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Foam board over plywood sheating?

| Posted in General Discussion on May 3, 1999 08:39am

*
I would like to put foam board over plywood sheating to increase the R value of a wall and then put cedar shingles or siding over the foam sheating and was wondering if it could be done and if I would be creating any moisture problems within the wall cavity.If so what would be the correct way to detail it?

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  1. Guest_ | Apr 24, 1999 05:25pm | #1

    *
    The problem with foam loosely touching foam such as hand layering them together in a wet climate is that the capillary space in between the layers if able to get a feed source of water will wet well and will then stay wet easily and the rot will follow soon after...Why does it work for some people...No water to feed in for whatever reason. So it can work...it can fail a little... or the whole house can rot to the core.

    Near the stream, and wary of capillary spaces.

    J

    1. Guest_ | Apr 25, 1999 01:29am | #2

      *David Zaborny. The correct sequence (structurally)is OSB or plywood directly to the studs then attach the RFBI to the structural sheathing.Will this combination: two so-called vapor barriers, one on the exterior and one on the interior, cause moisture problems? Many believe it will and many say no. Keep in mind that although foil-faced RFBI is a vapor diffusion retarder (VDR) its high R-value (R-7 per inch) raises the wall cavity temperature and limits potential condensation. GeneL.

      1. Guest_ | Apr 25, 1999 05:27am | #3

        *RFBI?

        1. Guest_ | Apr 25, 1999 06:18am | #4

          *generic for "rigid foam board insulation".

          1. Guest_ | Apr 25, 1999 07:03am | #5

            *Gene,Yes foam on the exterior helps with cavity condensation and insulation value retention with less delta T, but and a big but...if exterior water finds its way into the capillary sandwich, it's good bye sheathing and hello rot in just a few months in more humid climates...The Carilinas are so scared of foam now that one of the states banned EIFS systems with out drain backer for residential use.Capillary sandwiches that have a remote chance of water entry suck litterally, liberally, and slang englishly.Near the stream, trying to avoid the sponge effect,Lobster traps...use only with lobsters at sea, not in your building techniques,J

          2. Guest_ | Apr 30, 1999 10:13pm | #6

            *Since you mentioned foil faced RFBI, have you or anyone heard of interior application of such a material which posed serious health problems from offgassing? Someone suggested to me recently that a family died in a building with some sort of "uncured" RFBI. No fire involved. Didn't make much sense to me, but I was planning on a foil faced polyisocyanurate inside application and just wanted to seek feedback. Thanks.

          3. Guest_ | May 01, 1999 02:26am | #7

            *SteveI gutted the inside of my current house 9 years ago and installed 1" foil faced polyiso (Thermax Sheathing) over f/g in stud bays,taped seams with metal tape, then covered with d/wall.Still have a few sheets in storage out in the "back porch".Family and I are still fine as far as I can tell. . . neighbours etc. think I'm a raving sociopath but exhibited those tendencies pre RFBIdrool, drool, grunt-pm

          4. Guest_ | May 01, 1999 07:04am | #8

            *Patrick...So thaaaaat's the deal with you...RFBI outgassing and twisted braincells linkage....More bedtime strories...Has anybody seen the list of "Things your wife wouldn't normally ask?"Near the stream,J

          5. Guest_ | May 01, 1999 09:05am | #9

            *Kcajummm. . . what was the question???

          6. Guest_ | May 02, 1999 01:51am | #10

            *Jack. On eway to solve the capillary sandwich problem, is to eliminate one of the sheathjing materials.Steve Wright. Two problems with RFBI on the interior. (1) unhappy sheet rockers and plasterers.You'll pay extra because of the extra work RFBI causes on interior of exterior wall. (2) this combinatiuon creates an ice boax wall cavity. Exterior must be air-tight.I've never heard of anyone dying from out gassing of RFBI. How do we know it wasn't caused by CO? I lived in a house whose exterior walls were 6 inches of spaayed-in PUR foam. Lived there 10 years. No lung problems.Fred B. RFBI is rigid foam board insulation. GeneL.

          7. Guest_ | May 03, 1999 08:39pm | #12

            *Patrick,Thanks for your reply. I've started installing the stuff. No drooling yet, but I did grunt a little. Hmmmm.Did you make any special provisions (ie horizontal strapping) for hanging kitchen cabinets or tiled bathroom walls, etc. Steve.

  2. david_zaborny | May 03, 1999 08:39pm | #11

    *
    I would like to put foam board over plywood sheating to increase the R value of a wall and then put cedar shingles or siding over the foam sheating and was wondering if it could be done and if I would be creating any moisture problems within the wall cavity.If so what would be the correct way to detail it?

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