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Trying for R40/R60 in Colorado (zone 5)

DeusExMachinae | Posted in Green Building Techniques on

Sorry if this is in the wrong category, I’m a newbie.

Hi all. My partner and I recently purchased 2.5 acres in the Denver area. We’re in the final design stages of the modern efficient home we would like to build.

We are GCing/self-building this home. It’s our first, but we’re leveraging the experience of my FIL, who has built about 8 homes himself. We decided to use the “family” architect, who has been receptive and great to work with but may be old school.

I’ve been heavily researching the PGH philosophy for about 9 months now. We’re at the stage of deciding what to do about insulation and I’m getting the feeling that my ideas are somehow atypical despite being commonplace in the green building world.

I put my foot down on no spray foam…we don’t want to be responsible for that environmental impact nor the amount of VOCs in the house. 

I initially proposed (inside out): painted drywall>Rockwool batts (2×6 studs)>Zip sheathing>4″ recycled polyiso>furring strips (rainscreen)>siding. Similar design for the roof, we’ll have a vaulted rafter section where we’ll use 11″ batts or cellulose and the same recycled polyiso. Total wall: R-40, total roof: R-60+.

He immediately balked at this as it seems to be very nontraditional for the region. He was also very concerned about the 3 “trips” raising construction costs astronomically. He also pointed out we will have a large 6-panel bifold patio door, so exceptional effort might be wasted.

His proposal is (inside out again): painted drywall>batts/cellulose (2×6 studs)>Zip R-3 sheathing>siding. On the roof he’s suggesting: painted drywall>cellulose/wool batts in the trusses and rafters>OSB>6″ thick polyiso bonded to OSB>ice and water shield>roofing. Total wall: R-20, total roof: R-60+.

I’m not sure what I’m asking but is it worth combining the sheathing, exterior insulation and furring steps into one trip at a loss of significant insulation value and rain screen? Am I way overthinking insulation for our high desert climate?

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Replies

  1. creativedestruction | | #1

    Zip R9 for the walls would be a better compromise. Your proposal is best. He's not wrong that additional trips around the house adds up quickly though. Denver is a fairly dry climate; you can potentially forgo the rainscreen, especially if you have overhangs and a single-story design.

    It's difficult to make value judgements without knowing all of the parameters and goals of the project. Energy modeling can be helpful for this but most folks don't make that investment. Have to go with your gut. (Or write out a research paper's worth of Q&A's!)

    Best of luck,
    j

    1. DeusExMachinae | | #4

      We do have overhangs and a single-story design. My next question to him is if we can go thicker, did you say R-9 because the R-12 can require additional accommodations for shear?

  2. Expert Member
    Akos | | #2

    R60 roofs and R40 walls are only worth it if you are insulating with inexpensive insulation. Your plan of exterior rigid will have a very long ROI because of the cost of the assembly. You can go with rigid in both places, but I would do a bit of modeling first to figure out what R value makes sense. Generally for expensive roofs/walls you are proposing, the best is to make the walls and roofs about the same R value. When it comes to heat loss, there is nothing special about a roof, just another surface with a bit more loss there because of night time radiation.

    If you want high R value and cheap stick to blown in insulation and batts. You can get a pretty high R value roof with oversized I joists or trusses for rafters, install a baffle under the top flange and use that as a vent space. With 24" I joists/trusses very easy to get an R60+ roof.

    Your best bet for high R value low cost wall is 2x8 24 OC with high density batts. Won't get you to R40 but it will significantly cheaper to build that what you are proposing. You can also build walls with I-joists instead of studs for more depth but you would need your structural engineer to sign off on it. Dense packed double stud walls can easily get to R40 for extra cost.

    Either option is much cheaper materials and much easier to build than trying to hit studs through 4" foam with 7" screws.

    1. DeusExMachinae | | #5

      Yeah, I proposed this because there's a local seller of reclaimed polyiso and it's about $0.05 per R psf for 4" 4x8 sheets.

      2x8s were nixed as the cost is almost double that of 2x6s right now.

      1. Expert Member
        Akos | | #7

        You have to look at the labor costs as well. The 2x8 are more expensive, but when you add in the cost of hanging the 4" foam, the cheap foam gets very expensive.

  3. Expert Member
    BILL WICHERS | | #3

    You probably scared him off with the extra thick exterior rigid foam. Up to about a inch of exterior rigid foam is fairly common, up to 2” thick exterior rigid foam isn’t that much harder to do. As you get over 2” thick, things start getting tricky with hitting studs and trim details.

    Double stud walls filled with dense pack cellulose can get up to massive R values at reasonable cost, but a lot of builders aren't familiar with them either. A downside is you tend to get pretty thick walls this way, so your windows look a little like tunnels (to be fair, this is an issue with really thick rigid foam too).

    I would suggest going with Zip R9 or R12 as a happy medium. This keeps a rigid exterior sheathing surface to attach siding, so trim details aren’t much different from the usual. With Zip R12, you can get up around R33 whole wall performance, which is pretty good.

    Bill

    1. DeusExMachinae | | #6

      I think I want to steer away from Zip R12 due to questionable shear capabilities.

      As you and Jason suggested, I think I'll be happy with R-9 sheathing. That's about R-26. The siding system we're looking at installs with an air gap so that will satisfy a rain screen. And we'll cut our trips around in half.

      1. Expert Member
        BILL WICHERS | | #9

        You could look into the possibility of framing your exterior walls on 24" centers to reduce the thermal bridging a bit more. Run that by your architect as a possible option as it's an easy way to squeeze out a bit more thermal performance from the wall assembly.

        Bill

  4. walta100 | | #8

    Take the time to learn how to model your house in BEopt. You can enter your local cost to build each wall it will show you how much it will cost each month to finance and operate each wall in your location then you can select the best option. Be sure to find and watch the BEopt training videos.

    I could never build a model where the R40 wall was a winner.

    I could not stand the idea of Zip+R from a structural point of view.

    I built drywall 2x6 filled with cellulose, Zip 1 inch foam and siding.

    https://beopt.nrel.gov/home

    Walta

  5. user-6184358 | | #10

    In California a mandated energy calculation (Title 24) for a typical residences is about $300.00 - What does it cost in other states. It seems simple here, It gives you all the answers to your energy use questions.

    You have lots of land so why not a double stud wall?

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