for the specified 4" slab edge insulation and unspecified wing insulation, it can't be presumed to be 6" of EPS unless specified. Nothing is actually dimensioned in that drawing, and the "Do not scale from drawing" warning found on many fully dimensioned drawings should be applied here.

    With grade beam type slab on grade, any insulation that extends under the grade beam needs to be fully specified by an engineer, since the grade beam is supporting the full weight of the house.

  11. climb_on | | #11

    Ahh. I was wondering why so many of the drawings didn't show insulation under the grade beam. I will be having the slab design approved by an engineer.

    The attached garage will be conditioned to an average temp of around 45 degrees F, do you think it is prudent to reduce insulation in the garage area (R-19 wall, R10 sub slab insulation)? Should I insulate the common wall between the house and the garage the same as the house or same as the garage?

    Forgot to mention that I will be installing 10k of PV on the house as well. I was hoping to do 1.2k or 1.4k, but I got accepted for a killer solar rebate program that MN has and the max array is 10k. We are super excited about it. With the rebate and since I am installing, our expected payback is about 5 years.

  12. GBA Editor
    Martin Holladay | | #12

    Q. "Should I insulate the common wall between the house and the garage the same as the house or same as the garage?"

    A. This wall should get just as much insulation as any exterior wall in your home. (Remember, just because you intend to heat the garage next year, doesn't mean you will heat the garage in 10 years. It's also possible that you will sell the house, and the next owner will leave the garage unheated.)

    If I were you, I would insulate the garage slab (including the slab perimeter), the garage walls, and the garage ceiling with code-minimum levels of insulation, at least, considering the fact that you plan to heat the garage.

    -- Martin Holladay

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