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Floor system as part of vapor management system

StrikeKBT | Posted in Green Building Techniques on

Considering the transition of the vapor management layer from the lower (finished) level to the next, I find myself looking for an alternate method to tie the walls together than patching in between joists.

This appears to be a critical location and with good chance of failure. Why not use the ceiling as part of the system, whether with a paint or membrane vapor layer? Does anyone know of any information, monitoring articles on this method? thanks

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  1. GBA Editor
    Martin Holladay | | #1

    I don't understand your questions.

    What makes you think that you need a "vapor management layer"?

    To understand your question, we need to know your climate or your location.

    I assume that the "lower finished level" is an above-grade room of a house, but I'm not sure.

    I assume that "the next" means an unconditioned vented attic, but I'm not sure.

    You need to describe what you are talking about.

    Here is a link to an article that explains why most concerns about vapor diffusion are not grounded in science: Do I Need a Vapor Retarder?

  2. StrikeKBT | | #2

    Hi Martin,
    Thank you for your reply. This new residence will be built in zone 6. The Lower Level is a finished walkout basement. The level above is the finished main level. I'm interested in information addressing best means to join the variable vapor management layer (Intello PLus), of these lower and main level walls, at the perimeter of the floor assembly. Patching insulation and vapor layer in between joists well seems unrealistic considering all of the materials and joints, so questioning; why not consider using the painted ceiling as part of the system? The floor will be filled with blown in fiberglass.

  3. GBA Editor
    Martin Holladay | | #3

    I don't know why you have chosen to install Intello membrane. Some builders use it as an air barrier; others have wall systems that may benefit from the fact that this membrane is a smart vapor retarder.

    Some questions:

    1. Why do you need Intello?

    2. What layer is your air barrier?

    3. Please describe all of the layers of your wall assembly.

    4. What is your plan for insulating the rim joist area?

    If you can answer these questions, it will be easier to provide advice.

  4. StrikeKBT | | #4

    1. Using Intello Plus on the inside surface of 2x6 Cellulose packed stud wall, with two interior alternating layers of 2x3 strapping, as electrical service chase and to reduce thermal bridging, over the intello, with it filled with mineral wool batts. R31
    2. Air barrier is on the exterior of the wall sheathing with appropriate sealing
    3. Rim could be insulated with mineral wool or cellulose. The vapor management connection here is what I concerned with.

  5. GBA Editor
    Martin Holladay | | #5

    In Climate Zone 6, a wall insulated with cellulose does not require a vapor control layer on the interior -- especially if the wall has a good air barrier, as yours evidently will.

    The Intello is unnecessary but harmless. There is no need to be concerned with the continuity of the vapor control layer, since it is unnecessary.

    I prefer to insulate rim joists with rigid foam, but it is possible to insulate rim joists with cellulose, as long as you come up with a plan to contain the cellulose. Here is a link to an article about a house where the rim joists were insulated with cellulose: A Superinsulated House in Rural Minnesota.

    Another possible approach is to recess the rim joists 2 inches toward the interior. The recess provides room for 2 inches of rigid foam on the exterior side of the rim joist. That method helps keep the rim joist warmer and dryer than it would otherwise be.

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