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Community and Q&A

Sheathing – plywood vs Eclipse Foil Faced OSB

user-1104371 | Posted in Energy Efficiency and Durability on

We are looking to build our next home thisupcoming spring and I am trying to decide what sheathing I should use on this project. The first house we built was a PU SIP (thermocore) home and our current home is a more standard 2×6 stick built home with OSB sheathing (2″ PU spray foam then blown cellulose for walls and attic ceiling). We have been surprised that our heating/cooling bill on our current home is almost the same even though we have almost doubled the sqft of our SIP home – although we are protected from western winds during winter (northern MI) and have significant shade in summer (nestled in a maple forest) at our current location which was notthe case previously. We will have a similar setup with slightly less summer shade in the next house, and I had originally planned to switch from OSB to all plywood construction. (Sheathing and subfloors). However, the Eclispse foil faced OSB has me interested. I am contemplating it for the roof sheathing and the wall sheathing – both with the foil facing inward. That is the typical method they recommend for the roof but they suggest using the foil face as the WRB on the walls. I don’t want to do that as I am worried it would prevent the wall from drying if I use 2″ of PU spray foam then cellulose again for the wall insulation – instead I am wondering if it would make sense to put the foil facing inward then spray the foam onto it and use Typar as planned for the WRB on exterior. Thoughts?

Also, I use Grace Ice and Water for the entire roof. If I use that on the exterior and the foil face is facing the interior, would that prevent the roof OSB from drying too?

Any thoughts or feedback are greatly appreciated! The price on this Eclipse sheathing is very attractive currently as it is about equal to plywood, but i don’t know if it will be best long term to go with it or stick with my original plan of all plywood construction this time around.

Thanks in advance – Paul

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  1. GBA Editor
    Martin Holladay | | #1

    The usual application for foil-faced OSB is as roof sheathing, with the foil side facing down. The foil must face an air space to get the benefit of the radiant barrier.

    If you plan to install spray polyurethane foam against the underside of your roof sheathing, then there is no benefit to having a foil facing.

    It's also worth noting that the R-value of the spray polyurethane foam layer which you are proposing to install is too thin to prevent moisture problems in your planned roof assembly. In northern Michigan (Climate Zone 6), this type of flash-and-blow roof assembly needs a spray foam layer with a minimum R-value of R-25 -- in other words, at least 4 inches of closed-cell spray foam, not 2 inches as you are proposing.

    For more information, see this article: How to Build an Insulated Cathedral Ceiling.

  2. user-1104371 | | #2

    Thanks for the detailed responses. In regards to the attic, the plan is to PU Foam spray the top of the drywall then blow in cellulose for the remainder of the insulation. The roof sheathing will not be in contact with the insulation, however, I am still left wondering if the Grace Ice&Water on the non foil side and the foil underside will create an issue for drying as it sounds it will.

    For the walls we are looking to build it as cost effective as possible and had planned on using 24" OC 2x6 framing so we can limit the therma bridging and material costs as much as possible but also allow for standard window and door jambs. Also, there are not a lot of skilled builders in the area who are doing the exterior foam insulation, and those doing it are charging a premium for their services. We have actually planned to use prebuilt wall panels from our truss company to also improve the standard wall construction process and limo ate waste. I do not want to use SIPs again as we had several panels that "popped" every evening during the summer when they cooled.

    We live close to Lake Michigan in a hilly and shaded area so we actually only heat for about 4.5 months per year and cool for the rest but the cooling is offset by the shading of the trees during the hottest months (Lake Michigan also works as an insulator year round- warming in the fall-winter months (until it freezes over in Feb) and cooling in the Spring Summer months. The hills block us from winds and deciduous trees provide shade in summer and allow sun to help with our heating during the winter).

    Again our current home is insulated as described above with the spray foam sealing the building and providing the dew point protection for the wall cavity and bumping up the insulation value. We average about $200 per month to heat during the winter months 5600 sqft with 10' ceilings throughout. This next house will only be 2800sqft of conditioned space so I anticipate an even lower monthly bill. Thoughts? We are not trying go to extremes on this project just build a great building that is safe, comfortable and economical upfront and down the road. Side note: looking to improve windows this time (have ext aluminum clad wood in current house) and looking at foam filled fiberglass triple pane 1/2 fixed and 1/2 casements- any suggestions for manufacturers close to this area?

  3. Dana1 | | #3

    Specs say it's less than 1 perm, which makes it at least a class-II vapor retarder with very limited drying capacity:

    The thermal value of the foil is zero unless it's facing an air gap. Whether applying 2lb foam to it or stuffing the cavities with cellulose tight to the foil, it's buying exactly zero thermal benefit, while creating a potential moisture trap.

    On the roof deck it creates a bit of a moisture trap, but gives a very modest but measurable thermal improvement during the cooling season when the roof gets direct sun. If the roof has high shade factors, it's not worth the risk. With high sun exposure and a top-ventilated roofing (tile, cedar shingles etc) and without the Grace Ice & Water it may be worth using if it's cheap, but in your location & roof stackup it's a liability.

    Closed cell polyurethane is both expensive (17-20 cents per R per square foot) and environmentally unfriendly due to both its high polymer/R ratio and it's extremely high global warming potential blowing agent, HFC245fa (~1000x CO2 for it's 100 year global warming potential.) Where/when you can design it out of the stackup, it's nicer to the planet.

    The primary benefit to closed cell foam in stud bays is air sealing, which can be done much more cheaply and greenly using alternate methods. The raw R-value is severely undercut by the thermal bridging of the framing, adding remarkably little to the total thermal performance of the assembly beyond air sealing. A secondary benefit is as a vapor retarder, protecting the sheathing from interior moisture drives, but that also limits the ability to dry toward the interior. In almost all cases spending the wall-foam budget on rigid EPS or polyiso on the exterior is much better bang/buck, and far greener. Both EPS and polyiso are blown with pentane (~7x CO2). At 1lb density polyiso is about half the polymer/R of 2lb polyurethane, a bit more than half when properly derated for temperature in a cold climate exterior application. XPS is blown with HFC134a (~1400x CO2), and about the same polymer/R content of 2lb polyurethane. EPS and polyiso also run about half the installed cost of 2lb polyurethane.

    With 2x6 framing full of cellulose or other fiber in US climate zone 6 it takes a minimum of R11.25 of exterior insulating sheathing to protect the sheathing from interior side moisture drives. Even if you derate polyiso to R4.5/inch for it's average mid-winter performance (it'll do much better than that over the whole season) it takes 3" to get there, at a cost of about $1.80-2 per square foot, which is about the cost of 2" of polyurethane in the stud bays. With 2" of PU+ 3.5" of cellulose in a 2x6 studwall you're looking at about R16-17 whole-wall, after factoring in the thermal bridging of the studs. With 5.5" of cellulose and 3" of exterior polyiso you're well north of R25 whole-wall (north of R30 for a seasonal average), and a lot more moisture resilient.

  4. user-1104371 | | #4

    Sorry I forgot to add that we did 2.5" of PU spray foam for the walls on our current home after looking at my specs more closely to get 15rvalue before the dense pack cellulose. So we would do 2.5-3" on walls again as is suggested to prevent dew point issues as was stated above.

    Also if we did go to 3" polyiso on the exterior what is the suggested method for siding insulation if I don't need a rain screen? We will be using a combination of polymer siding (Novik Shake) and vinyl (batten) - woodpeckers up here destroy wood and fiber cement siding - strange but true on the fiber cement!

  5. GBA Editor
    Martin Holladay | | #5

    Thanks for clarifying what you meant by the "attic ceiling." I thought you meant the surface above the attic; you meant the surface below the attic.

    You don't really need to install 2 inches of spray polyurethane foam above the drywall ceiling of your top floor in order to seal air leaks. You can address air leaks more selectively, with caulk or canned spray foam -- and if you do, you will have chosen a more environmentally friendly approach.

    I don't usually recommend installing Ice & Water Shield over the entire roof, unless there are unusual circumstances. That's an unnecessary expense, and in some cases the Ice & Water Shield impedes drying.

  6. Dana1 | | #6

    For clarity, yes having the foil on the underside of the roof deck is a potential moisture problem with MOST roofing material types. It's not just Grace Ice & Water. Even #30 felt + asphalt shingles has a vapor permeance down around 0.1 perms, and cannot dry toward the exterior at a reasonable rate even under the best of circumstances. (No roofing dries toward the exterior through a layer of snow, or when it wet with rain or dew.)

    Going all-in with advanced framing techniques (not just 24" stud spacing) buys you another ~ R1.5 of whole-wall performance, not more. The performance difference between 2.5" and 2" polyurethane before dense-packing in the stud cavities is about another R0.2 in whole-wall R. If you did 3", it's really not much different, (and it's way over-kill for dew point control on the R7.5of fiber insulation in the rest of the cavity.) Even if you did 5" of R6.5 /inch ccSPF AND did the 24" o.c. stud spacing you're only looking at ~R20 whole-wall.

    The IRC calls out R20 cavity fill + R5 continuous, or R13 +R10 for zone 6, both of which come in at a bit over R20 whole-wall with 16" o.c. spacing. R20+ 5 has some mold risk due to insufficient exterior-R, but R13 + R10 had plenty of foam. If you're looking to build at only slightly better than code min without a thick wall, go with 16" o.c. 2x4 / R15 (rock wool or HD fiberglass), and 2.5-3" of exterior polyiso, which comes in at R23-R25 whole-wall after fully derating the polyiso for climate (and over R25 whole wall at the labeled R-value.) Either would have MORE than ample dew point control for setting it up to dry toward the interior with only standard latex paint as the interior side vapor retarder. If 2x6/R20 + foam it really needs the 3" for adequate dew point control, which makes for a somewhat thicker wall.

    Taking the 2x4 + 2.5-3" foam route, doing the foundation with a 2.5" + 2.5" insulated concrete form and the stud plates set on the exterior edge of the concrete the 2.5" EPS is roughly co-planar. With windows mounted "outie" you can then seal the polyiso to the EPS with can-foam, for a single continuous thermal break. Mounted "innie" you'd have to install Z-flashing at that seam, which is a thermal bridge cutting into performance a bit. Since it's not a very thick wall there aren't much of any aesthetic or functional issues with taking an outie approach to the windows- it would look pretty much like any 2x6 framed house.

    Siding on 3" of exterior foam needs to be mounted on furring through-screwed to the studs for mechanical/structural reasons, even if the extra 3/4" of ventilation space isn't really needed for vinyl siding.

  7. user-1104371 | | #7

    Thank you Mr. Holladay and Mr. Dorsett- very thorough and great advise.

    Looks like I will just do plywood sheathing on the roof. What is the recommended caulk and can foam for sealing the attic? Great Stuff normal consumer cans seem to be more hassle than they should be- is there a better alternative?

    In regards to the walls, I am bringing SIPs back into the equation but do have some questions about the exterior foam route as well. I have been looking at the window and door installation cross sections and wondering if there is a setup to use a ripped standard dimensional lumber buck for the bump out to make installation easier - I like fewer moving parts as a rule. In other words does it make sense to build a 2x4 interior wall + 1/2" plywood and then have the window and door framing bucks to be ripped to 7-3/4 wide to be flush with the 3/4" furring strips on top of 3" of XPS or Polyiso on the exterior. Or I guess a standard 2x8 would work perfectly if I used 2.5" of exterior foam - correct?

    On a side note, we have had great luck in our past two homes with our Superior Walls precast basement walls. They make finishing very simple and now have 2" of foam installed as thermal breaks and the "studs" also have a thermal break under the steel faces. It makes it easy to run wiring, you can easily add unfaced fiberglass or the cavity ( easily accepts 2x6 insulation) and it is more economical than a poured wall at 10' tall - which makes it truly feel like it is not a basement.

  8. wjrobinson | | #8

    1- start with a home design
    2- add location and altitude and other data, Solar exposure, shading etc.
    3-Some one or group is in charge, You, GC, plan designer, architect, energy and green advisors, etc
    4-interview subs, get them on board, part of deciding materials and methods integrated with all other materials and subs.
    5-Part of early stage poke around the web like here for ideas, of which I see many try to combine too many ideas that do not need or may not even be compatible assemblies to integrate.
    6-As you said, asking anyone to do your specs. who is not doing your methods successfully for years with great results and no warranty issues is IMO asking for trouble.

    Some of my thoughts:

    -Standard plywood is great stuff. I also like Zip sheathing. Zip is almost the only sheathing used in my area now. Foil for roofs is a Florida, southern location product.
    -Nothing is environmentally great, having no kids is environmentally great. So I have no issues with which foam to use. I do know that if you have a spray foam done right your home will be perform darn good as it will be air sealed well or at least better than homes built for the past thousand years. One thing about sealing a home up better is you we are now building homes with moisture problems caused by our new methods. It is very easy to spend lots of money improving the energy efficiency to then create a home that needs more money spent to make it not rot and mold all that money away all this to save a few hundred dollars a year, or not when you end up running a HRV or ERV system so much... don't get me started...

    Your last home performed well it sounds. Build that again, switch to plywood, use Grace products for the right reasons, which has to do with dormers, side wall roof intersections, bad roof designs for a snowy area, eaves, valleys, sky lights, roof penetrations such as chimneys. Use your sprayfoam if you are comfortable with the last guy that did your last home, it does seal all well, Have your framers seal with caulks the frame which the spray foam does not, like the sill to the subfloor etc, etc.

    To repeat some of this post... taking a bunch of ideas off the internet, and the street and building a house from some one off combination of ideas... IMO is really nuts!

    I have been building for decades. This new world of fishing for ideas on the internet and leaping the return of the "wild west period" but instead of gun slingers running rampant for 20 years we have homeowners gone wild with a million Home Network shows that are fantasies and web pages with enough good, ok and bad info to almost assure a home is built with some insane combination of half baked ideas.

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