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What is an acceptable air barrier separating showers and tubs as referenced in the 2009 IECC Section 402.4.2.2?

LeonM | Posted in Green Building Techniques on

I live in Zone 5 (No.NV) and it has been the local custom to place unfaced batt insulation in outside walls between showers and tubs in new construction. Line item 13 of the referenced Inspection process under the 2009 IECC specifies an air barrier to be used. I’m a certified HERS Rater performing these inspections, but having much resistance to this item.

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  1. user-1087436 | | #1

    No response from the pros, so this amateur will give it a try. I think it's called Thermo-Ply. It's thin and foil-faced, and I've seen it referred to elsewhere on this site. One builder (Matt Risinger in Texas, I believe it was) called it "glorified cardboard," but it's what they seem to use in this situation. Prof. Lstiburek also has referred to it.

  2. davidmeiland | | #2

    I guess that would be better than nothing. It looks like rigid material and it looks like it could be taped.

    The reason you're getting resistance is that it's a part of the job that doesn't belong to anyone on the typical jobsite. The plumber wants to come in and slap the tub/shower against the framing, nail it in place, plumb it, and leave. The insulator wants to come in and shove insulation behind it. The drywaller wants to lap his board over the flanges and tape it.

    Who's going to add the air barrier layer? The insulator would have to come in early, put his material in the tub area, then leave. Then someone (the insulator? the drywaller? the plumber?) would have to bring a few sheets of Thermo-ply or whatever to the job, and install it. Then you're back on track and the plumber can do his thing. Oh, except he has to rough in the water supplies before the board goes up.

    Yeah, it's going to be a headache on a production oriented site. You might get some traction if you can lay out for your builders an exact way of approaching this. Right now, it's no one's job.

  3. GBA Editor
    Martin Holladay | | #3

    ThermoPly is the product usually used for this purpose, but many other sheet materials can also be used, including gypsum drywall, plywood, or OSB.

    I'm attaching a photo of a worker installing ThermoPly over fiberglass at the location of a shower. I took the photo in Las Vegas.

  4. LeonM | | #4

    Thank you for the responses. I was not familiar with Thermo Ply, but did some research after Gordon's reply. Like Martin indicated, I might have some luck in suggesting the use of OSB or plywood since it is more commonly available. David saw right through to the problem.

    While we are on the subject of the Inspection process, another local custom is to install open faced batts in contact with the underfloor, but with the crawl space still vented. This to me is worse than a Grade III install as in Grade III at least all six sides are enclosed? Usually a plastic film is placed on the ground to manage ground moisture, but the film is left loose around the edges and not sealed to the stem walls. The benefit of insulation doesn't match the time and cost?

  5. GBA Editor
    Martin Holladay | | #5

    Your instincts are correct. Fiberglass batts don't belong in a crawl space. And unless the batts are enclosed by an air barrier on all sides, they aren't doing much. It won't take long for them to start falling down under the influence of gravity.

    Here's more information on insulating a crawl space: Building an Unvented Crawl Space.

  6. ohioandy | | #6

    I just witnessed another behind-the-tub air barrier solution in a Habitat for Humanity build that is closely monitored by a HERS rater: 30-lb roof felt.

  7. LeonM | | #7

    Thanks Andy, I'll do some research.

    I looked at a house today where the insulation company stapled on the face of the studs a clear plastic sheet over the unfaced batt insulation. That doesn't work as there was moisture on the plastic on the insulation side. With our cold nights in the mid 20's, I have to assume the moisture inhabits all of that area in the 2 x 6 wall.

  8. GBA Editor
    Martin Holladay | | #8

    You wrote, "I just witnessed another behind-the-tub air barrier solution: 30-lb roof felt."

    This is not a good solution. Unless you can seal the seams of your air barrier material, it's not an air barrier. There is no good way to seal the seams of asphalt felt. Moreover, it is easily damaged during installation. I vote for a solid panel material like ThermoPly, drywall, plywood, or OSB -- with sealed seams.

  9. ohioandy | | #9

    Martin, I checked with the supervisor on that HfH build, and apparently the roofing felt passed muster. He sealed the seams with Dow foamboard tape! Now I think you'd have to be pretty willful to damage 30-lb felt during installation, but sealing felt with ANY tape is a bizarre leap of faith. (For that matter, how can tape be trusted to seal ANYTHING more than temporarily? That's a question for another thread...)

    The trouble with a solid panel is that it has thickness, and unless you've designed for that from the outset, there isn't room. Solution: design for that from the outset!

  10. davidmeiland | | #10

    The thickness of the thermo-ply is going to be a non-issue if the installer takes one small added step when he works the tub area--furr out the rest of the studs on the affected wall(s) with strips of scrap thermo-ply material, or something similar. In Martin's photo above, he could get another sheet, slit it into 1-1/2" strips, and staple them along the studs above and to the left of the shower area. That way the drywall will lay flat instead of bowing out when it gets to the shower flange.

    A good drywall installer is going to use "butt strips" on that wall--they are strips of cardboard 1/16" x 1-1/2" x 48 and are very easy to use where the studs need to be built up behind drywall.

    A very small amount of extra work... something production builders really don't want.

  11. LeonM | | #11

    My original question centered on the placement of "snap together" tub or shower surrounds placed over the unfaced fiberglass. The surrounds may be "water proof" due to the design of the flanges to shed water, but the joints are not "moisture proof" which allows the unwanted vapor to enter the wall.
    As pointed out, any barrier with thickness will have to be designed in from the beginning or shimmed if needed.
    Again, thank you for the comments.

  12. Expert Member
    MALCOLM TAYLOR | | #12

    Leon, It's a fairly common practice to use asphalt building paper or roofing felt on shower, especially those which are tiled and have used a permeable substrate such as cement board. The felt is there as a barrier to water infiltration, much as it is used on exterior walls behind siding. These installations are essentially relying on the shower surface itself, not the felt, to act as an air barrier. Poly used as an air barrier works as well behind a shower as it does elsewhere on any exterior wall. If there is adequate drying capacity to the outside and it is not in a climate where cooling is used in the summer it should be fine. In the wall systems commonly talked bout here on GBA which rely on drying to the interior it will be problematic and subject to the moisture build up you observed.

  13. LeonM | | #13

    Thank you Malcolm. I agree that the tiled surrounds afford more moisture protection and should not be a problem. What I'm seeing are the mid-priced production builders installing the plastic/fireglass surrounds for time/cost benefits, disregarding the long term affects.

  14. NateFournier | | #14

    I can't find anywhere that supplies Thermo-Ply anymore... will 1/4" Foamular XPS work as an air barrier behind a shower surround, with Tyvek taped seams?

    1. GBA Editor
      Brian Pontolilo | | #15

      I've never seen it used in that application Nathan, but I don't see why it wouldn't work as long as it is detailed right.

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