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Community and Q&A

New sheathing or not?

joelebright | Posted in Energy Efficiency and Durability on

I have a 1950’s era ranch house that is in Delaware (zone 4). I am starting a renovation where I have to replace the roofing, siding, windows and doors.

The sheathing on the house is 1×6 t&g boards that have large gaps between them. I also do not have any insulation in the walls. I should also add that my inside walls are not conventional drywall or plaster lathe. It is some type of 16″ wide strips of 1/2″ drywall-like material that is covered with another 1/2″ of mortar. It is strange (and hard as concrete) but I have seen it in a few houses in my area.

My question is, should I keep the sheathing and blow cellulose into the walls, then cover with tyvek? OR, should I remove the old sheathing and replace with Zip sheathing and then blow the cellulose. Does anyone have any better ideas? I would love to use closed cell foam, but it is probably out of my price range.

If I kept the t&g sheathing, would it cause problems with air infiltration?

What if I filled the framing cavities with foam board and used canned foam to fill in the gaps? I haven’t done the math from a cost perspective, but would that be a better idea?


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  1. prominentjustin | | #1

    I would dense pack with cellulose through the existing sheathing then add a min. of 1" xps foam with taped seams and finally add tyvek and your siding choice. Before adding any insulation to your stud cavities you need to be certain that your home does not contain any knob and tube wiring and if so eliminate it in the walls before insulating.

  2. GBA Editor
    Martin Holladay | | #2

    Unless there are signs of rot, there is no reason to remove your existing sheathing. As long as the gaps between the boards aren't too big, your insulation contractor should be able to insulate your stud walls with cellulose. If you are worried about the gaps, talk to your insulation contractor.

    If the gaps are a problem, you can install new OSB, plywood, or ZIP sheathing directly on top of the existing boards without removing them. You can also install a layer of rigid foam, as Justin suggested.

    Q. "If I kept the T&G sheathing, would it cause problems with air infiltration?"

    A. No matter which route you take, you want to pay attention to air sealing. If you keep the existing sheathing, you'll want to install a layer of housewrap with taped seams to lower the rate of air infiltration.

    Adding new OSB, plywood, or Zip sheathing with taped seams would, of course, provide a tighter layer with less infiltration than the old boards with taped housewrap. But either approach would work.

  3. joelebright | | #3

    Thanks guys for your help!

    Justin- Why XPS and not polyiso? And if I did go that route with a 1" layer of rigid foam, would I have to add furring strips before siding? I am using vinyl and worried it might look wavy. Maybe place the furring strips every 8" instead of 16"???

  4. GBA Editor
    Martin Holladay | | #4

    If you install rigid foam, I strongly urge you to install vertical furring strips. If you pay attention to what you are doing, and check occasionally to be sure your furring strips are plumb and co-planar, everything will be fine. Your siding won't look wavy.

    There is a minor controversy about installing vinyl siding on furring strips. Most experts have concluded that there is no problem with vinyl siding installed over furring strips, but a few sources have muddied the waters by advising against it. Read about the controversy here: Can Vinyl Siding be Applied Over Furring Strips?

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