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Extra insulation costs reasonable?

benhood | Posted in General Questions on


We are planning to build a new home in climate zone 4A (DC metro area), and I’m wondering about the best place to invest funds for more insulation.
The builder includes this insulation:
Ceiling R-Value: 38
Wood Frame Wall R-Value: 15
Floor R-Value: 19
Basement Wall R-value: 10 (continuous insulation)
Basement Wall R-Value: 13 (cavity insulation)
Slab R-value: 10, 2 ft

And there are some more options that we are considering:
Closed cell spray foam, 0.5 inch thick, in the wall cavities, in additional to the standard batt insulation (hybrid? They already use some spray foam between floors, behind switches/outlets, but this adds to walls): $5500
Change attic insulation to R-49, (and change roof trusses): $1500
Change first and second floor walls to 2×6 with R-21 insulation: $5500
Add R10 rigid foam insulation under basement slab: $4900

The builder has been emphasizing that they create really efficient homes already (HERS rating of 58?), but it seems like during build time is the best time to put in more insulation.

Apologies if I asked something stupid. All of this is pretty new to me. Where would you spend money? Especially to get the biggest bang for your buck? Or are all of these additions overkill? We are hoping to keep utility costs down, with a comfortable home.

Thank you!

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  1. Expert Member
    ARMANDO COBO | | #1

    2015 & 2018 IECC R402.1.2 requires in CZ4A, ceiling R-value of 49 min. and walls R20 cavity (2x6 studs) or R13 cavity (zx4 studs) + R5 continuous.
    I prefer to specify 2x6 walls @24"o.c., with 5.5" R21 dense pack cellulose + 1" R6.7 polyiso rigid foam on top of the sheathing, instead of flash and batt, and less money! Tape all sheathing and rigid foam, and you'll get a better wall, with a tight envelope.
    I would not use rigid foam under basement slab, since you have good basement wall insulation, unless its a walk-out, so I would use 48" under slab at exposed edge.
    "Slab R-value: 10, 2 ft"? Is your foundation slab, basement or both. Slab on Garage?
    Give this to your Builder: (he could buy or google codes)

    1. benhood | | #2

      Thank you so much, Armando. This is great information.
      Unfortunately, it looks like Virginia requires 2015 IECC, but they specifically lower the required ceiling value to 38, and wall value to 15 or 13+1:
      So I will still need to decide if increasing the ceiling to 49, and the walls to 21 is good value for money (though it sounds like the flash and batt isn't as good value as the rigid foam on top of sheathing, if they will do that.)
      Thank you!

      1. GBA Editor
        Brian Pontolilo | | #3

        Hi Ben.

        Flash-and-batt usually isn't worthwhile in new construction. It is expensive and does nothing to mitigate thermal bridging.

        I would think you would at least want to hit the R-value minimums of the current 2018 IRC, which most high-performance builders choose to exceed. However, to know what is a good value and to find the point of diminishing returns, you'll need to have some energy modeling done. Otherwise, it's guess work. You're windows and doors and air sealing details are just as important as the insulation values.

        A good builder will also understand that the money spent on the building envelope can often be saved on mechanicals. But again, this requires someone with experience and/or the ability to use energy-modeling software.

        1. Expert Member
          Dana Dorsett | | #5

          Most of the described insulation IS the IRC 2018 code minimum for zone 4A (which did not change from IRC 2015):

          R38 attics are usually allowed under IRC 2018 on a whole-house performance basis, but for a vented attic the cost delta between R38 and R49 is really negligible. The more important energy efficiency at the ceiling would be air tightness, and keeping all of the mechanical systems (including ducts) under the attic fluff, rather than above the insulation in a hot/cold attic.

          That said better than code is still cost-rational, if applied correctly. (The R10 under the slab is a misapplication of resources.)

          A 2x4/R13+ R5 c.i. wall outperforms a 2x4/R15 wall by quite a bit, or even a 2x4/R21 closed-cell foam insulated wall. ANY closed cell foam between the studs is a waste- do the math:

          Save the high R/inch foam budget for the continuous layers.

          A 3/4 layer of foil faced polyiso on the exterior of an R13 wall would meet code, and be much more moisture resilient, keeping the sheathing partially inside the thermal envelope (for higher wintertime average temp = drier), and with only standard interior latex for the inside vapor retarder the drying capacity toward the interior is great.

          Be sure that the contractor understands that R10 continuous insulation on the basement walls extends all the way to the foundation sill. I see lots of projects with 2" XPS on the exterior that gets trimmed at grade, leaving the above grade portion uninsulated. Inspectors often let that fly if there is insulation in the joists between the basement and first floor, but that's a pretty crummy way to go.

          Installing 1.5" (R9-R10) foil faced polyiso on the interior side of the basement is plenty, as would 3/4" polyiso or 1" faced EPS trapped to the wall with a 2x4/R11 studwall, with no interior side vapor barrier (kraft facers are OK if the flooding risk is low.)

          R10 foam UNDER the slab is complete overkill for a zone 4A location. An continuous inch of EPS (~R4) is plenty for keeping the "musty basement smell" at bay in summer by keeping the slab temp above the summertime dew points, and is even enough to allow putting carpet or a wood subfloor on the slab.

          Any use of XPS under the slab or on the foundation is the opposite of "green". The lifecycle energy savings would never make up for the extreme CO2e footprint of the foam + HFC blowing agents. R10 XPS has about 10x the footprint of R10 EPS or R10 polyiso. If insulating on the exterior of the basement walls(and at the slab edge), 2.5" of Type-II EPS (R10.5) is enough, and a heluva lot greener. See:

    2. Expert Member
      ARMANDO COBO | | #4

      Knowing what you know about cars, would you buy a car without seatbelts or airbags? Just because VA allows a softer code, it doesn’t mean you should accept it. Quality is quality! Always remember, a code house is the minimum LEGAL house you can buy.
      You have a choice to lower size of the house by 10%, or the budget in finishes and décor, but I would never recommend to skip in structural issues, building tightness, insulation and quality installations.

  2. walta100 | | #6

    The only way I know to understand if a give R value will pay for its self in fuel savings over time, is to model your house, in your location, with your local fuel costs, using your guess at inflation and entering the installed costs of different R values. It is no simple equation but luckily our government has made a computer program to do the math and will give it to you at no cost. To use it you will need to watch the training video and enter your data. My guess it is that it will require at least 20 hours to get an answer.

    Once you have done the work you can answer a lot of question quickly with real answers.
    Like is the $2000 up charge to go from R13 to R19 a good investment?
    Is the $4000 up charge to go from U.35 to U.22 windows a good investment?
    Is the $6000 up charge to go from a 10 to a 20 SEER AC system a good investment?
    If I turn the house to face south how that will change my fuel bills.
    Several different heat pumps compared to several different propane furnaces.

    The name of the computer program is BEopt


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