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Best value for building green?

Jeremy Monroe | Posted in General Questions on

I have a limited budget but would like to build my new home as green as possible but need to know where the money is best spent.

I live in climate zone 5 (southern idaho) and will be building ranch style with full basement.

my site is pretty awesome, south facing and no trees to block any passive solar gain.

-exterior rigid insulation: for my climate zone it would need 1.5 inches , is this worth it? I have gotten answers from local insulation contractors that are all over the place , would it be better to spend more on closed cell foam ? then would not have to spend the money on changing the window frames.

-insulating the slab: I was planing on doing 2inch of XPS under the entire slab, but is doing the outside edge just as effective ?

-insulating the exterior or interior of the basement wall? I would love to avoid having to refinish the rigid foam. has anyone done rigid form on the exterior but stopped just short of the grade level ? lets say 6 inches

-Air sealing: planing on Flash and batt method, but would using using ecoseal or owens corning energy complete be more cost effective ?

-what should my end goal be for insulation be for walls and attic? does the 10/20/40/60 rule sound good?


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  1. GBA Editor
    Martin Holladay | | #1

    I suggest that you research these topics on GBA. Most of the answers won't be yes-or-no answers. These will be judgment calls.

    Yes, exterior rigid insulation is a superior approach. Unlike spray foam between the studs, exterior rigid foam addresses thermal bridging through the framing members.

    To insulate the slab, you need both types of insulation -- a continuous layer of horizontal insulation under the slab, and adequate vertical insulation at the perimeter of the slab.

    It doesn't make any sense to insulate only the below-grade portion of your basement wall. After all, most of the heat loss happens at the top, since the exterior air is colder than the soil. Your method would leave a massive thermal bridge -- right through the concrete. For more information, see How to Insulate a Basement Wall.

    There are lots of ways to seal air leaks; flash-and-batt addresses some, but not all air leaks. Ecoseal and Energy Complete are both useful, but most builders use simpler air sealing methods.

    If you manage to hit 10/20/40/60, you will be doing much better than most U.S. builders. It is an admirable goal for your climate zone.

  2. Expert Member
    Dana Dorsett | | #2

    The difference in "whole wall" performance between a 2x6 wall filled with open cell foam and closed cell foam is less than R2, all due to the thermally bridging framing. That performance difference is MUCH cheaper when implemented as exterior foam.

    R7.5 XPS on the exterior will only be about R6.5 in 50 years as it outgasses it's HFC blowing agent. It's much better to go with 1.5" polyiso which will underperform it's rated R9 in mid winter, but won't have long term performance declines. At roughly the same performance point, 2" 1.5 lb density EPS would be R8.4 nominal, but over R9 in mid-winter, and would not have long term performance slide that XPS experiences. The blowing agent for both polyiso & EPS is pentane, a much lighter molecule that is lost more quickly, but a much more environmentally benign gas than HFC134a too. The labeled R value of EPS is the fully-depleted value, but whereas the labeled R-value of XPS is a long term average (IIRC 25 years), but as the blowing agent leaks out performance drops to that of EPS of equal density & thickness. So thinking for the longer term, you'd need at least 2" of XPS, not 1.5".

    The Pretty Good House R-10/20/40/60 rule is "whole wall" value, not center-cavity, and more appropriate to a zone 6 climate than zone 5.

    In zone 5 it's possible to build a Net Zero Energy house with an PV array that fits on the roof with 8/15/30/55, if you keep the windows at or under U0.25 (R4-ish). See Table 2, p10 of BA-1005:

    If the roof is designed and oriented on the site to take advantage of the sun, Net Zero energy with rooftop PV will be cheaper energy than the nega-BTUs of going higher-R than that.

    An example of an R30 wall would be a 2x6 /R20 studwall wall with 3" of exterior polyiso. That would outperform current code-min by 2x.

    Don't- overdo the passive gain or you'll be roasting yourself out even in winter. To limit peak cooling loads, shrink or eliminate west facing windows. Run the roof ridge east-west, and limit any dormers to the north side, to maximize the size of your photon-farming field.

    At 2015 pricing the installed cost of residential rooftop PV is averaging about $3.50/watt (before any incentives or subsidies- see: ), or about $35 per square foot. Utility scale arrays are running $1.50/watt, all-in, and that's a likely price point for residential rooftop at some point in the early to mid 2020s, given the 40 year learning curve trends for PV. (At the anticipated mid-2020s efficiency of new PV it will take less rooftop to get there too.)

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