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Community and Q&A

Installing Boral Siding Without Furring

user-6294636 | Posted in GBA Pro Help on

We are building an addition in Western Pennsylvania. The plans called for cedar siding but a change order now specifies Boral Bevel siding installed vertically over 1/2 inch plywood +Tyvek House Wrap + 1 inch XPS T/G foam insulation on the outside. All the Boral literature indicates installing vertical siding on horizontal drainable furring like CoraVent. The architect talked to the Boral rep and she stated that it was OK to install directly on the wall without furring because Boral won’t swell or rot. Any suggestions?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, it's quite unusual to install bevel siding vertically. Are you sure you want to install it that way?

    I am a strong believer in the advantages of a rainscreen gap behind siding, especially when the substrate is rigid foam. That said, I have never installed Boral siding before, so it's possible that you can install it directly over rigid foam.

  2. user-2310254 | | #2


    Is this a 2x4 or a 2x6 wall assembly? The 2x6 framing needs R-7.5 of exterior foam in Zone 5. (I'm assuming you are not in one of the Zone 6 counties.)

  3. user-6294636 | | #3

    Steve, walls are 2x6 with fiberglass batts + 1/2 " plywood sheathing + Tyvek housewrap + 1" T/G XPS foam insulation.

  4. GBA Editor
    Martin Holladay | | #4

    Steve is correct to raise this issue. One inch of XPS is rated at only R-5.

    If your house is in Zone 5, your exterior rigid foam needs to have a minimum R-value of R-7.5.

    If you house is in Climate Zone 6, your exterior rigid foam needs to have a minimum R-value of R-11.25.

    For more information, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  5. user-6294636 | | #5

    I'm not sure I understand your concern. Is your concern the overall R value of the wall. As designed, mine is R-38.

  6. user-2310254 | | #6


    Here the key issue as stated in Martin's article:

    "Thin foam is dangerous, because it reduces the ability of the wall to dry to the exterior without warming the sheathing enough to prevent moisture accumulation (a phenomenon that is usually but incorrectly called “condensation”)."

    Read more:
    Follow us: @gbadvisor on Twitter | GreenBuildingAdvisor on Facebook

  7. Expert Member
    Michael Maines | | #7

    Boral trim is rated for ground contact, so it won't hurt the product any to be installed directly on the WRB. Best practice is to provide a drainage gap behind the siding, and if you go with your stated wall assembly you definitely want it to be able to dry to the exterior, for which you need the gap. If you read the article Martin linked to, the concerns about your exterior to interior foam ratio should become clear.

    One thing to be aware of is that Boral shrinks when the temperature drops. Not a lot, especially compared to vinyl or PVC trim, but a lot more than wood.

  8. Expert Member
    Dana Dorsett | | #8

    When the average wintertime temperature of the structural sheathing is much below the average dew point of the interior air (~38-40F, typically, but it varies), the sheathing risks having mold-worthy amounts of moisture by the springtime, when temperatures are warm enough to promote rapid mold & fungus growth.

    Adding sufficient exterior-R to keep the sheathing a few degrees warmer, or using a Class-II vapor retarder on the interior side to limit the amount of water vapor diffusion mitigates against this risk. The IRC building code prescribes the minimum amount of sheathing R it takes to be able to get by with just latex paint (or other Class-III vapor retarders) on the interior in this table:

    Most of western PA is zone 5, but cooler parts are zone 6. The green counties are zone 5, blue are zone 6:

    If it's not too late, polyisocyanurate is a much greener product than XPS, due to the extremely high global warming potential of the HFC blowing agents used in the manufacture of XPS. As the HFC diffuse out over a few decades the performance of 1" XPS drops from the labeled R5 toward an eventual R4.2.

    An inch of foil faced polyisocyanurate runs R6-R7, and if there is an air gap between the exterior foil facer and the siding it adds another ~R1 of average performance. While most vendors' goods have a derating factor that has to be estimated when used on the exterior side of the assembly Dow claims to have beaten those issues, and Thermax would be the right product to use if an inch is all you have to work with. See:

  9. PeterRS | | #9

    My experience with Boral was a nightmare. Within a year, the product started to warp profoundly under certain atmospheric conditions. I contacted the TruExterior/Boral/Westlake and wrestled with them for six months. My experts tell me the Boral just lacks the rigidity of a HardieBoard product and warps in harsh sunlight. Boral gave me the runaround, stating both that my installation was “beautiful” AND ridiculously that the contractor should have used materials that EXCEEDED those specified in Boral’s written specs. Specifically, Boral told me post-facto that Tyvek House Wrap is an INADEQUATE WRB for Boral and will cause warping. They refused to honor their warranty. Take a look.

    1. Expert Member
      Michael Maines | | #10

      Peter, that sucks. I consulted on a project with the same problems. As far as I can tell, Boral's solution was to sell that part of their business.

      1. PeterRS | | #16

        Since my experience, I've found enough stories nearly identical to mine to make me believe Boral knows there's a problem and refuses to alter course. I seems there are positive experiences out there but so many negative one that the product shouldn't be sold.

    2. Expert Member
      Akos | | #12

      Since the problem is thermal expansion, it can be fixed but won't be pretty as all the siding needs to be removed. At that point, I think you are better off with residing with something without expansion issues.

      1. Expert Member
        Michael Maines | | #13

        On the project I consulted on they replaced all of the siding and the same thing happened again, and not just on the sunny sides. After quite a bit of research I'm not convinced that the problem is only thermal expansion. It's not a product I recommend using as siding.

        1. Expert Member
          Akos | | #14


          What was changed for the reside?

          My thinking would be to reduce the max length of the pieces which adds more gaps thus room for expansion and to hang it with either clips or over something like the thicker cedar breather which allows the nails to bend a bit to allow for movement.

          Kind of strange that this is even an issue as the coefficient of thermal expansion for this is actually pretty low (at least on the spec sheet).

          1. Expert Member
            Michael Maines | | #17

            Sorry, I can't discuss details, and probably already said more than I should.

          2. PeterRS | | #18

            I'm not an expert in these matters. I'm just a guy who regrets using this product and has listened to a number of experts on site explain to me what happened. At the end of the day, I think the problem is that Boral does not have the rigidity of a HardieBoard type product. You can hold both in your hand and one bends where the other does not. It seems in many circumstances, it just warps with any stress on it.

      2. PeterRS | | #15

        I was hoping Boral would refund the material cost and I'd either go with Hardie or stucco but Boral is completely uncooperative.

  10. jimkas | | #11

    I think there are probably also good experiences with Boral. Attached pics are a project in CZ7 of Colorado. Extreme sun, winter to -20 etc and the material has performed flawlessly. It was chosen for fire resistance as this is in an area with a fire risk

    It is straight and true through out. This is installed over furring over blocking over 3" of mineral wool. It is vented top and bottom. Vertical is 1x12 boards and 1x3 battens, horizontal is one of the lap siding profiles. This has been up through two summers and winters with no issues

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