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No Sheathing with Vinyl Siding

conor_mc | Posted in General Questions on

During my basement air sealing and insulation project, I realized that the bumpout/overhang I have doesn’t have any sheathing on it at all!  When I look in from the basement I can see the vinyl siding.  I could feel the cold air pouring in during the winter.  There’s no good reason for this lack of house sheathing right?  My plan was to remove the vinyl siding locally, cut and install zip siding, use zip flashing tape to tape from the old siding to the new siding, and reinstall the vinyl.  Does this sound right?  How would I properly closeout the new piece of siding to the house/concrete foundation?

Here are some pictures to show the issue:

Climate zone 5A, southern NH.

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  1. Expert Member


    No reason at all for the missing sheathing.
    You may find this article useful in deciding how to proceed:
    Rather than trying to air-seal the bump-out, concentrate on air-sealing between the bay and the house at the foundation.

    1. conor_mc | | #5

      Thanks for the reply Malcom and thanks for linking that article! I'm definitely onboard with air-sealing between the bays at the house foundation. The article calls out solid blocking for this but can I also use rigid foam instead? I'd spray foam the perimeter of the blocking either way. One bay has ventilation going through it so that one will certainly be a lot more difficult. Any good recommendations to air seal that?

      Also, the article recommends putting the rigid foam on the bottom and allowing the vapor to dry inward, but given the 'leaky' looking framing I have, I think I would prefer to cut and cobble the rigid foam on the inside up against the rim joist and the subfloor, and then install regular zip (not zip-R) on the underside and allow the vapor to dry outward. Do you see anything wrong with this plan? some more pictures to illustrate

      1. Expert Member
        MALCOLM TAYLOR | | #6


        Yes rigid foam would be fine as blocking. I'd probably use strips of peel & stick membrane around the pipe - and I agree your plan for insulating the bay so it can dry outwards makes sense.

        1. conor_mc | | #7


          Oh that's a good idea that I wouldn't have thought of! Something like ZIP flex tape? Or something else?

          Thanks for the feedback on my plan!

          1. Expert Member
            MALCOLM TAYLOR | | #8


            I use a lot of this around pipes and other penetrations:

  2. user-5946022 | | #2

    There are two issues here
    1. Air sealing
    2. Insulating the building envelope at the underside of the bumpout

    It looks to me that they used soffit material on the underside of the bumpout.
    I think I would try to buy one of those online kits and spray foam the underside of the bumpout. I would then cut sheathing, attach it to the underside and seal it up everywhere. Since the thickness of the sheathing may lower the underside, I would then finish the underside with Hardi board. I would be concerned about critters getting into the underside and eventually into the house.
    Others on this site will have ideas on the best way to ensure you don't trap moisture & water when you do this,

    1. Expert Member
      MALCOLM TAYLOR | | #3


      insulating and sealing the underside of the bay is unlikely to stop air moving into the conditioned house through the blocking at the rim joist. All the articles describing how to deal with these situations stress the importance of including air-sealing there.

  3. walta100 | | #4

    Seems to me if the spray foam was allowed to come in contact with the vinyl siding, the spray foam would prevent the siding from expanding and contracting in its normal way and the siding would buckle and get wavey (not a good look)

    In my mind the question is we know the installers were cutting corners by skipping the sheathing and water-resistant barrier what else did they cheat.

    When I look at the framing in the photo it seems clear the wood is getting wet sometimes. The way it is with no air barrier and insulation the water maybe carried away by the heat and air leaking thru the bay. There is a real danger if you insulate and air seal that mold and rot could destroy the bay in a few years.

    I see the bay as a can of worms. I would leave it as is because it seems to have found a balance where it is not rotting away. Seems to me if you are determined to make changes you need to be prepared fix every short cut the siding installers made and seems likely you find lots of ugly stuff once you see behind the surface. Are you prepared to replace 100% of the siding flashing and water resistant barrier on the whole house?


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