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Sheathing, exterior insulation, and interior poly in Climate Zone 3A

Chris_in_NC | Posted in Energy Efficiency and Durability on

I’ve got a lovely 1989 house on a moderately encap’d crawlspace, complete with wood lap siding and single-hung aluminum windows (not thermally-broken) .  House needs work.
The windows have all had the IGUs replaced….poorly. All windows need to be replaced because they’re leaky, are racked because of bad IGU bonding, and are clear glass with high solar heat gain.  That’s a story for my next post.

I’m thinking about replacing the siding along with the windows, for reasons of best practice for WRB and flashing details.  Also, the siding has some pretty soft areas that are shedding paint, especially on the South side of the building, and around splash areas.

 Now, for the fun part.

Problem number 1:  ThermoPly.  It’s ripple-y under the siding, which is not a great sign, and none of the penetrations for light fixtures, etc., are flashed or air sealed at all.  Standard procedure for the time, I know. Currently I’ve got flat sills and some type of black poly sheet for flashing, which I assume is taped somehow to the ThermoPly.
I can’t think of a great way to flash new windows to existing ThermoPly without removing the siding, and at that point I’d rather replace with decent sheathing and a modest amount of sheet insulation.  

Problem number 2: Poly sheeting on the interior side of the wall.  Climate Zone 3A.  It does not seem to be a issue at the moment (air leakage likely prevents problems), but with a proper WRB, etc., I’m wary of creating a moisture problem in the wall cavity.

What is the solution here?  If I replace the sheathing and add a modest amount of exterior insulation, what would be the proper stackup of sheathing/WRB/insulation (insulation to the interior or exterior side of sheathing), and can the poly retarder be left as-is with no other changes depending on that order of stackup?  Am I overly concerned with creating a moisture problem?

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  1. Expert Member
    Dana Dorsett | | #1

    With the exception of foil types, most WRBs are quite vapor permeable, and won't impede drying toward the exterior. Tyvek is north of 25 perms, Typar is about 10 perms which is still more than 100 x more vapor permeable than 4 mil polyethylene.

    The primary issue would be the vapor permeance of any added sheet insulation. At 1" most unfaced Type-II (1.5lbs per cubic foot density) EPS runs betwen 2.5-3 perms, which is fine- only slightly tighter than standard interior latex paint on wallboard. But foil faced polyiso would run 0.03 perms or less, tighter than your interior side polyethylene.

    At any given thickness XPS has about 1/3 the vapor permeance of Type-II EPS (about 1 perm @ 1") but should be avoided in almost all circumstances anyway on environmental grounds. The HFC blowing agents used in all North American XPS that give it an early-years performance boost over EPS are extreme greenhouse gases, making it's CO2e footprint about 10x that of EPS at any given R-value- by far the least-green insulation in common use today. As the HFC dissipate over the decades it loses performance, and is only warranteed to 90% of labeled R @ 20 years. At full depletion it's performance is the same as Type-II EPS.

    If the new window flashing can be extremely well detailed, the interior polyethylene sheeting isn't likely to cause any issues even with foil faced polyiso, but any bulk water incursions will take forever to dry. If the foam (any type) is thin, installing the WRB and flashing on the exterior side of the foam adds a degree of protection. This is referred to as a "outie" mounted window. If the glass is going to be roughy co-planar to the sheathing rather than the siding it's an "innie" mount, and WRB & flashing would between the foam & new sheathing. Using a crinkle type housewrap, to give it a bit of capillary break and drain space would be advisable (eg Tyvek Drainwrap, Benjamin Obdyke HydroGap, etc.)

  2. Chris_in_NC | | #2

    Thanks for the info Dana; you have my utmost respect since I first started learning from GBA a few years ago.

    You've touched on the unstated part of my question, which is what materials to stay away from to avoid problems, so choice of materials sounds like the priority.

    For this situation, what are your thoughts on things like ZIP panels or the insulated ZIP-R panels? The ZIP-R is polyiso (non-foil), and would be the easy choice from an install standpoint, but there seems to be differing information about perm rating for the coatings on the ZIP family, and for OSB in general. If it's going to be marginal, I definitely fall into the better-safe-than-sorry camp and will avoid that family of products if perm is going to be called into question at all.

    Given the scope of work regardless of the products, and the existing poly barrier, would it be prudent to use plywood sheathing for extra perm margin, and a well-selected WRB to compliment that? I don't want to attack it with overkill, but don't want to fall into a situation of "it'll probably be fine".

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