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Rigid Foam as Exterior Sheathing

txkevin | Posted in Construction Techniques on April 18, 2005 08:39am

On a recent episode of Home IQ, i noted that the IBACOS sponsored home used rigid foam insulation as its only form of exterior sheathing (rather than plywood or OSB).  I think they also used a House Wrap (Tyvek) type product.  They stated that the use of rigid foam insulation as the sheathing provides better insulation and you lose NO structural stability.

Has anyone had experience with this type of construction?  Here in the Hot and Humid Austin, Texas area, I dont think I have seen any construction that only used rigid foam for the exterior sheathing.  Perhaps I just have not noticed.

In particular, I wonder if you structural stability is truely comparable to more traditional Plywood or OSB and if there are any moisture or insect concerns that would be different from using the wood products?

Thanks

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  1. reinvent | Apr 19, 2005 01:14am | #1

    I hope for the homeowners sake that the builders at least put in some diagonal metal bracing before that foam 'sheathing' went up. Otherwise I think they are full of shi$$. Unless the foam sheathing were SIPS panels and I doubt they were. The foam sure as hell wont be able to resist lateral loads if a big storm blows thru.
    Reminds me of a time when they use to say that drywall could be counted as 'structural sheathing' in earthquake zones. That got quickly tossed after the 89' trembler. The BS some people try to sell.
    Do you think IBACOS will put there money where their mouth is in 10-20 yrs if that home dose'nt survive a storm. I doubt it.
    As far as insulation goes, yes it should help. As for insects and moisture problems only if the flashing detailing is not done correctly. I refer you to synthetic stucco as an example.

  2. DanH | Apr 19, 2005 01:25am | #2

    For awhile here you saw the blue foam used a lot for sheathing on low/middle homes. Usually flakeboard would be used on the corners, and I assume that braces were cut into the studs.

    Haven't seen it recently, though. Maybe the code changed.

    I did wonder how they managed to hang vinyl on this -- the stuff certainly wouldn't hold nails, and it's hard to always arrange to hit a stud with vinyl.

    I also wondered how hard it would be to break into a house built this way. Why go through the door when you can so easily kick through a wall?

  3. DANL | Apr 19, 2005 02:57am | #3

    When I was a framing carpenter we built many houses with blue (Styrofoam) "sheathing". But, all those houses had plywood or OSB at the corners, or kerf (let-in) braces at the corners. Styrofoam would make slightly more sense in a humid climate because it can, if properly done, be a vapor barrier, but up north, it seemed like the wrong side of the wall for insulation. Plus everything everyone else said about it not resisting wind loads, not holding nails and so on is true. One thing that my boss was amazed by was the strength of the foil-faced (both sides) isocyanurate foam (not the blue stuff). It acts like a stressed skin panel and is quite strong as long as the foil isn't damaged.

    Edited to add: Foam won't prevent racking either--plywood and OSB will (in fact it is nearly impossible to plumb a wall after plywood/OSB sheathing is nailed on).



    Edited 4/18/2005 8:05 pm ET by Danno

  4. blue_eyed_devil | Apr 19, 2005 05:10am | #4

    Around here in Michigan, we've been putting foam on houses since the early 80's. We use a Simpson t brace on each corner and that prevents racking.

    Weve done this on 100k houses and one million dollar houses and five million dollar houses.

    All are still standing.

    Usually, the foam is used in brick veneer applications. I don't remember many times when I've put LP siding or hardie or vinyl over it. If you did, you'd have to hit every stud with your siding nailings and be very careful about over pounding the siding and creatine waves.

    blue

    Just because you can, doesn't mean you should!

    Warning! Be cautious when taking any framing advice from me. There are some in here who think I'm a hackmeister...they might be right! Of course, they might be wrong too!

    1. txkevin | Apr 19, 2005 05:26pm | #5

      Thanks for the replies.  Perhaps a brick or stone exterior is required for the equivalent stability though this is not mentioned.  There appears to be no special bracing and the rigid foam is simply nailed up as if it were plywood.

      <!----><!----> <!---->

      If in fact the rigid foam does in fact provide comparable stability as they claim, then it seems like a great choice for our humid TX region (with proper ventilation of course).  It seems like a particularly good choice when used with a spray foam insulation such as Icynene or Sealection which would benefit from the added R-value of the rigid foam (no Corbond in Austin Tx to my knowledge).  I just wonder why I don’t see this type of construction in this area and I am not particularly inclined to gamble on it and be first.

      <!----> <!---->

      Here is a link and excerpt from the article…

      <!----> <!---->

      http://www.diynet.com/diy/hm_he_walls/article/0,2085,DIY_16823_3278207,00.html&lt;!---->

      “Sheathing (figure B) is an added protective layer on the outside of the exterior wall. It's essentially plywood that is nailed or screwed into the studs of the building. Sheathing provides structural integrity, and its other function is to close off the studs. <!---->

      Note: Sometimes sheathing can by foam insulation -- or rigid board (figure C). There's no structural loss using rigid board, and it's basically a better insulator for your home. “<!---->

      The rigid foam sheathing is also detailed in the Owens Corning Cultured Stone Installation Instructions along with Plywood and Concrete as exterior sheathing.  It must be being used somewhere.  <!---->

      Anyone else have experience using the Rigid foam and Brick or Cultured Stone?  How did it perform?<!---->

      Thanks All <!---->

      <!----> <!---->

      <!----> <!---->

      1. CombatRescue | Apr 19, 2005 06:01pm | #7

        There's no structural loss using rigid board

        That is a bunch of hooey, imo.  I'm sure it will be sufficient in Texas and other parts of the country that don't have stand up to significant wind-loads (100mph+), but there is no way that rigid foam will provide equivalent strength to rated ply or OSB.  Here in Florida, where pretty much everything is built to withstand 140mph wind loads, 1/2 or 5/8 rated plywood is required, with a strict nailing schedule and blocking at all joints.  Foam is sometimes applied over the plywood, but it is never a substitute.

        1. outofnails | Apr 20, 2005 04:18pm | #8

          I totally agree. Leaving out sheahing plywood would be insanity. OSB also to consider.$$$?

        2. txkevin | Apr 20, 2005 04:34pm | #9

          Any condensation issues to battle in FL when Rigid foam is installed over the Plywood sheathing?  Also, does the rigid foam do away with the need for a home wrap type product or do you double up the moisture barrier?

          1. DanH | Apr 20, 2005 04:43pm | #10

            In theory, in FL one is trying to keep humidity OUT, so the vapor barrier goes on the outside.

          2. CombatRescue | Apr 21, 2005 05:45am | #14

            As amazing as it may seem, there is no requirement for a vapor barrier in the Florida Building Code.  As a result, they almost never get used. 

            Air infiltration barriers are required, however, so Tyvek is used quite often on wood structures. Nothing is used on masonry structures, which is the norm down here (though most 2-story houses are masonry on the first floor and stick on the second). The only places any sort of vapor barrier is required are below slab foundations and attic spaces that can't, for some reason, meet minimum venting requirements, which is pretty rare.

            I'm not sure why they aren't required, but for some reason it doesn't seem to be much of a problem down here.  Although my experience is limited, I've never seen rotted sheathing or similar problems as a result.  Maybe it's the relatively low temperature difference between the inside and outside of a house (about 25-30 degrees in the summer), as compared with up north.  It's weird to me (I'm a Colorado native), but then again, there are a LOT of weird things in Florida.

            To answer your question though, Tyvek is most often used.  Rigid foamboard, installed over the plywood sheathing, is less frequently used, though the foil-faced ones are sometimes used in attics as extra insulation + a radiant barrier.

            Edited 4/20/2005 10:48 pm ET by Andy

        3. txkevin | Sep 21, 2005 05:58pm | #15

          Lets revisit.  In general when I asked this question in April I began to think I was mistaken about the use of rigid foam for external sheathing.  If you have seen the latest FHB issue, they discuss this new construction technique of using 2x6 - 24"centers and rigid foam as sheathing with some strategic bracing. 

          Any thoughts on the article if anyone has read it yet?  While I was not prepare for 2x6 on 24's I was considering going with 2x4 on 16's with rigid foam as sheathing to benefit from higher R value and better air seal.

          1. Caleb | Sep 21, 2005 07:19pm | #16

            After a substantial amount of research in regards to building an energy efficent house using "conventional" methods, this is how I did mine. 2" of ridgid foam on the outside of 2x6 walls on 24" centers. I used a let in metal bracing to strengthen the walls and keep them from racking.I notice now it's actually becoming quite common here in Ottawa on the higher end construction.A few random thoughts:
            It makes applying the outside finish (siding / brick) more difficult as you have to hit the studs with your nails.
            The glue for this stuff sticks to everything (particularily leg hairs).
            It's difficult to cut tight enough to not get some gaps that would allow water penetration. This means you either have to cover it with tyvec (making hitting the studs with for your outside finish even more difficult) or use a lot of expanding foam to fill any gaps.
            If I wasn't DIYing, I would have gone with insulated contrete forms which price out about the same, and in the end (in my opinion) provide a better (stronger & more energy efficient).
            If used behind vynle siding, allows someone to break in to the house using only their fist or a knife.

          2. CombatRescue | Sep 21, 2005 08:29pm | #17

            A lot depends on your local area, climate, and codes.  In my area of Florida, wood construction wall are relatively rare, but generally call for 2X6 on 16" centers with a minimum of 1/2 structural ply on the outside with a strict nailing schedule.  Up north, where you don't have to worry about wind requirements, and the insulation requirements are much stricter, the practice protrayed in the FHB article makes sense to me.

  5. outofnails | Apr 19, 2005 05:42pm | #6

    I covered my entire house in 3/4" extruded polystyrene, but with a 1/2" covering of plywood first. Tape the seams and cut neatly. No problems here in the S.E. doing in that way. Window and door jams will have to be built out unless you can custom them.

  6. JohnT8 | Apr 20, 2005 06:50pm | #11

    On a recent episode of Home IQ, i noted that the IBACOS sponsored home used rigid foam insulation as its only form of exterior sheathing

    Always nice when you only need a utility knife to break into a house.  Makes it easier for the bad guys. ;)

     

    jt8

    Never put off until tomorrow what you can do the day after tomorrow. -- Mark Twain

    1. txkevin | Apr 20, 2005 10:21pm | #12

      Actually it was a brick home so that pretty much protects the walls.  You may want to go with the rock and the window before tackling the brick with the knife.

    2. gordzco | Apr 21, 2005 12:00am | #13

      >>"Always nice when you only need a utility knife to break into a house."<<

      Who needs a utility knife when the crooks are packing 18volt skillsaws, drills, recipro saws and angle grinders? Nothings safe anymore.

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