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Austin ranch house insulation

deerefan | Posted in Energy Efficiency and Durability on

I was able to convince my house design team to review the sizing of the HVAC equipment needed for the project. I now need to finalize the insulation parameters which I would like to use. I was hoping to get some input on this area from the experts on the forum. Here is what I am thinking about going with, please suggest changes if you think differently.

Roof – 1.5″ polyiso on top, 8.5″ open cell foam underneath roof deck

Walls – 5.5″ open cell foam in wall cavity (I considered cellulose but I am not sure about my contractors experience in doing this well), XPS foam on outside. I am not sure about the optimal thickness, I am considering 2″ but is this excessive?

Floor – 2.5″ XPS underneath

I am pretty certain that my architect and mechanical engineer think that I am crazy for wanting this extent of insulation, they do not feel there is a payoff in our climate zone. Am I? Please chime in on specific recommendations/changes for the 3 components.

Thank you, as always.

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  1. GBA Editor
    Martin Holladay | | #1

    First things first. You are in Climate Zone 2. In your climate zone, it's OK to use 1.5 inch of polyiso on top of your roof sheathing -- it's enough, because your climate is mild. So far, so good.

    The total (nominal) R-value of your roof assembly is about R-40 -- near code minimum. It will work.

    Green builders try to avoid the use of XPS for environmental reasons. I advise you to use polyiso for your walls, not XPS, if you decide to install rigid foam on your walls. More information here: "Choosing Rigid Foam."

    You don't describe your foundation or floor assembly. Is this a house on posts? Or do perhaps have a slab on grade? Not sure what you mean by "Floor – 2.5 inch XPS underneath." If it is a slab on grade, and you want continuous sub-slab insulation, EPS is preferable to XPS for environmental reasons. For more information on insulating a slab on grade, see these two articles:

    "Insulating a slab on grade"

    "Determining Sub-Slab Rigid Foam Thickness"

    In general, a continuous layer of sub-slab foam isn't a cost-effective approach in Climate Zone 2 -- stick with vertical foam at the slab perimeter if you care about a return on your investment. It's also probable that it's hard to justify exterior rigid foam for your walls in your climate zone, if you are looking at the investment from an economic perspective.

  2. Expert Member
    Dana Dorsett | | #2

    In zone 2 putting insulation under the slab will probably increase energy use. The deep subsoil temperatures in Austin are a nearly ideal 71-72F, so USE that. About 1.5-2" of EPS at the slab EDGE down to the stemwall footing or bottom of the grade beam would get you there. Finish the above-grade portion of the EPS with any EIFS. In termite zones use a copper clad plastic sill gasket for the framed walls that extends out over the top of the slab edge EPS.

    In Austin it's possible to hit Net Zero Energy with a 2x6/R20 open cell foam wall even without the exterior foam. A 2x4/R13 wall with just 1" inch of foil faced polyiso (if rainscreened siding) or 1.5-2" of EPS (stucco or EIFS) would beat the 2x6/R20 wall's performance by a bit, and would keep the structural wood at a more stable temperature & humidity. An alternative would be a 2x4/R13 wall sheathed with Huber's R6 ZIP-R or 2x6/R20 sheathed in R3 ZIP-R if the detailing of a foam-over terra incognita for the builders.

    The walls & slab may have been overkill, but the roof is not. 1.5' polyiso is good, but bringing it up to 3" of foil faced polyiso would not be insane.

    Take a look at the Zone 2 row of Table 2 on p. 10 of this document:

    Those are "whole assembly R" numbers that account for all the thermal bridging of the framing, the R-values of the interior & exterior sheathing/siding/roofing and interior & exterior air films, not just the insulation elements. A well thought out 2x6/R20 24" o.c. framing package can hit that R15 whole-wall goal without exterior foam.

    Going much further than those numbers is more expensive on a lifecycle basis than rooftop solar leveraged by heat pumps, and at about those numbers it's possible to hit Net Zero Energy with a PV array that fits comfortably on the roof, if the house doesn't have a gazillion hips valleys & dormers.

  3. Expert Member
    ARMANDO COBO | | #3

    The roof specs for your house are great, and that's what we use in DFW and south TX.
    The wall assembly, we use 1" taped and sealed polyiso on the outside in DFW and 1/2" in the south, to avoid the thermal bridging in the summer, 1" is good too.
    I can't believe that you guys can't find a good dense-pack cellulose installer in Austin, not that OC foam is bad, but cellulose, being a hygroscopic material, can help with humidity, plus is recyclable material, and costs less.
    We don't use rigid foam under the slab or outside the perimeter wall in south TX, because I can't justify the cost of installing it and protecting it from termites and ants, besides, Austin has mild soil temps. If we were installing a sealed crawl space, then we install 1" polyiso on the inside of the perimeter. I do think you are getting there.

  4. deerefan | | #4

    Thank you kindly for the replies.

    I will stick to my roof plan.

    I will take your advice and use 1" polyiso on the exterior aspect of wall framing - just to confirm - this is enough? The insulation contractor is trying to discourage me from filling the entire 2x6 frame with open cell, he does not think that the financial return is there. Any thoughts?

    As far as the floors, I should have been clearer, the 2.5" XPS will only be present under the floors of the cantilever portion of the house, not the entire slab.

    I will be meeting with my mechanical engineer to review the HVAC plans. Apart from the window and insulation specs, is there anything he has to know or I should check on before he can make more precise recs?

    Thank you again.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "I will take your advice and use 1 inch polyiso on the exterior aspect of wall framing - just to confirm - this is enough?"

    A. Re-read Dana's answer -- in your climate zone, you don't really need exterior rigid foam on your walls if you don't want to install it. The rigid foam may not be cost-effective from an energy-savings standpoint, but if you want a high-R wall, and you're willing to pay for it, the decision is clearly yours.

    Q. "The 2.5-inch XPS will only be present under the floors of the cantilever portion of the house, not the entire slab."

    A. Will this be cut-and-cobble rigid foam, or a continuous layer under the floor joists? Continuous insulation will perform better than a cut-and-cobble job. In general, green builders try to avoid the use of XPS (because it is manufactured with a blowing agent that has a high global warming potential). Either EPS or polyiso is preferable to XPS.

  6. Expert Member
    Dana Dorsett | | #6

    >As far as the floors, I should have been clearer, the 2.5" XPS will only be present under the floors of the cantilever portion of the house, not the entire slab.

    IRC code min for exposed exterior floors in zone 2 is R13 (presumptively between joists.) Using 3" of continuous Type-II or Type VIII EPS (R12.6)clad with EIFS or something would be higher performance and usually cheaper than 2.5" of XPS, without the performance degradation that XPS experiences over time.

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