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Community and Q&A

Zip or Zip R?

arkitkt | Posted in Green Building Techniques on

I’ve been doing a lot of research trying to find the best option for residing by 1920 house located in Climate Zone 4 – mixed-humid. Existing plaster and lathe walls on the interior. I’ve asked a few questions on this website and the responses have been very helpful so I’m asking one more.

I’m planning to remove the existing vinyl siding and wood clapboards and spray foam the walls from the exterior with medium density spray foam to give the stud cavity an R value of about 21.5.

I’ve then debated using Zip sheathing or possibly Zip R to get an additional 3.6R and reduce thermal bridging and then a rainscreen under Boral Truexterior siding. My question is, Zip R has a vapor permeability of less than 1 perm – Zip sheathing has a vapor permeability of 12 – 16 perms.

Will the permeability matter since the spray foam is vapor permeable? Or do I need to allow the wall to dry in both directions (and will the existing plaster walls reduce drying to the interior?) Thanks

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  1. GBA Editor
    Martin Holladay | | #1

    You are wrong about the vapor permeance of Zip sheathing. (The manufacturer, Huber Engineered Woods, is deliberately confusing on the issue. The manufacturer trumpets the vapor permeance of the overlay rather than the vapor permeance of the overlay plus OSB, which is more relevant. The manufacturer wants purchasers to think that Zip sheathing is more vapor-permeable than it really is.)

    The vapor permeance of Zip sheathing is between 2 perms and 3 perms. As John Straube says, this is about the same vapor permeance as other brands of OSB (in other words, ordinary OSB).

    The vapor permeance of Zip R is irrelevant, since a wall with exterior rigid foam is designed to dry to the interior. Whenever you install exterior rigid foam, you can't assume that the foam layer is vapor-permeable. It isn't. But that's not a problem, for the reasons explained in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. user-6184358 | | #2

    The maximum shear wall aspect ratio for the Zip R is different than for the plain zip sheathing. This should be checked if is to be used as bracing for the building.

  3. arkitkt | | #3

    Thank for the replies. I hadn't thought about the structural aspects of the Zip compared to Zip R but I didn't necessarily plan to use it for structural purposes anyway because the existing inside of the exterior walls are braced with 3/4" plaster boards.

    That said, the interior walls are original plaster on the 3/4" boards as opposed to gypsum board (at least in most of the house). And I've read somewhere that old plaster is not that vapor permeable. So will the plaster cause problems drying to the interior?

    Regarding thickness of the exterior rigid insulation, I'm in climate zone 4 so any thickness will work, I'm just trying to figure out if I need my wall system to dry to the exterior with interior plaster - or if the interior plaster is vapor permeable.

  4. GBA Editor
    Martin Holladay | | #4

    Don't worry. The interior plaster is vapor-permeable enough.

  5. Expert Member
    Dana Dorsett | | #5

    The plaster may be vapor permeable, but in a 97 year old house there's a chance that it has been painted over with a fairly low-permeance alkyd (or even more vapor tight, leaded) paint. As long as the framing is dry and the flashing details are right it's still pretty low-risk though.

  6. arkitkt | | #6

    Just got the price back for removing the siding, adding Zip, new siding, etc. Wow. Still might go through with it but I'm considering other options. Like even new vinyl over the old wood.

    I've learned on this website that having the blown in behind the clapboards could cause problems. And in a few areas where we had some spray foam retrofitted in they sprayed directly to the back of the clapboards with open-cell - which I've now learned is also a bad idea.

    So here are my choices:

    1. New vinyl over the old wood, basically what I have now, not very well insulated but passable. As far as I can tell there are no issues behind the existing vinyl but I've only removed on small area. I really hate vinyl but I'm a little scared of the price to do the other options. With this option, is it possible for condensation to occur in the wall cavities?

    2. Remove the vinyl and paint the wood. I understand there can be issues with this but what are the possible consequences of leaving the wood with most of it having blown-in cellulose behind it and a few areas having 4" of open-cell spray foam. I'm guessing the paint won't stick very long and there may be moisture issues in the walls?

    3. Bite the bullet and take the old siding off, new spray-foam, new Zip sheathing (I've decided against the Zip R), rainscreen, and new siding. One concern is getting the siding off where they spray foamed to the back side and leaving the foam in place. The area is above in an open attic so it will be a little more difficult to spray from the outside. I guess we'll just have to redo those areas from the inside if needed.

    Also thought about phasing the project (because we're also looking at over-roofing our unvented roof). Would there be any major problems with removing the vinyl, painting the wood and then removing the wood, spray-foaming, etc. in a few years when the paint starts to fail and we've saved up a few more bucks?

  7. Expert Member
    Dana Dorsett | | #7

    New vinyl over old wood works, since the vinyl is inherently back ventilated, and the wood can dry.

    Paint over old wood is likely to result in peeling paint no matter what gets installed in the wall cavities.

    Leaving the old siding in place and using 2x furring through-screwed to the studs with pancake head timber screws, with open cell foam sprayed between the furring (then trimmed flush) would tighten up the wall considerably and add some R without inhibiting the ability of the wall to dry toward the exterior. A vapor permeable WRB and standard siding (any type) would then be attached to the furring. Building it with a 1/4" rainscreen gap would be even better. There will be window flashing details to attend to for properly directing the bulk water to the exterior side of the new WRB.

    Filling the stud bays with cellulose would be protective of the structural wood, and cheaper than the mid-density foam solution. High R/inch foam in stud bays is a waste, since the thermal bridging of the framing robs it of it's potential performance. An ~R20 center-cavity R-value doesn't usually beat a R13 by more than R1 for "whole wall" R, after the thermal bridging is factored in. You'd get more performance at a lower price out of 3/8" perforated fan-fold siding underlayment. (Perforated underlayment only, in this climate & stackup, if going that route.)

  8. arkitkt | | #8

    Thanks Dana. You mentioned fan-fold underlayment - if I leave the wood and go back with new vinyl (which I really don't want to do but may be what budget allows). I would patch in a few areas with cellulose that I've found with thermal imaging. This would be blown in with the 2-hole method. Would it then be beneficial to use a thin fan fold insulation to provide an air barrier over the old wood before the vinyl goes on? Or just use a house wrap?

  9. Expert Member
    Dana Dorsett | | #9

    Rigid foam or fan-fold foam or housewrap detailed as air barrier applied over the wood clapboards doesn't do much for air tightness- there is too too much bypass channel created by the clapboards. But it's possible to air-seal clapboards from the exterior using with spray foam.

    Sheet goods detailed as air barrier over plank sheathing can be effective though. Most clapboard sided homes built in 1920 had plank sheathing on the exterior of the framing, followed by rosin paper or asphalted felt or other broadsheet flexible material as something of a drain plane/air barrier, then the clapboards. If that's how your house is built, the window flashing (if it exists at all) would normally be lapped to whatever underlayment they had for the siding. If you strip the clapboards it probably gives you enough room to add up to an inch or so of sheet foam under the vinyl siding.

    Fan fold insulation isn't an air barrier unless detailed as such, and PEFORATED fan fold is never an air barrier. But 3/8" fan fold would add ~R1.5 or so to the whole-wall performance, which is more than the difference between R13 cellulose vs. R20 mid-density closed cell foam in the wall cavities.

    A layer of half-inch foil faced polyiso over the plank sheathing would add twice that much R, and can be detailed as an air barrier, taping the seams with foil tape. It would take about 3/4" of polyiso to bring the performance up to current IRC code minimums for zone 4.

    If you're not changing the windows but stripping the clapboards to accomodate sheet foam, a crinkle type housewrap between the sheathing and the sheet foam, lapped properly to the window flashing would be in order. A shiny foil facer adjacent to the air space behind vinyl siding adds another R1 or more of average thermal performance too.

  10. arkitkt | | #10

    Our house does not have sheathing, the clapboards are nailed directly to the studs. So if I go with vinyl I was going to leave the clapboards in place. You mentioned an air-sealing the clapboards with spray foam - are you referring to a fluid applied WRB?

    And were not changing the windows, having the interior wood windows rebuilt, weatherstripped, etc. and installing new Allied Storm Windows, probably with Low E glass.

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