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Community and Q&A

Foamboard over cedar

Samdbtto | Posted in Green Building Techniques on


I will be starting a fiber cement and new window install soon had questions regarding the install.  My current walls are 2x4s with rockwool bats followed by a 3/4″ fiberboard with felt like facing on the exterior side (thinking its celotex or similar) followed by cedar lap siding.  My original plan was to remove the cedar lap, leave the fiberboard, cover with 1″ foamular tounge and groove, tape seams, followed by tyvek.  The replacement windows will be new construction so was going to install the windows over the tyvek/foam with a piece of 5/4 cedar under tyvek used under the window for additional support.  This all would ve followed by furring on 16s and cement board over it.  Now I am considering leaving the current cedar in place, placing foamboard over it and using spray foam to fill the gaps at edges to seal the space between the foam and original siding.  I feel this will save a lot of time, some money and be less disruptive.  Cedar siding is in good shape and where i have pulled a lap to check, it looks good. Looking for thoughts on doing it this way and if its not a good idea, how does theoriginal plan sound?   Im Illinois, zone 5.  Thanks!

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  1. seabornman | | #1

    I did the same thing and regretted it at every step. The thickness of the installed lap siding varied a lot and, added to less than perfect framing, gave me widow openings that were very irregular. Maybe your situation is more regular.

  2. GBA Editor
    Brian Pontolilo | | #2


    First, to answer your question, I agree with Joel. If you are going to do this, I'd remove the existing siding. Taking down siding isn't difficult, goes pretty quickly, and gives you a clean and even surface to start your work.

    Otherwise, you're plan sounds okay, but here are some thoughts:

    If your siding is in good shape, why are you doing this in the first place? To upgrade performance? Have you already air-sealed and insulated your attic, air sealed around windows from the interior, etc.? There's a lot of low-hanging fruit when it comes to energy upgrades. Adding exterior foam on walls is an excellent upgrade, but maybe not a priority if your siding is in good shape. If everything else is done, go for it.

    I'd also consider installing the WRB (Tyvek) and windows before the foam. I think it is easier to get the flashing details right this way. If you install your WRB over the foam, all of your flashing details need to be at this plane. Have you seen this article:

    1. Samdbtto | | #4

      Thanks for the reply!

      To answer why I am replacing, I am in serious need of new windows and the current windows are unique (1960 "sashless" windows built into wall) extra exterior and interior work. The siding is in good shape that they are all attached and straight but appearance wise there is a lot to be desired and since I am doing windows already I am taking opportunity to improve insulation and exterior appearance with new siding / fresh paint. Cost isn't bad since I am doing all the work. I have air sealed 75% of the attic (some areas unreachable without holes and drywall work) and upgraded insulation. I will also be insulating rim joists while doing this work.

      I am planning to install windows over foam and Tyvek (WRB) flashing right to the Tyvek. So "outtie" windows per the article. Outtie window will give me a nice drywall return on the interior as well.

      I think I will go ahead and do the tear off. As you said, clean and even surface. Makes too much sense, so thanks for that.

      In regards to my rainscreen, what fasteners can i use to attach 1 by furring to the studs? I need to go through 2 1/2" of material to hit studs. I was planning to use a coated washer head lag screw from Spax but they are about 20 cents a screw. Wondering if 1/4" lag screw is overkill and something cheaper will work but maybe using more of.

      1. Expert Member
        1. Samdbtto | | #6

          That's perfect, thanks.

          Looks like #10 and above screws will work for fiber cement with up to 2 1/2" foam.

          1. Expert Member
            MALCOLM TAYLOR | | #7

            I'm always surprised by how little support the furring needs.

  3. Samdbtto | | #3

    My siding is pretty consistent. Unless I inspect almost parallel to a wall its very hard to detect anything and when you are almost parallel all that can be found are slight waves in long runs.

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