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Community and Q&A

Exterior rigid foam and vapor concerns in Climate Zone 4

88Clayton | Posted in General Questions on

Hi, guys!  I’ve been enjoying this website for a while now.  Treasure trove.  This is my first post.

I’m doing a new construction project in Tennessee, Climate Zone 4.  I’d like to use about 1″ of an exterior rigid foam, but am worried about trapping vapor between OSB and the rigid foam.  Being in a mixed-humid climate, my assumption is that I need the wall to dry in both directions.
The details:
1.  2×6″ framing at 24″ o.c. with insulated corners.
2.  Wall cavity insulation will either be Grade 1 installed fiberglass batts, or wet blown cellulose.  I’m still pricing things here.  I’m not very fond of spray foam.  Just not that impressed with the overall value in an otherwise well-sealed home with 2×6 insulation potential, especially with exterior insulation to break thermal bridge.
3.  Interior sealing around rim joists, plates, and other necessary areas will be done, including use of gasket type product under bottom plate to concrete.

4.  My planned house wrap would be either a fluid applied, Tyvek drain wrap, or perhaps a self adhered product like Delta Vent SA or Blueskin.  I want really good envelope sealing qualities with my WRB system.

5.  House will have combo of lapped horizontal siding and board+batten.  Most likely fiber cement.

6.  I’m trying to budget for Energy Vanguard to do the HVAC design and calculations.

7.  This area has been getting 55-60″ of annual rain.

8.”  I’m most likely going with a ducted VRF system, which may or may not make a dehumidifier (like Ultra-Aire) unnecessary.  Thoughts?


The main questions:

A)  Is this vapor issue a valid concern with exterior insulation on a Hardie type siding product, especially board and batten?

B). If yes to above, is the concern alleviated by using Rockwool instead of an XPS or polyiso?  I’m thinking perm rating is the big factor here.

C)  Which of the WRB choices are best in this case?  I’m thinking the Tyvek Drain wrap would assist in drying between OSB and innermost side of exterior insulation board.  Since exterior insulation requires what is effectively a venting rain screen on the outermost side, I guess I’m good there.  I’m worried about a smooth product like Delta Vent or fluid applied WRB.  The insulation board would sit flush against the OSB.

I want to showcase this home in some local publications for the purpose of highlighting smart, cost-effective, “green” building techniques.  They are not building houses correctly around here!!

Bonus question, if I haven’t worn you out yet…. Are there any better options than 1×4″ wood for use with exterior rigid foam?  Cor-a-vent?

Thank you!

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  1. Expert Member
    Dana Dorsett | | #1

    Your concerns about "...rapping vapor between OSB and the rigid foam..." and presumption that "...that I need the wall to dry in both directions..." are unfounded.

    Independently of siding types, in your climate zone an inch of foam is more than sufficient for dew point control, keeping the OSB warm enough during the winter to prevent excess moisture accumulation in the OSB, and the low vapor permeance of the foam blocks summertime moisture drives from making your drywall moldy/soggy. The assembly will work just fine to be set up for drying only toward the interior.

    Any reasonably vapor-permeable type of cavity insulation would be fine, even with 0.05 perm foil faced insulation on the exterior.

    Tyvek Drainwrap would be the WRB of choice for "innie mount" windows. With only an inch of foam it's really a matter of how the window flashing is set up.

    1. 88Clayton | | #3

      Thank you.

  2. BrianPontolilo | | #2

    Hi Clayton,

    Glad to have you posting in the forum. Seem like Dana covered most of your concerns. And you mentioned you've already done a bunch of reading, but I did put some links to what seems like the required reading list for this topic below.

    As far as your bonus question is concerned, I only have experience with using 1x4 furring strips for, well, furring strips, but I'd be interested in hearing other users experience. This material is affordable and available in long lengths, or at least in my area it is, but hitting studs with fasteners through foam can be tricky. Ideally, once you have located a stud at the top of the furring strip, you can get the strip plumb and have a better chance of hitting studs as you work your way down. Unfortunately, the furring strips I had delivered were anything but straight, so setting them plumb wasn't that easy. Hopefully some others chime in with their experience on this because I think it would be helpful for anyone tacking their first (or second) crack at installing furring strips over foam.

    Anyway, here's those links:

    1. 88Clayton | | #4

      Thank you for the links!

  3. 88Clayton | | #5

    Would like a couple opinions on the following... Now that I've had a couple responses, my wall system is shaping up to look like this:

    5/8" drywall
    cellulose in 2x6 cavity
    7/16" OSB ($5 cheaper than 5/8")
    High quality WRB... either sprayed, self-adhered, conventional with cap nails and taped seams
    1" rigid foam insulation board
    Furring strip on 24" o.c. to line with studs
    Siding (lap and B&B)

    Will leave weep holes at bottom and maybe vent at top.

    Does this look good? Is anything overkill?

    Is it better to tape OSB seams or WRB seams (where applicable)? I plan on sealing up the sheathing to foundation gap. Do you guys like a big tape for that or a fluid applied?

    Considering it's half the cost of 1/2" plywood, is the OSB going to be fine?


  4. Expert Member
    Dana Dorsett | | #6

    Detailing the OSB as the primary air barrier would be worthwhile, and more rugged than the WRB. A 7/16 OSB leaks more air direcly through the board than you might think , but with a spray-applied WRB I suspect it would meet spec. If using DrainWrap, definitely detail it for air tightness as well. Taping the seams of the foam board is also going to be worth it.

    The wall described is going to come in at about R20 "whole wall" or slightly higher, after factoring in the thermal briding of the framing, and the R values of the sheathing/siding/wallboard/air-films etc. That's better than code, but it may also be economically rational to go for about R25 whole wall (say 1.5" of foil faced polyiso instead of 1" EPS), the recommended starting point for zone 4 in Table 2 of this document:

    At those "whole assembly R" values it's usually possible to hit Net Zero Energy using heat pumps for HVAC & hot water with a PV array that fits comfortably on the roof, if you've designed the house carefully to take best advantage of the sun, and limit excess cooling loads by keeping all ducts inside the thermal & pressure boundary of the house, and avoiding large amounts of west facing window area (which isn't easily shaded with overhangs.) Simulating the house with a free download tool BeOpt is useful for teasing out the best performance for the least cost while it's still in the design phase.

    1. 88Clayton | | #8

      Very cool. Thank you, Dana.

      If I go with a fluid applied like PolyWall, or liquid Tyvek, should I still tape the polyiso seams? Or does the completeness of the monolithic fluid applied WRB take care of that?

      The only foil faced polyiso I'm familiar with right offhand is Atlas Energy Shield and RMax. I'm seeing the 1.5" RMax product is over $900 for 1,032 sq ft (32 sheets). I'm pretty sure the Atlas Energy Shield is also expensive. I'd end up spending $3,000 just on foam boards. Is this typical? If not, are there more cost effective products?

      And yes, I planned on a hybrid hot water heater and ducted VRF HVAC. No immediate plans for PV array on roof. Not opposed to it, I just don't have another $15-20k right now. But I love low utility bills!

      1. Expert Member
        Dana Dorsett | | #12

        >If I go with a fluid applied like PolyWall, or liquid Tyvek, should I still tape the polyiso seams?

        Yes. Foam board changes dimension over time, temperature & humidity- you don't want air to be able to flow into gaps that open up, undercutting performance.

        Reclaimed fiber-faced roofing polyiso can often be had for less than the cost per R as batts. A common thickness is 2", which is usually labeled R11 or R11.5, sometimes R12, but who cares if it's less than half the cost per sheet (usually 1/3 the cost in my area)?

        Run this search on local and nearby craigslists occasionally and you'll find some. You might find some 1.5" foil faced for dirt cheap too:

        This outfit in Atlanta has 24 sheets of 2" foil-faced labeled R13 for $300:

        That's $12.50/sheet. Two bundles of that would be 48 sheets, over 1500 square feet for well under $900.

        When you find it you'll have to figure out a place to store it. Up on pallets covered by tarps or housewrap is OK, laying on the dirt weighted down with cinder block , open to the rain is not.

        Note: 2" /R13 foil faced and a 2x4/R15 rock wool wall would come in just under R25 whole-wall too (a solid R25 if using advanced framing) at the same wall thickness as a standard 2x6 wall that had no exterior insulation.

        1. 88Clayton | | #14

          Fantastic. I did find some kind of reclaimed foam board place in Georgia last week, but I wasn’t sure about the consensus on using those and was skeptical of how they had been stored.

          I did not think of checking Craigslist. Thanks!

          I’m guessing performance of an R11 recycled sheet could be better than a new R5-6 foil faced. The foil simply reflects heat, right?

          What if corners/edges are damaged? Any practical fixes for that?

          1. Expert Member
            Dana Dorsett | | #15

            Most lots of the foam board I've used have some amount of damage. Buying a minimum of 15% more than you need has usually been enough. Badly broken corners pieces can still be used for fitting around windows & doors etc, but there will likely be at least a few sheets that are complete scrap. If you're running short using can-foam or 2-part polyurethane foam kits (Froth Pak, etc) to fill big dents in the edges or missing corners is fine.

            The foil facers both reflect heat, and emit less heat during the heating season. When facing a 3/4" air gap it adds about R1 to the average performanc. On a deep energy retrofit project I was involved with a handful of years ago they did the walls in reclaimed 3" fiber faced roofing foam in some areas, 3.5" in others, then installed a layer of 0.5" or 1" factory seconds foil face polyiso to bring the total thickness to a fairly uniform 4.0" with foil facing the rainscreen gap. The roof got 7", reclaimed foam only (no foil). The net savings of going with reclaimed foam on that project was over ten grand.

  5. aaronbeckworth | | #7


    I am also located in CZ4, although I’m in a much drier climate in high elevation New Mexico. Our average annual precipitation is 16 inches. The assembly you are considering is very much what I had in mind until reading about Carl Seville’s (Green Building Curmudgeon) use of Zip R sheathing on his own house in Georgia. The Zip R panel provides the exterior foam (in different thicknesses) you desire for breaking the thermal bridge, and a WRB/tape system that seems like just about the easiest way of achieving an exterior rigid air barrier. Seems like a system that the typical framing sub would have an easy time with compared to going around the house multiple times for all of the separate layers in the assembly you listed. And window installation would be easy, or at least more like what your builder is probably used to.

    Obviously I’ve become excited about incorporating Zip R in my future build. But I’m purposefully holding back on that project as I continue to learn from the generous and thoughtful GBA community, as well as those of FHB and JLC. I’m interested to hear if anyone can comment on my suggestion for considering Zip R for either of these very different CZ4 locations.


    1. 88Clayton | | #9

      Aaron, would you still be excited about ZipR if you learned it was $47/sheet for R6?

      I'll look up the Curmudgeon's article on it. Thank you!

    2. 88Clayton | | #10

      Also, the framers around here charge up to $0.50 more per square foot to install regular Zip. Possibly more than that with Zip-R. Then the tape is a total ripoff, IMO. It's over $300. If I went this route, I would Liquid Flash it for 80% less than the tape cost.

      I've seen some bad Zip tape jobs too. It's amazing how rare it is to find people who do things right! Quite depressing. You can't trust any of the crews. There's always at least one weak link among them.

  6. aaronbeckworth | | #11

    I’ve heard that Zip and Zip R sheathing varies in price quite a bit in different areas. I do not have current prices for my nearest supplier, which is a couple hours from me. I’m sure the price would be much more than commodity OSB, but I would expect that once you account for the cost of the WRB and rigid foam, the cost difference would be similar. And I’d be willing to pay a premium on materials if it brings down the cost of labor and keeps the builders happy.

    If taping is the weak link in your opinion, there is now a liquid flashing option from Zip. I wouldn’t use liquid flash for easy to tape sheathing seams, but would definitely consider it for transitions and rough openings.

    1. 88Clayton | | #13

      In my particular area, with multiple local suppliers, OSB is $7.99. Zip is $14.99. Zip-R3 is $42 and Zip-R6 is $47.

      A big roll of Tyvek Drainwrap is about $250. Less than a single 90’ roll of Zip Tape.

  7. brianvarick | | #16

    What do you mean less than a single roll of Zip tape? There are cheaper systems than Zip, but the fact that it's go around the house with twice is huge in my mind. Also, the sheathing being exterior makes window installation much easier. I bet they are similar in price when you consider the time it takes and a far simpler system lessens the chances for mistakes a long the way.

    1. 88Clayton | | #17

      What I mean is a big roll of Tyvek is less than a mere 90' feet of Zip Tape last I checked. 90' length of Zip tape is over $300. Probably need 3-4 of those to complete house. Then figure in the extra $0.25-50/SF framers charge for Zip. They apparently would rather wrap a house than tape it.

      Yes, window insulation would be quite a bit easier, but what kind of $$ does that translate to? I'm sure Zip has their product priced very strategically, but I'm willing to bet it comes out a few thousand dollars more for less R value than alternatives.

      Then what can be done about rain gap between siding and Zip? I'm not so sure nailing holes in the Zip that cannot be sealed is a good idea. Zip is not self sealing like some of the self-adhered WRB's are.

  8. brianvarick | | #18

    I guess it comes down to how much is your or your crews time worth when it comes to a system like Zip. I am planning on using it because It's my first time doing exterior insulation and the ease of installation is worth the extra money to me.

    I'm not sure what kind of tape you're looking at. The 90' rolls cost $25 for they 3.75" tape.

    1. 88Clayton | | #19

      You are correct. The guy who told me it was $300 was estimating the amount I’d likely need rather than the roll price.

      I think it’s definitely easier for those wanting exterior insulation, especially in the South, where crews have likely never done exterior rigid foam. Could be worth the extra money considering that.

  9. aaronbeckworth | | #20


    If you’ve had a chance to read the Curmudgeon’s project posts, what did you think about him continuing the Zip sheathing over the ceiling joists and then setting the roof over that decking? That is a method that I’ve only seen mentioned a couple of times on this site. John Brooks, an architect in Texas, once provided a nice detail for this idea using trusses (see the first several comments following the blog post, Lstiburek’s Rules for Venting Roofs).

    I really like the continuous air barrier and the service chase this would create. My project will be small, ~900 ft^2, and I am interested in knotty pine ceiling, which otherwise would need some other air barrier. So for me stumbling onto this idea has me back at the drawing table. Just thought I’d add this idea to the conversation.

    1. 88Clayton | | #21

      I was unable to find the article you reference. Can you link me to it? Thanks!

  10. Jon_R | | #22

    Robust moisture design of partitions is not as black and white as is often presented on GBA. Every building/situation (and even every stud bay) is different and even recommended designs can fail in unusual situations. So it's not crazy to be concerned about using an extra robust design (eg, drying in both directions or non-wood sheathing).

  11. aaronbeckworth | | #23

    Here is the link to Carl Seville’s blog post discussing his choice to use Zip R.

    1. 88Clayton | | #26


  12. aaronbeckworth | | #24

    And here is the link to the other article I mentioned.

  13. Jon_Lawrence | | #25


    Zip tape is $30 for a 90' roll.

    1. 88Clayton | | #27

      Yes, quite expensive, no doubt, but not $300 a roll like I was first told. I’ve actually seen it for $25 recently. Thanks!

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