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Good WRB over beveled lap siding? – no sheathing

adepaoli | Posted in General Questions on

Seattle, WA  – 4c

Wondering what the best WRB to use over top beveled lap siding? Residing a house that has no sheathing, nor WRB – very leaky, but also high drying potential haha. Will be replacing almost all the window, and residing 90% of the house. My thought would be to apply the WRB over the existing beveled lap siding, then add furring strips over the studs, then the new Hardie lap siding.  The existing lap siding obviously seams like a very poor substrate for any WRB, but we are most likely going with this assembly to save money over taking the siding down to studs and installing sheathing.

Previously I’ve tried to come up with a cost effective way to take the existing siding completely off, that way we could add insulation into the stud bays (currently none), add sheathing, rainscreen, and the siding. This route seems much easier to achieve a better air barrier and bulk water control layer, but obviously much more expensive.  Any more thoughts on this matter would be very appreciated!

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  1. Expert Member
    Akos | | #1

    I can't see of any way of air sealing this assembly. About the only thing that might work is covering the house in a layer of thick rigid insulation, tape the seams of the rigid for air barrier and install the WRB over the rigid. You would have to move your windows out to match.

    Insulating and air sealing an old house like this is once in 50 year opportunity. I would take off the siding, insulate, re-sheath with OSB/CDX. You can now easily air seal the sheathing. You'll end up with a much more comfortable house and a more robust assembly.

    1. adepaoli | | #2

      In order to make up some labor cost, any reason I shouldn't suggest using zip system sheathing? That way I don't have to sheath and then add a WRB. Additionally, since the walls are 2x4 framed, do you know if we required to get the r-value of the assembly to current code levels (r-21), or can we just improve the assembly and add r-15 mineral wool batt? If we need to get it to r-21, maybe using the zip-r6.6 sheathing would be good here?

      1. Expert Member
        Akos | | #3

        Any sheathing and WRB combination that is detailed properly will work. I'm partial to CDX with taped seams covered in house wrap, maybe call me olde school. Zip can certainly work as long as proper instalaltion is followed. I've seen a Zip around me installed without seams taped or taped using vapor barrier tape.

        As for R value, you would have to check your local code. Ours has exemptions for certain renovations. I would like to say, that increasing your R value is worth it but in zone 4 it is hard to justify. Maybe strapping out your studs to 2x6 depth and insulating with R22/24 batts might be a simple upgrade, just watch your bracing as it would still need to be installed directly to the 2x4 studs where needed.

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