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Installing LP Smartside over 3″ foam

northman56751 | Posted in Green Building Techniques on

Question: I am putting an addition on my house and have a question about fastening the LP Smartside.

My wall details are as follows (from inside to outside).
* 5/8″ sheetrock
* 3/4″ plywood
* 2×6 wall with r-19 or 21 fiberglass insulation
* 3″ foam
* Furring strips, probably 2×4 – has to be at least 1 1/2 thick per LP
* LP Smartside

My question is, what is the best way to go about preventing the siding from sagging (over time) when installing. With a 3″ space between the furring strip and 2×6 stud wall (styrofoam), there is a lot of unsupported weight there. Worried that with all of the weight, over time, the screws will eventually bend down/sag.

I am getting some different opinions on the best route to go……some of them are:
* Move the wall assembly in a little ways onto the floor so the bottom of the furring strip will be supported on the floor (subfloor/rim joist area).
* Move the whole rim joist/floor detail in a ways on the foundation (poured 8″ thick x 5′ deep) and then run the furring strip down to be supported on the foundation itself.
* Install the system in the normal fashion (if there is one!) and not support the bottom of the furring strip. Instead, use large lag bolts every so often on the furring strips to help support the weight, or even run the furring strips up higher and secure to the rafters for additional support……kind of a hanging system

To add to the mess, it sounds like i will have to have 2″ or 3″ of foam on the exterior of my poured foundation walls!! I want to avoid having the foundation foam being proud of my whole wall/siding detail.


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  1. GBA Editor
    Martin Holladay | | #1

    You are worrying needlessly. I suggest that you read this article:

    Fastening Furring Strips to a Foam-Sheathed Wall

    Also, I suggest the you read John Straube's comments on this page: How to Attach a Thick Layer of Exterior Insulation.

  2. Chaubenee | | #2

    Place you HeadLok screws closer if you are afraid. But if you get a very good price, you are still looking at about 55 cents eachassummh you are using 6" screws. But you may want 6.5 inch screw which are even more money. Why is your plywood inside and hit outside?

  3. northman56751 | | #3

    Martin and Joe, thanks for the info.

    I am placing the plywood on the interior wall as I have quite a few large objects and some taxidermy mounts to hang. Using the plywood (3/4") allows me to position them wherever I want instead of directly on a stud.

  4. Chaubenee | | #4

    Is there OSB or plywood on the outside of studs or just the foam?

  5. Irishjake | | #5


    Why 3/4" plywood? You only need 3/8" plywood or 1/2" OSB (less expensive). I did a whole bar/tavern this way, and the benefit was not only being able to hold up beer signs and moose, deer, fox, elk, etc on the walls, but when someone decided to punch the wall, they got a real surprise and it reduced the damage.

  6. user-5946022 | | #6

    It sounds like your furring strips are vertical and line up with your stud walls, correct?

    Are your 2x6 walls 24" oc or 16" oc? Be careful on the LP Smartside. Their Series 38 (standard product) require 16" oc fastening or installed over a SIP; you need to go to their premium Series 76 product to get 24" oc fastening. The info is well hidden on their somewhat annoying website:

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