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Community and Q&A

Boral–Proper installation on vertical siding

Brian Magan | 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. User avatar 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. Steve Knapp CZ 3A Georgia | | #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. Brian Magan | | #3

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

  4. User avatar 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. Brian Magan | | #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. Steve Knapp CZ 3A Georgia | | #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:
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  7. User avatar
    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. User avatar
    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:

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