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Installing New Sheathing over Old Sheathing

JohnG2023 | Posted in General Questions on

I am renovating a heated garage built in 1974. I am in zone 5.  It has texture 111 as the sheathing that was painted in the 90’s and is peeling off.  The interior is dry and the T111 is in good shape with no apparent rot or water damage on either side.  I am planning on replacing the man door, garage doors, and windows and the interior walls will be rough cut kiln dried shiplap left natural (interior) as well as the final layer painted (exterior).  There is currently no insulation in the building cavities so I can do whatever is most efficient.  Here’s where I need the help/advice.    Does it make sense to use an insulated sheathing Zip R product and go over the existing sheathing or just Tyvek house wrap and insulate from the interior?    I’d like to go over the existing sheathing rather than strip it.  Is the rough cut for the interior/exterior even a factor with any of the products?  Any advice would be appreciated.

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  1. Expert Member
    BILL WICHERS | | #1

    You probably don't need to go with Zip-R here, you could just put sheets of polyiso or EPS up and go over those with regular OSB (or plywood). Since the T111 is already your structural sheathing, you don't need the structural rating that Zip-R has. I would only consider Zip-R here if it's cheaper than building things up yourself in layers, or if you want the integrated WRB (although you could consider "regular" non-R Zip in that case, with a seperate layer of polyiso or EPS).

    Adding some extra rigid foam won't hurt you here, and will help reduce the energy used to keep your garage warm in the winter, so it's not a bad idea.


    1. JohnG2023 | | #3

      Hi Bill thanks for the info. So just so I am clear you are saying sandwich the polyiso or EPS between the existing T111 and new OSB or plywood then either use Zip or Tyvek WRB before I apply the rough cut as the final layer. Do I need a rain screen or not?

      1. Expert Member
        BILL WICHERS | | #5

        Your stackup would be (from inside to outside) T111, polyiso/EPS, OSB/plywood, WRB (which could be the integrated WRB if you use Zip for the exterior OSB), then your rain screen gap, then siding.

        Rain screens are usually considered to be a good idea, but are generally not required. I would build in a rainscreen if at all possible though as it makes for a more robust assembly due to the much better drying possible with the rain screen.


    2. richmass62 | | #4

      I have a similar question. The building (my house) has a first floor that is 92 years old. The original sheathing is not a plywood type product; it is planks -- probably 1x8 pine.

      So I can add a layer of zip or something structural, maybe Zip r9. But how structural does it have to be? The second floor of the house is pretty new, 18 years old. I am thinking it would be good to add structure but perhaps not necessary to add that much structure. Also I considered stripping the building down to the studs but that would require too much work reinsulating the inside of the walls and there are risks that it would make the whole house unstable.

      Is ZipR9 the way to go, or are there other structural products that insulate, provide limited structure, and are actually available from distributors?

      1. Expert Member
        BILL WICHERS | | #6

        Ye olde way to build a house was with plants on the exterior (often placed diagonally) where we would use plywood or OSB today. There is nothing wrong with the old way. I would double check that you have the top and bottom plates pinned to the studs with those planks though. Diagonal planks almost guarantee this, but if you have horizontal planks, double check the top and bottom planks to make sure things are tied together. The most important thing is that the ends of the studs tie into the top and bottom plates.

        You probably don't need anything additional for structure here, but you'd need an engineer to check things to be sure. If it's just a small area and you're concerned, you could put a layer of plywood or OSB right over those planks to beef things up, and that might be cheaper than an engineers consulting fee. Worst case, you'd be going overboard with the structure that way, but it won't hurt anything. You could then put up exterior insulation in the usual way, you don't really need Zip-R here, although there is nothing wrong with using it -- it's just more expensive is all.


        1. richmass62 | | #7

          The last time I looked at any detail was in an area that was wet and had rot, so you are reminding me that in those parts of the structure I definitely should open up the bottom layer of wood sheathing to see what is going on.

          1. Expert Member
            BILL WICHERS | | #8

            The most common places for moisture/rot problems are the bottom edge near the ground, and around flashing for things like windows and doors. If you've had issues in some areas before, it's worth checking other areas of concern as you move forward with your project.


      2. nynick | | #9

        You'll want to consider what is going to happen to all your window openings, eaves, gable ends and building corners when using any ZipR product. This could add substantial labor to extending each of those to accommodate the ZIP and trimming accordingly. Not difficult, but definitely a consideration.

  2. Deleted | | #2


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