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Community and Q&A

R47 Wall and R 66 Roof – “Mind the Gap – the Almost Perfect Wall”

howard_road | Posted in Energy Efficiency and Durability on

I would really appreciate some feedback from the group on my version of the “Almost Perfect Wall” for my home in Southeastern Michigan (Climate Zone 5)

http://www.buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh/files/BSI-038_Complex_Mind_the_Gap.pdf

One of my big questions is…what to tape? The Rigid Insulation, the House Wrap…the Plywood

Wall Components from the Inside out:
1/2” Gyp
Dense Pack Cellulose Cavity Insulation ([email protected]=R21)
2×6 Wall Studs @ 16” O.C.
1/2” Plywood Sheathing
Crinkled House Wrap
2 Layers of 2” Polyiso Rigid Insulation ([email protected]=R26)
Furring Strips
Corrugated Metal Siding

Roof Components from inside to out: (3:12 slope)
5/8” Gyp
Dense Pack Cellulose Cavity Insulation ([email protected]=R46)
11 7/8” I Joist
3/4” Plywood
Fully Adhered Sheathing Membrane (Grace Ice & Water)
2 Layers of 2” XPS Rigid Insulation ([email protected]=R20)
1/2″ OSB
30# Felt Paper
Corrugated Metal Roofing (or Asphalt Shingles)

This system makes a lot of sense to me…but I’ve never been credited with having excess amounts of good sense.

The Structure stays warm and dry behind the bulk of the insulation
The ‘innie’ windows can be installed with the flanges integrated into the House Wrap
The built-up system with staggered joints means less reliance on tape
The ability of the Plywood and Cellulose to redistribute moisture and aid inward drying

Thanks,
howardroad.blogspot.com

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Howard,
    You are good to go.

    The air barrier of your roof assembly is the Ice & Water Shield.

    I would tape your plywood wall sheathing if I were you, and make that the air barrier for the wall. At the connection between the top of the wall and the bottom of your roof assembly, make sure that there are no air leaks and the air barrier is continuous.

  2. user-1137156 | | #2

    Howard,
    Your wall is a good bit less than r47, that is the cavity value. It is r33.5 where the wood framing is which will be 20 to 25% of the area. The average r will be r 43 to r44. These numbers are accepting the value of r 6.5/" for poly iso which it will be near initially but it's aged value will be closer to r 5.5/" Accounting for the age degradation of the poly iso leaves r39 -40 for the wall r.
    I respectfully diss agree with Martin and would advise that you use airtight drywall to avoid condensation issues on the inside of your plywood as it's located with about 1/2 the insulation on either side.

    I prefer a double stud wall with no foam and use Roxul comfort bat. I prefer to locate the plywood on the outer face of the inner wall and detail it as the primary air barrier giving a service cavity and eliminating any need for sealed drywall. There is really no need for exterior sheathing (but I'll use fiberboard if required), house wrap under furring is a sufficient WRB. With 3 layers of r 15 Roxul I'd have a wall ( 11" from furring to drywall backside) 1/2" thicker than you are proposing for significantly lower cost with about the same thermal performance after a year or so of aging. 16" OC framing costs more in both material and reduced thermal performance. I'd prefer 24" OVE framing if at all possible.

  3. howard_road | | #3

    Thanks Martin,
    The Spray Foam was meant to ensure Air tightness of the Roof to Wall Connection. I have redrawn this detail a few times. I settled on Spray Foam on the exterior to keep as much of the structure as warm and dry as possible.

    Do you think the Spray Foam is a good solution for this application? Any alternates I should consider?

  4. howard_road | | #4

    Thanks Jerry
    Agreed…My title contains a bit of R Value Hyperbole.

    About the Air Tight Drywall. Most of my research tells me these assemblies will need to dry to the interior. In the article Joe uses Spray Foam in the cavity. I switched to Dense Pack Cellulose because of its benign nature and ability to redistribute moisture.

    My hypothesis is that the Cellulose will handle the small amount of water that will inevitably make its way into this assembly, as long as it can dry to the interior of the house.

  5. snfh | | #5

    Martin,

    Question. Why tape the plywood when the polyiso can be taped? It seems to make more sense to create an air barrier at the exterior plane than 4" deep into the insulation. Also, how many layers of tape are necessary with foam sheathing on any house? If the seams of the foam sheathing are staggered from the plywood sheathing seams and the foam sheathing is taped and covered with house wrap I'm thinking that taping the rigid foam sheathing should be enough. Is it?

  6. user-1137156 | | #6

    Howard,
    Making the drywall air tight will lessen the moisture carried by air movement but not the moisture that moves by diffusion. In other words air tight drywall doesn't remove the ability to dry to the interior. But it does eliminate the transport of moisture by air movement.. With cellulose you will have moisture "buffering" capacity but air tight drywall will further reduce the risk of moisture issues.

  7. GBA Editor
    Martin Holladay | | #7

    Bruce and Jerry,
    Although I suggested that Howard should tape his wall sheathing, it's not the only way to go. I have no problem with builders who choose to follow the Airtight Drywall Approach, and taping rigid foam can work well. The proof is in the pudding: if you hit your airtightness goal when you run your blower door test, you have succeeded.

    I don't think there is going to be much air leakage into this type of wall assembly, assuming that the builder pays attention to airtightness at the sheathing level, so worries about air infiltration through drywall cracks is something of a red herring. That type of leakage is most common in homes where no one was paying attention to airtightness when the home was being built.

    That said, choose any approach to airtightness that you prefer.

    One caveat: there is some evidence that many types of rigid foam are not dimensionally stable over time. That's why I prefer to see the air barrier at the sheathing level rather than at the level of the rigid foam.

  8. howard_road | | #8

    Thanks for the comments everyone,

    What about the OSB in the roof assembly? Could this be omitted? If cost drives me to asphalt shingles, then I do believe that the OSB is necessary. However, if I am able to keep the corrugated metal roofing do I still need the OSB? I keep thinking that the OSB is the week link, the one item that is most likely to suffer water damage and require maintenance before the other components.

  9. GBA Editor
    Martin Holladay | | #9

    Howard,
    If you want to install metal roofing on top of rigid foam, you don't need OSB. Instead, you can install 1x4 or 2x4 purlins, 24 inches on center, parallel to the ridge, on top of the rigid foam.

  10. snfh | | #10

    Martin,

    I've not yet tried taping plywood. Plywood expands and contracts with temperature and moisture (quick google search does reveal clear calculation for coefficient of linear thermal expansion or hygrometric coefficient of expansion). What is the risk of tape failure on plywood over time assuming a high quality tape for that application was installed properly?

    Back to my original question about how many tape applications are necessary. It only makes sense to tape foam sheathing as it is installed. Is there a consensus at GBA on the necessity to tape foam sheathing and plywood if used together? A blower door test will reveal real-time results, but won't reveal air leakage in the future due to dimensional instability. Staggered joints of foam over plywood and house wrap seem to offer additional protection from air leakage should the foam move. Yes? No?

  11. GBA Editor
    Martin Holladay | | #11

    Bruce,
    You raise many questions, and we don't have good answers to some of them. For more information on some of these topics, you might want to read:

    Backyard Tape Test

    Using Rigid Foam As a Water-Resistive Barrier

    Plywood moves, and so does rigid foam. If you choose a high-quality tape, the tape should also be able to move -- up to a point. When the movement exceeds the limits of the tape's flexibility, the seal will fail.

    Ultimately, this is a judgment call. Many experts -- but not all -- have concluded that it makes sense to tape the plywood or OSB sheathing. If you prefer to tape the rigid foam, nothing is stopping you. Some builders tape both, and it makes sense that taping both should provide a (small) incremental improvement in airtightness. Whether this tiny improvement is worth it depends on your budget, your airtightness goal, and whether or not you are a congenital worrier.

  12. jinmtvt | | #12

    Why half-assed solutions ...
    save the wrap the tape and the labor
    and just peel stick all of the sheathing

    we all know it is the best solution, why try to derivate from it ?
    At the end you will probably not save much ( it is very fast and easy to apply peel stick in large rolls and much more labor consuming to tape all little details ...u can still tape where u wish water/air to be sealed off at the top layer ... )
    And it protects all of the framing from exterior future water damage.

    Then, as suggested by some, make sure u don't have too much insulation inside wall cavity.
    U can skip on the crazy gypsum sealing techniques...

  13. jinmtvt | | #13

    Also this goes on with what Sifu Martin teaches us here :

    """ That's why I prefer to see the air barrier at the sheathing level rather than at the level of the rigid foam. """

  14. snfh | | #14

    Martin,

    Thanks for the links. After a little more research I now see the big trend toward taping plywood. However, if anyone uses foam sheathing, especially a significant amount of foam sheathing (say 2-4"), it does not make sense to me to allow outdoor air to penetrate 2-4" of insulation before it met an air barrier. This does not seem like a "small" matter if someone is going to the trouble and expense of installing foam sheathing. Taping the foam sheathing seems like the only prudent way to install it absent a peel and stick WRB. Please comment.

  15. GBA Editor
    Martin Holladay | | #15

    Bruce,
    You wrote, "Please comment."

    In my last response, I noted, "Ultimately, this is a judgment call. ... If you prefer to tape the rigid foam, nothing is stopping you." It sounds like you have made a judgment. The call has been made. So it doesn't seem as if you have any further questions.

  16. howard_road | | #16

    Thank you everyone for the comments and advice.

    I did the math and for my house it would be about $1,100.00 (not including labor) worth of Grace Ice and Water Shield to cover all wall exterior sheathing. This is the direction I’m leaning, as opposed to taping the plywood sheathing seams.

    This methodology does however bring up some concerns.

    With no wrinkled House Wrap I have now lost my “Gap” between the Polyiso and sheathing.

    I have now wrapped my entire house in rubberized asphalt, so are there off gassing / interior air quality concerns from the Grace? Yes there will be mechanical ventilation…build tight and ventilate right.

    By omitting the taped seams in the 2 layers of Polyiso am I opening myself up to water issues with the Polyiso absorbing water thru the untapped seams? My siding will be mounted to vertical furring strips so I will have a rain screen to allow for exterior drying.

    Thanks,
    Howardroad.blogspot.com

  17. jinmtvt | | #17

    Why not use Tyvek at the exterior point ??
    will help with some "air/draft" and keeps water out of assembly ...

    Happy to see that you've opted for the wise choice :)
    Don't worry about the asphalt off gasing, since there won't be any leaks there won't be any draft from the envelope to the interior, to transport any possible scents ...
    You should still consult manuf/paper of the product you choose to use to make sure it is ok .

    What have you decided to use inside wall cavities ??
    Don't forget to do the math of the dew point for your temperature design to make sure it will always stay outside of the sheathing.

  18. GBA Editor
    Martin Holladay | | #18

    Howard,
    I think that it's fine to put peel-and-stick over your plywood wall sheathing, but only if you are installing all of your insulation on the exterior side of the peel-and-stick. If you insulate your stud bays, however, that approach becomes riskier -- because the insulation in the stud bays cools off the wall sheathing. Without the insulation in the stud bays, the plywood stays at room temperature -- warm and dry.

  19. howard_road | | #19

    The current wall assembly does call for Dense Pack Cellulose in the Stud Cavity. This may get omitted due to cost issues. I have been torn between the convenience of having an open service cavity and increasing the R value of the assembly. I have received a few comments that given the quantity of glass and the fact that some exterior walls are ICF, super insulating the few wood stud walls is a waste of resources. Either way I will have the WUFI calcs re-run.

    So is tyvek or tar paper needed between the furning strips and the Polyiso? This is just one more layer in an already complex and expensive wall assembly.

    Thanks,
    Howardroad.blogspot.com

  20. jinmtvt | | #20

    Howard : i would use tyvek , some would not ..
    if your poly iso has an aluminum facing, you could only tape it and use some kind of flashing
    at the top ...
    again, the labor difference VS using a 1 step home wrap is usually not much ... take a few moments to do the cost maths !

    As for interior insulation in zone 5 .. may not be worth the trouble ..

    i'd consider using something like a 3.5" thick Roxul as it is very quick to install ..shouldn't pose any problem with the dew point if you are @ 4" of polyiso

    are you still going for 2X6 ??
    that would leave some free space for wires etc..

    cutting 1-2" of XPS and gluing it on the cavity with expanding foam adhesive could be a nother tactic, but will probably cost alot more in labor and adhesive
    again do the maths...

    :)

  21. howard_road | | #21

    Thanks Jin!

    Yes the walls are 2x6 primarily due to structural considerations. They are rather tall walls (12’-6”) and supporting significant roof load.

    I like the concept of partially filling the cavity with Roxul for some additional R Value.

    So what about swapping XPS for Polyiso? Less money, much less concern about water absorption, but higher GWP and lower R value. It should allow me to omit the Tyvek and reduce the cost of the entire assembly…correct?

    The Polyiso decision was primarily based on the need to get the external R value high enough to insulate the cavity without condensation issues. Eliminating the dense pack cellulose and/or partially filling the cavity with Roxul would lower the required external R Value.

    I would just go Roxul all around but the added cost, weight and thickness are…problematic.

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