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Will open-cell spray foam require an additional vapor barrier?

Terry Blackwell | Posted in Green Building Techniques on

I am planning to apply open cell foam to the interior ceiling and walls of my metal building. Currently, the building has metal siding with no vapor barrier and creates some condensation. The contractor is planning to apply the open cell foam directly to the metal siding. Will this be a problem? What will happen to the condensation? Will I need to add a vapor barrier?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "The contractor is planning to apply the open cell foam directly to the metal siding. Will this be a problem?"

    A. The main problem with this technique has nothing to do with vapor diffusion. The main problem is that when some of the metal panels that you are using for siding begin to rust -- a phenomenon that usually starts at the bottom of the panels, which are subject to regular wetting due to splashback -- it will be very difficult to replace the rusting panels. The spray foam will glue everything together.

    Q. "What will happen to the condensation?"

    A. There shouldn't be any condensation. Because the metal panels won't absorb moisture, I doubt if you will have any problems. But the usual advice for buildings with wood sheathing, OSB sheathing, or plywood sheathing would be that you need an interior vapor retarder if the building is located in Climate Zone 5 or anywhere colder.

    Q. "Will I need to add a vapor barrier?"

    A. No, you won't need a vapor barrier. But you will definitely need a layer of gypsum drywall on the interior side of the spray foam for fire safety reasons. (Most codes require this barrier.) The drywall should be taped. It wouldn't hurt to install a layer of vapor-retarder primer (paint) to the gypsum drywall. A vapor retarder is a less stringent layer than a vapor barrier.

  2. Terry Blackwell | | #2

    Thank you for clearly addressing my questions. Each of these questions have been a concern for me as I have contemplated my decision as to what method of insulation to use. I hadn't considered the difficulty of replacing the panels should the need arise. Your answers were very helpful.

    We live
    in Southeast Texas and it get's pretty hot here. The other insulation method that has been suggested to me is fiberglass batts with a vinyl covering as a moisture barrier. These are commonly used in metal buildings, but I am concerned they won't offer enough in the way of keeping the building cool in the summer and warm in the winter. Any thoughts?

  3. Terry Blackwell | | #3

    I forgot to mention that we are building out the interior of the building as a living space.

  4. GBA Editor
    Martin Holladay | | #4

    There are many challenges involved whenever anyone tries to transform an uninsulated steel building (a building type associated with barns and unheated warehouses) into living space. In most cases, the best approach is to build a separate building inside the steel building, with new walls and a new ceiling (and probably a new floor -- depending on how you intend to insulate the existing slab).

    The major problem is that steel buildings have no air barrier, and every conditioned building needs to control air leakage.

  5. Terry Blackwell | | #5

    Martin, The scenario you stated is exactly what we are doing. We are into the project too far now to change directions and you are correct in stating that it presents many challenges. You bring up a couple more questions for me. First, how do you suggest insulating or sealing a slab that was not previously sealed? Secondly, would you recommend insulating the new walls and ceiling rather than the original barn enclosure?

  6. GBA Editor
    Martin Holladay | | #6

    If 100% of the old building will be transformed into a conditioned space -- in other words, if you aren't partitioning the building into two halves, with a house on one side and a garage/barn on the other -- then you can insulate the slab with vertical insulation at the slab perimeter. This vertical insulation needs to be protected on the exterior from sunlight and physical abuse, and it needs Z-flashing at the top of the insulation. The Z-flashing needs to be integrated with the WRB or siding.

    If the building will be partitioned into two sections (conditioned and unconditioned), you have no choice but to install insulation above the slab, followed by a new plywood subfloor or floor framing.

    To create an air barrier for your walls -- assuming that you have abandoned the idea of installing spray foam directly against the steel siding -- you pretty much have to install new 2x4 or 2x6 walls on the interior side of the steel walls. You can sheathe the walls with plywood or OSB before they are raised.

  7. Terry Blackwell | | #7

    Thank you for your advice. This gives me a lot to think about.

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