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Community and Q&A

WRB and SIP walls

taybill | Posted in General Questions on

I’m designing a 3 season cabin at 8300 feet in the Colorado mountains.  Climate zone is 4a, with yearly  13 inches of rain, and 64 inches of snow.  Humidity averages 60% over the year.  Typical temperature swings over 24 hours will be 35 degrees.  IOW a dry, cold, semi-desert.

The structure will be built of SIPS, consisting of a sandwich of two pieces of 7/16” OSB wrapping 3-7/8” of closed-cell foam.  The sandwich is rated at R28, with 2 perms/inch.  All the joints will be taped.

The exterior will be cement lap siding.  (James Hardie indicates I don’t need a wrb in Colorado.)  The roof will be standing seam metal.

I’ve been considering Benjamin Obdyke’s Hydrogap SA on the walls for both a WRB and air gap.  However, I’ve got a tendency to over design things, and wonder if I’m doing too much. Not using it would simplify construction and lower the cost.  I’m also concerned about the high altitude UV exposure on any WRB I put on. 

One interesting opinion can be found here.  Written by some folks from Edmonton, Alberta, Canada.

Should I install a WRB on the walls? 

Should it be drainable?  

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  1. Expert Member
    ARMANDO COBO | | #1

    I don't know what CZ map you are looking at, but the 2021 IRC N1101.7(R301.1)Climate zones map shows most CO mountains, specially at 8.300 feet, to be in CZ 6-7. Having said that, I would never design a house with out a rainscreen. If you are on a high-fire zone, BO's Hydrogap is a great option.
    I've never seen a 4" SIP panel to have R28, and the colder the climate/weather, the worst it gets. See the following chart for information...

  2. taybill | | #2

    Picked out the climate zone from Maybe I should be looking here, which gives numbers much more like you suggest.
    SIP panel info from here:
    Turns out I'm in an aspen grove, so no special fire protection needed according to Colorado recommendations.
    Interested in why the Hydrogap would impact fire issues. Is it just the additional "gap".

  3. Expert Member
    Joshua Salinger | | #3

    SIPS can be temperamental. It is critical to get the seams air sealed and to flash and manage bulk water if one is to use them. I have had to 'save' a number of failing SIPS buildings due to poor workmanship. This isn't to say SIPS can't be highly successful, but it is to say one should pay attention to the details as they don't tend to be as resilient as other types of building if not done correctly.

    The four most important aspects of any assembly are, in order of importance, bulk water, air tightness, vapor control and thermal control. Benjamin Obdyke offers a self adhered HydroGap product that will take care of the first 3 of these items on the list with one product. It will also be the easiest and foolproof way to make sure the SIPS are done correctly.

    I would not skip the WRB on your project. I would also highly recommend a rain screen gap.


  4. Expert Member
    Akos | | #4

    The R28 is a bit optimistic for any foam assembly. Most closed cell foam has a long term R value somewhere between R5.5 and R6 per inch once the blowing agent has dissipated.

    Most likely your 4.5" SIP wall with drywall and siding over rainscreen on the outside including air films will end up around an R25 assembly. Not too bad, but nothing special. 2x6 stud wall with R5 rigid will get you similar assembly R value for less cost and none of the SIP issues.

    If you are using the same SIPs for your roof, to avoid ridge rot, make sure to figure out your warm side air barrier as it needs to be perfect plus ensure there is some drying potential under your metal roof panels. Even though you are not in heavy snow country, permeable underlayment and top venting your roof SIP would be a good idea.

  5. Expert Member


    The confusion is probably mine, but it seems like there are two issues getting mixed up here. Your question is about whether to include a WRB, but the links are all about whether the siding requires a rain-screen gap.

    I'm not that familiar with US codes but Martin has always answered questions about this by saying that a WRB is required under the IRC. Most sheet WRBs are rated for 4 to 6 months UV exposure, and once the siding is on there will be no degradation.

    Do you need a rain-screen gap? Cement siding is a reservoir cladding, and OSB isn't great with repeated wetting. Given that the skin of SIPS panels is structural, I would do everything I could to protect it.

  6. plumb_bob | | #6

    I too am of the opinion that SIPs are prone to problems and should only be installed by a contractor with a proven track record with SIPs. I also agree that a WRB is always recommended, even if not required. And rain screening simply makes for a higher performing wall.

  7. Chris_in_NC | | #7

    Some SIP suppliers offer their product with Huber ZIP exterior skins instead of standard OSB.

    1. Expert Member
      MALCOLM TAYLOR | | #8


      That might alleviate a number of concerns.

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