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WRB for painted T1-11 plywood sheathing prior to new siding?

bgabel2 | Posted in General Questions on

We live in Climate Zone 5 in Southeast Michigan and want to add 3 inches of exterior Rockwool insulation, 1×3 furring, then new fiber cement siding over existing, painted plywood T1-11 siding. We believe the T1-11 was installed in 1989, when the house was significantly expanded from the original 1932 lake cottage. In the old section of the house, the ¾” T1-11 is installed over original ¾” horizontal T&G wood siding, and there is 1/2” foil-faced board insulation between the studs and the T&G siding. In the “new” section of the house, there is only ¾” T1-11 and does not appear to be a WRB. My question is, what do I use as a WRB on top of the T1-11, prior to rockwool, rain screen and new siding, since the T1-11 has at least 3 coats of paint on it and acts as a vapor retarder? Do I skip the WRB and just try to seal the joints and seams of the T1-11 as best as possible? Or do I add a vapor permeable WRB? The house is not very air tight currently but this siding upgrade will be the start of a lot of effort to tighten it up as much as possible (including new fiberglass windows, attic insulation and sealing, etc…). Thanks in advance for the advice.

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  1. GBA Editor
    Martin Holladay | | #1

    The existence of 3 coats of paint has nothing to do with whether you need a water-resistive barrier (WRB). The purpose of a WRB is to stop wind-driven rain, not to stop vapor diffusion. (In fact, almost all WRBs are vapor-permeable.)

    For more information, see "All About Water-Resistive Barriers."

    The most common type of WRB in your case would be plastic housewrap, although other types of WRB would also work. Remember that your WRB has to be integrated with your window flashing.

    In 1932, the use of foil-faced insulation board was unusual. Aluminum foil is a vapor barrier, so the presence of this aluminum foil may complicate your insulation plan, depending on how you intend to insulate these walls.

  2. bgabel2 | | #2

    Thank you so much for replying to my question and for all your contributions to GBA. It’s really a fantastic resource. Invariably when I have a question Google points me to content that you’ve written. I have read “All About Water-Resistive Barriers” a few times.

    I should have been more clear with my original question and provided more info. My original thought was to install a liquid-applied WRB (Tremco ExoAir 230) or vapor-open peel-and-stick (Henry Blueskin VP100) to ensure air-tightness, stop wind-driven rain, and allow the assembly to dry to the outside (which is part of the reason for selecting exterior Rockwool board rather than foam); however, after contacting the two manufacturers, neither would provide any assurances that their WRBs would stay adhered to the painted T1-11 over time. That led me to these options, with concerns about each…

    1. Use a liquid-applied or peel-and-stick vapor-open WRB over the painted T1-11. There is risk that it wouldn’t adhere well or for long and its vapor openness doesn’t help since there would still be paint on the T1-11 behind it. Provides air and water tightness, but is not vapor open due to the paint.
    2. Remove all the paint from the T1-11 to assure adherence and make the assembly vapor open to the outside. Then add the liquid WRB. Obviously, lots of added work to remove the paint. Provides air and water tightness and is vapor open.
    3. Add regular, vapor-open house wrap. That’s the easiest option but it is not an air barrier. Would I seal joints and seams of the T1-11 with tape and sealant to provide air tightness under the house wrap? That would provide a water barrier and maybe air tightness but still not vapor open to the exterior.
    4. No house-wrap between the T1-11 and Rockwool, but seal seams and joints. For the most part the latest coat of paint (8-yrs old) has held up well except in one small area that is peeling which I would need to fix, and understand if there is moisture behind it. Still not vapor open.
    5. Tear off all the T1-11 and install Zip or regular OSB with a liquid-applied WRB. I would hate to throw away all that T1-11 (sheathing) that doesn’t appear to be damaged by water or insects upon initial inspection.

    I will be flashing the window and door rough openings with Henry Air-Bloc LF and will integrate to the WRB accordingly.

    I will double-check that the foamboard insulation under the original T&G siding is in fact foil-faced. The whole assembly could very well be vapor open to the inside if it’s not foil-faced.

    Which option would you suggest, or can you think of any others? I would gladly pay more for something that is less work to install since I am doing most of the work myself.

    Thanks again!

  3. GBA Editor
    Martin Holladay | | #3

    I think you are overthinking things a bit. A few layers of paint aren't a true vapor barrier -- they are more of a vapor retarder than a vapor barrier, and therefore not worth worrying about.

    I vote for plastic housewrap. If you choose a high quality housewrap -- Tyvek Commerical Wrap or one of the European housewraps from 475 High Performance Building Products -- and if you tape the seams, the housewrap can act as an air barrier.

  4. bgabel2 | | #4

    Thanks very much, Martin! I intend to take your advice and go with a quality house wrap and tape the seams of the T1-11 and house wrap.

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