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Community and Q&A

Zip-R panels for new home construction in Climate Zone 7?

amanto | Posted in Green Building Techniques on

I’m designing a new home in the Upper Peninsula of Michigan — still in the drawing phase, but quickly moving towards a set we can build from. The home is a ~1000 square foot, single story over a full conditioned basement. My builder has proposed the following wall section for the 1st floor, from inside to out:

5/8″ GWB
6mil poly sheet – air barrier 
2×4 studs with R-15 fiberglass insulation
2″ XPS rigid foam
7/16″ OSB sheathing
Taped Housewrap – air barrier and WRB
LP SmartSide horizontal siding – bulk water barrier

Initially, I thought I’d like to go with liquid flashed Zip sheathing and 2″ of exterior XPS and a rain screen, but my builder says that unprotected exterior insulation will be vulnerable to bugs and rodents. Has anyone else run into this issue?

From what I’ve read on GBA and elsewhere, the 2×4 with 6mil poly seems like a tried and true technique for my area, but isn’t that redundant and even counterproductive once exterior continuous insulation is added, since it acts as an air barrier anyways?

After doing more reading, I came across Zip-R, which seems like it might be the best of both worlds. They recently released R-9 and R-12 versions of the product, which would allow me to hit the 13+10 outlined in the IECC in CZ 7. Are there any issues if the wall section looks like the following?

5/8″ GWB
2×4 studs with R-15 fiberglass insulation
Liquid flashed Zip-R – air barrier and WRB
Vertical 1×4’s rainscreen
LP SmartSide horizontal siding – bulk water barrier

I’m planning on using taped OSB on the underside of the roof trusses as an air barrier and tying it into the wall over the top sill. Would the Zip-R panels need sealant applied between the panels as they were nailed in place to keep air from getting between the foam and to the panel edges?

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Replies

  1. rockies63 | | #1

    Hmm, no expert yet has replied so I'll give it a shot.

    In climate zone 7 I think you'll need some exterior insulation in order to keep the sheathing warm enough that moisture in the wall cavity doesn't condense on the inside surface of the sheathing. A 2x4 wall on its own won't contain much insulation so I doubt it will meet code for your climate.

    https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

    Click on the main picture at the top of the article and go to image 3 to see a guide for calculating the amount of exterior insulation you'll need for your climate zone.

    If you do install a ZIP-R system (or exterior rigid insulation) then you shouldn't have 6 ml poly on the interior side since it will create a wall where moisture can't escape in either direction. If your inspector insists on a barrier on the interior try Membrain.

    https://www.certainteed.com/building-insulation/products/membrain/

    I would probably do:

    5/8″ GWB
    Membrain– air barrier
    2×6 studs with R-15 fiberglass insulation
    7/16″ OSB sheathing
    Taped Housewrap – air barrier and WRB
    2″ XPS rigid foam - taped at seams
    Vertical 1×4’s rainscreen
    LP SmartSide horizontal siding – bulk water barrier

  2. Jon_R | | #2

    Spend substantial time thinking about air barriers and blower door testing. Two air barriers (interior and exterior) are better than just an exterior one.

    You can use these regarding vapor barriers/retarders:

    General info: https://www.victoria.ca/assets/Departments/Planning~Development/Permits~Inspections/Example~Plans/Illustrated-Guide-R22-Effective-Walls-In-Wood-Frame-Construction.pdf

    Various climate zones:
    https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers

  3. AaronBeckworth | | #3

    Andrew,

    There is no reason to shun Zip tape; it’s a tried and true method for attaining an effective air barrier if detailed correctly. Save the Zip liquid flash for flashing window and door rough openings and other difficult to tape transitions.

    As for OSB on the underside of the trusses, I remember seeing a photo posted on this site awhile back that elegantly solved the problem of tying the air barrier at the exterior wall to the interior ceiling. The owner/builder had strips of OSB nailed onto the top plate and taped to the exterior sheathing before trusses were set. The strip of OSB was a bit wider than the top plate and allowed for the ceiling OSB to continue the air barrier uninterrupted from interior to exterior.

    Good luck,
    Aaron

  4. Aedi | | #4

    Your builder's assembly is a little strange. It is uncommon to see 2" XPS between the structural sheathing and the studs, and I would be worried that the foam would inhibit the ability of the structural sheathing to do its job bracing the walls. I'm not even entirely sure it is up to code. 2" is about the maximum foam thickness of Zip-R panels, but they do a lot of engineering to make sure it is structurally sound. I think your assembly is superior, and will have better air sealing.

    Some people have had issue with ants in rigid foam specifically. I do not think it will be an issue Michigan, so long as you are not completely careless. Include the usual insect screen details, and make sure your walls deal with bulk water appropriately (so the foam is not constantly wet), and you will be fine. In particular, spec wide roof overhangs, 2-3' if you can. If you are really concerned, you can look into mineral wool board insulation. Not only is it fire- and insect-proof, it is also draining, so you can place the WRB underneath it.

    While your assembly is better than your builder's, but both are basically code minimum insulation. In climate zone 7, it almost always* makes sense to beat code, as the extra insulation cost will be quickly offset by energy savings. The best way to do that depends on prices in your area, and what the local builders are comfortable with. In particular, see if there are any vendors of reclaimed foam in your area, as that makes the extra insulation very affordable. Here are some possible assemblies (note that all of these will also have interior drywall and rainscreen/siding):

    2×6 studs with R-19 fiberglass insulation -> 7/16″ OSB sheathing (taped) -> 3" of rigid foam (taped) -> WRB
    2×4 studs with R-13 fiberglass insulation -> 7/16″ OSB sheathing (taped) -> 2 layers of 2" rigid foam insulation (outer layer taped) -> WRB
    2×6 studs with R-19 fiberglass insulation -> R9 Zip-R Sheathing (taped) -> 1.5" mineral wool board insulation.

    Aesthetically, the last one is my favorite, but it is probably the most expensive. Like I said, it all depends on prices and builders in you area.

    *The primary exception is plans with lots of glazing.

  5. amanto | | #5

    Thanks for the feedback ya'll.

    @Scott Wilson: You're not the first person to mention MemBraine for an interior vapor retarder. It looks like Intello or ProClima are also options. I've also heard that regular craft-faced fiberglass batts can also do the trick.

    @Aaron Beckworth Thanks for mentioning the tape -- that's best/manufacturer's-specified way to go. It seems like the LiquidFlash is probably best used around doors and windows, although I've seen it used on nail holes and panel seams. Using liquid on the seams might just be more expensive there, or even take the system out of warranty?

    @Aedi Thanks for the suggestions. We don't have any tall walls, and we'll have 24" overhangs on all sides, so that should help a bit. I think we're set on the 2x4 wall, but finding a method for adding exterior insulation/creating a tight envelope that our builder is comfortable with will be the primary goal.

  6. Expert Member
    Dana Dorsett | | #6

    While not a super-risky assembly, 2" XPS on the exterior and sheet polyethylene on the interior creates a moisture trap. Ideally you'd have enough exterior R to be able to skip the interior vapor retarder and use only a Class III vapor retarder such as standard interior latex paint. In US climate zone 7 R10 is the absolute minimum necessary for dew point control at the sheathing for 2x4 framingif using a Class III vapor retarder on the interior (instead polyethylene):

    https://up.codes/viewer/wyoming/irc-2015/chapter/7/wall-covering#R702.7.1

    At 2" XPS is only warranteed for R9, and is likely to eventually hit R8.4 at full depletion of it's climate damaging HFC blowing agents. It's not really enough, but with a smart vapor retarder or half-perm paint on air-tight drywall it would be OK. A 2" XPS + 2x4/R15 wall DOES (barely) squeak by the IRC 2018 code minimum for thermal performance, but.

    An improvement would be going with 2" of just about any vendor's polyisocyanurate, which would deliver R5-ish performance even after derating for temperature. At 2" of Dow Thermax polyiso would be a true R12+ across temperature, and would not need to be derated. Trading the interior polyethylene for 2 mil nylon (Certainteed MemBrain) would give it an order of magnitude more drying capacity toward the interior than 4-6 mil polyethylene, while still performing as a Class-II vapor retarder whenever the foam/fiber boundary temperature drops below 35F, which is sufficient to limit moisture accumulation over the winter to reasonable levels.

    To put the foam layer between the studs and sheathing will require shear panels or let-in bracing. Without the shear panels or let-in bracing structurally more sound to put the foam on the exterior side of the sheathing such the sheathing is structural, and keeps the sheathing warmer/drier, and less likely to rot. If going with the original stackup insist on the bracing or shear panels, and be sure to include a rainscreen/air-gap no thinner than 3/8" between the SmartSide and OSB.

  7. Expert Member
    Peter Yost | | #7

    I don't think your concerns regarding insects and rodents and rigid foam insulation are misplaced. The thin I like about Huber ZIP systems is their attention to detail. I am attaching a couple or so pdfs from Huber on how to terminate their panel systems, including for ZIP-R.

    In addition to Huber's ZIP tape and Liquid Flash, you should consider their Stretch tape. Pretty amazing for window rough opening sills and even treating wall penetrations.

  8. jrsevy | | #8

    I've looked at many similar options for our new home to be built this spring. I've settled on Hardie siding, Benjamin Obdyke Sliker rainscreen, Zip-R12 all Zip taped, stretch window flash and use liquid flash where needed, 3" closed cell between studs with no vapor barrier between studs and drywall.

    I've read anything less than 2" of exterior insulation in zone 6/7 could cause condensation in batt insulation. Our ceiling will have 2" closed cell on top to seal can light foam covers and then blown-in insulation on top of that for a R-50+ roof.

    I wouldn't count on the Zip insulated sheathing to be 100% air tight and since the ceiling will be spray foamed, why not have the walls sprayed too for probably better air tightness?

    I'm currently comparing a poured foundation to an ICF foundation from quotes coming in.

    I've attached my wall detail.

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