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Whether or not install house wrap on existing unique wall assembly

kyle_r | Posted in General Questions on

I purchased a home two years ago (Climate Zone 5) that was built as a passive solar home in 1985-86. I add this detail, because it may explain some of the unique wall assemblies.

The basement (from interior to exterior) is 12” block, core filled, with 6” of “Dow Blue Rigid Foam Board” on the exterior, below grade.

The North, East, and West walls above grade (from interior to exterior) are 8” block core filled, 6” of “Dow Blue Rigid Foam Board”, and a 2×4 stud wall (strapped to the block) with fiberglass batt insulation and ¾” plywood sheathing. The plywood has tar paper and original cedar siding.

The South wall is primarily windows and is constructed (from interior to exterior) of ½” plywood sheathing, 2×4 stud wall with “Dow Blue Rigid Foam Board” in between the studs, 2” of “Dow Blue Rigid Foam Board” continuous insulation, and ¾” plywood sheathing. The plywood has tar paper and original cedar siding.

After living in the home for a year, I found the home to be quite drafty. I had a blower door test done and found that almost all of the windows had severe air leaks.

Now to my question, I am replacing the original cedar siding and windows. My initial thought was to wrap the exterior sheathing with Tyvek house wrap and use their system to flash the windows. However, after reading several articles on GBA I became concerned about possible condensation issues. Should I be concerned wrapping the walls in house wrap?

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Replies

  1. MattJF | | #1

    Interesting house you have there.

    House wrap is a waterproof, but vapor permeable material, so condensation is not specifically a concern with that material. It's purpose is to be a water resistant barrier.

    You have rigid foam on the exterior of your assembly that is a potential condensation surface as it is not vapor permeable. In zone 5 you want that exterior rigid foam to make up at least 40% of your total R value, and it seems likely good there from what you describe.

    Your wall should provide the following functions:

    1) Air barrier - This is the first step, define what the air barrier is. This might be your exterior sheathing or interior drywall (sounds like you have interior sheath as well). With all the siding off, you can potentially tape the sheathing. You could have a blower door done after sealing everything as well as possible to ID anything else that can be done.
    2) Vapor barrier - Limits moisture vapor movement through the wall. Your foam should do this well.
    3) Bulk water management - This is the house wrap and integrated flashing for all penetrations. The goal is to keep outside water out of the wall assembly. There are fully adhered house wraps that can be used to accomplish 1 and 3 together.

  2. Expert Member
    AKOS TOTH | | #2

    Tyvek is vapour permeable, you'll have no condensation issues from it. Although the folks at dupot call it an air barrier, it is hard to detail it as such.

    Since your big issue is air leaks, the best spot to fix those is to tape the seams of your sheathing and tape your windows/doors. Make sure you do a blower door test after all is done to confirm the leaks are fixed before re-siding.

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Kyle,

    As both Matt and Akos said, the simple answer to your questions is: No, you should not be concerned at all about installing a permeable houswrap like Tyvek. Installed properly and integrated with flashing, it will protect your walls from rain and runoff, and though you shouldn't rely on it as your primary air barrier, it will help some.

  4. kyle_r | | #4

    Thank you both for the quick response. I will definitely tape the sheathing seams on the South wall. Would you recommend taping the seams on the other walls as well, considering there is a block wall on the interior? I assume block would be air tight.

    1. MattJF | | #5

      The block wall might be air tight, but you will need a strategy at the perimeters. You will need a strategy there. Your air barrier should be continuous, every section of it should be connected to the adjacent section enveloping all 6 sides of your house.

    2. Expert Member
      AKOS TOTH | | #6

      Kyle,

      Painted or plastered block wall is reasonably air tight. The issues are where there are transitions from block to wood. Typically there will be some pretty ugly leaks around rafters/ceiling joists and floor joists. Probably also the connection form your south wall to the blocks.

      Without a blower door it is hard to tell where the problems are, you need to find them first. The big ones are usually easy to find and deal with, the smaller ones are always the problem. With a complicated sounding construction you will probably have some pretty circuitous air paths in there that will take some effort to fix.

  5. Expert Member
    Dana Dorsett | | #7

    Particularly in a retrofit, using FULLY ADHERED highly permeable WRBs such as Blueskin VP100 or Delta Vent SA is easier make air tight than non-adhered housewraps duch as Tyvek. Think of it as really wide air sealing tape. It's more labor intensive to install and more expensive, but not necessarily more expensive than retrofit-detailing the sheathing of a 30+ year old house as an air barrier. With fully adhered WRBs the 10,000 nail holes of cedar shingle siding become a non-issue.

    I'm currently advising on a project on a board & batten sided house that has no sheathing, and the owner wants to tighten it up and re-side with cedar shingles (but will not be installing continuous exterior insulation, which would require reworking too many details around windows and roof overhangs.) The approach is to first dense pack the cavities with cellulose from the exterior, then strip the battens and apply a fully adhered WRB detailed as an air barrier, then a mesh underlayment for the shingles for better drainage. Most of the cavities currently have low density fiberglass batts, most of which will stay in place, dense packing over them. Some of the cavities have brick nogging on the bottom ~2' which will have to be removed. On those sections of the house the plan is to cut the board siding horizontally just above the nogging, and if possible remove any pre-existing batts, then cut 3/4" plywood to fit prior to insulating.

    Without fully adhered WRB the house would probably have to be stripped to the framing and sheathed with plywood or OSB to make it sufficiently air tight.

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