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Community and Q&A

Zip System vs Rigid foam WRB

Julian Gurau | Posted in Energy Efficiency and Durability on

Hi all, we are planning on building a house to sell, nothing fancy, just a square  1800 sq ft 2 story house. I was planning on using zip system, but another idea  is bugging me, what if i used 1/2 inch of foam as a WRB and just use regular osb, extend the window openings forward and flash  over the foam and tape all seams. This is a budget build, but i would like it to be a bit better than all the builders that use cheapest stuff. Would this be worth the hassle or using zip and carefully taping all seams will be a better decision. I was also thinking of using reclaimed rigid foam and skipping the batts entirely, but at 2.5 inches thick minimum it kinda makes everything harder and more expensive with the rain screen on top. Code is R13 in my area. Sale price should be about 200k. Most people at this price point care about interior more than the building science, but i would just like to be a bit better than the rest.

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Julian,

    I'm guessing that the standard house where you are uses regular OSB, and a sheet WRB like Tyvek. I'm not sure that switching to either of your options yields much. Taping the OSB as the primary air-barrier would make it comparable to the Zip but still allow drying to the 0utside. 1/2" foam adds a lot of labour for very little gain.

    1. Julian Gurau | | #2

      Thank you, for some reason i have more trust in zip then regular osb + tyvek , even if it comes out about $1k more, the house has about 110 sheets. i just thought there might be a cheap way to cancel some thermal bridging, but i guess a tighter house will give better gains. I was comparing pretty much because with the price difference between zip and regular osb + housewrap i might get 3/4 sheets of rigid foam, then only labor would be extra.

  2. Deleted | | #3

    Deleted

  3. Andy S | | #4

    You didn't mention what climate zone you're in, but it sounds like one of the milder ones.
    I really like Zip myself, and I've found that the quality of the OSB that Zip uses is much better than commodity OSB. On the cost side, one of the things that Huber was pushing is that if you factor in the cost of Tyvek and the labor time to hang it, that it should be similar to Zip in overall cost. The nice thing about Zip is that you can tape your walls while they're on the ground so very little ladder work.
    Doing a few more air sealing details and paying attention will give you a better house using Zip. Hopefully enough that it would be noticeable on the inside with no drafts, bugs, and lower energy bills.

    1. Julian Gurau | | #10

      Thank you, i am in climate zone 3 but i would like to try and build something above average what is sold here, without a big extra investment. Comparing to tyvek labor zip is still more money, its about 1k extra in just in material cost. With 1k i could get probably get enough exterior foam to use for the whole house

  4. Max E | | #5

    well the half inch foam would provide more insulation than the zip system, and provide another barrier for the osb, and likely be a little bit more cost effective...

  5. Jon R | | #6

    A fully adhered WRB replaces some tape and provides better resistance to water. Consider 1" unfaced EPS (vs no foam) for much improved moisture performance and a useful R value increase.

    Wall moisture safety is critical - I'd exceed the minimums in the 2021 code, especially with commodity OSB.

    https://www.continuousinsulation.org/content/2021-ibc-and-irc-adopt-improved-vapor-retarder-requirements

    1. Julian Gurau | | #9

      I am trying to stay on a very tight budget, it is a budget home in a budget neighborhood, fully adhered WRB and tape will get too costly, it was either zip or regular osb and foam, and use the foam as a WRB

  6. Expert Member
    Armando Cobo | | #7

    IMO, if you are starting in the business, it all depends whether you want to be known as an above code Builder, one that cares about their clients investment, or a just for profit, same as the rest. You get one chance to start your reputation.
    Charlotte, NC is on the edge of CZ3 and a step away from CZ4, which I would design for CZ4. That said, I normally specify taped OSB with taped 1” rigid foam on top, 2x6 @ 24” o.c. in CZ3, and 1.5” rigid foam in CZ4 with 2x6. Going with 2x4 walls, you can use .5” rigid foam in CZ3, and 1” rigid foam in CZ4. On either wall assembly, we use dense pack cellulose. Better insulation and air sealing.
    Whichever way you decide to go, the most important thing is air sealing. Good taping, caulking, and a good seal at the plates and openings. You can get to 1ACH50 really easy, if you pay attention to detail.
    FYI, the cost increase to a Zero Energy Ready home in the USA is between 1%-3%, according to the RMI study, https://www.greenbuildingadvisor.com/article/insights-from-successful-net-zero-builders, and all my clients.
    Read this: https://rmi.org/insight/economics-of-zero-energy-homes/

    1. Julian Gurau | | #8

      Thank you for the insight,i would like to be known as someone that does better than most, even on a budget, a $1k extra on a $200k sale price is not a lot but i think 1 inch foam will make a difference in comfort and bills.
      I see you are recommending taped osb and taped rigid foam, you think taped rigid foam is not enough of an airseal or this is just in case. Also do you use a WRB under the rigid foam? This is a buget home, so it will get vinyl siding,

    2. Expert Member
      Armando Cobo | | #11

      One step at a time. You cannot sell a better than code house, unless you educate yourself and learn to point out the advantages your house has over the next door Builder. It’s all up to you. People will know when you are BS-ing and when not, so educate yourself, be confident in your work, and highlight your advantages. Today, buyers are not stupid, and often, they are better educated than many builders.
      Must surveys today show at the top of their answers, energy efficiency, healthy house, low maintenance, etc. Every single one of those items, that starts with a tight building envelope… and yes, that means tapping both the OSB and rigid foam. The rigid foam, if taped with their approved tape, it can be a WRB. Some folks use a housewrap between the foam and the sheathing, and some other will install it on top of the foam, to protect it from UV. A lot has to do with how long you are waiting on the siding contractor.
      As I described above my go to wall assembly, and paying attention to air sealing, we’ve gotten around 1ACH50 on all of our projects for the last 22 years, with the exception of one house, at 2.2ACH50, about four years ago, and that was a complicated project… it still gets under my skin. Haha!

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