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Exterior Insulation / cedar siding in marine zone (4c?) – Seattle

Jayraja | Posted in General Questions on


We are building a house in the Seattle area (Marine Zone 4c?). Our proposed wall build up on the exterior is as follows :

Plywood Sheathing
Prosoco CAT5 liquid applied air/water barrier
2” of Rockwool (min requirement R3.75)
3/8” Rainscreen
5/8” VERTICAL cedar siding

We wanted the T&G siding to be blind nailed to hide all the nails/screws for a clean look. Apparently Rockwool is under a severe backlog here and he can’t get it until mid-February. The siding contractor is proposing a slightly different buildup. They are proposing :

– 1” foil faced polyiso (taped at all seams)
– 1”x2” horizontal pressure treated wood nail base every 2’ nailed to studs.
– 1” foil faced polyiso installed in resulting pockets for a flush face
– 3/8” Rainscreen attached to horizontal nail base
– cedar siding T&G blind nailed to nail base

I believe the minimum requirements in our zone is R3.75 for the exterior insulation if we R15 or R23 Rockwool batt insulation in a 2×6 wall cavity.

Am I compromising the wall build up if we go with the siding contractors proposal? Alternate strategies?

Any thoughts on this would be really appreciated.

Much Appreciated.
Pacific NorthWET 🙂

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  1. Expert Member


    I'm assuming the rain-screen battens are horizontal for both assemblies.

    Starting with your preferred stack-up in case you go that route:
    - You need to beef up the thickness of your rain-screen battens for a couple of reasons. 3/8" is too thin to keep straight on Rockwool, and provides inadequate holding power for the siding fasteners.

    The wall assembly proposed by your contractor would work with minor tweaks:
    - A 1" thick layer of foam between the battens is 1/4" thicker than the 1"x2"s, meaning it doesn't end up flush, which has the knock-on effect off reducing your cavity depth.
    - Unless you are planning t0 add a WRB over the foam, why have two layers of battens (1"x2" then 3/8"on top) rather than one of the depth you want?
    - In a similar vein, why have two layers of foam, rather than one thicker one with the battens on top?

    The two assemblies differ in a fundamental way as the first allows great drying potential to the outside, and the second doesn't. The implications of that need to be thought through in terms of the rest of the assembly.

  2. Jayraja | | #2

    Hi Malcolm,

    I am a bit confused with the reply. We are installing vertical cedar siding. Perhaps my explanation was not clear. I’ll try and upload a picture to make things a bit more clear.


    1. Expert Member
      MALCOLM TAYLOR | | #3


      Vertically oriented siding requires horizontal battens. Whichever way the siding goes the battens have to go the opposite direction.

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