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Wall assembly advice for climate zone 3A

V5Farm | Posted in General Questions on

Hello Team,
Thank everyone for the amazing content on this site, it has been very helpful in making decisions for a new house build I have going.  I am at a cross-roads on the exterior wall assembly, and I would like to get everyone’s advice;
The details;
Currently the walls are 2×6, with 5/8 plywood for sheathing, and an impermeable peel and stick WRB with the windows flashed to the WRB. The house is on a slab-on-grade foundation, and have a 6 1/2” brick ledge since the cladding will be stone up to a water table sill then Hardie Board panels for board and batten.  My question is should I install exterior insulation, and if so what kind and depth, and should I be concerned about a drainage plane behind the insulation since the WRB is impermeable?  Looking for the best design for longevity.

Thanks in advance!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi LP Vernon,

    Why have you decided on an impermeable WRB? What type of insulation are you using within the walls? What is the interior finish? What climate zone are you in? If you are not sure, here is a map. Knowing where you are located will help us offer opinions on exterior insulation.

    1. V5Farm | | #3

      Thanks Brian for the reply, I am in Zone 3A and given that 70% of the cladding will be stone, and the remaining 30% fiber cement...both claddings are reservoir type claddings and my hope was to stop inward vapor drive. I haven’t installed any interior insulation, but was planning on using 5 1/2” of Open Cell Foam to help with air sealing and to possibly allow for inward drying?

  2. Expert Member
    Dana Dorsett | | #2

    In zone 3 there is a lifecycle financial rationale for an R20 "whole wall". See Table 2, p10:

    With 16" o.c. stud spacing the framing fraction is typically about 25%, which reduces the whole-wall R of a code min 2x6/R20 wall to about R14-R15. Adding a continuous 3/4" - 1" of foil faced polyiso would bring that up to about R20 whole-wall.

    As long as the cut bottom edge of the polyiso has a hint of air between the flashing and foam (1/4" is plenty), and the windows are flashed to the exterior (brick cavity) side of the foam it doesn't need to be a strong drainage path between the WRB & foam. If the windows are already flashed to the WRB, there needs to be a thin mesh underlayment between the foam & WRB for managing bulk water.

  3. V5Farm | | #4

    Thanks Dana, Yes, the windows are already flashed to the WRB. My hope is to get close to Net Zero to go off grid in the future. I found a 1” XPS with drainage channels behind it, which would give my R4 exterior insulation ( This might kill two birds with one stone, curious if anyone has used it for this purpose?

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