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On an exterior walls sheathed with 1/2″ ply, asphalt felt, and ribbed steel exterior siding – Where will condensation form?

rocket190 | Posted in Energy Efficiency and Durability on

I’m in the northern part of climate zone 6B, approx 80 miles south of climate zone 7B.

I’m in the process of finishing my insulated and heated workshop. The walls are 2×8 framed with 2×4 cross strapping, so the insulation thickness of the walls will be 8.75″ at the full depth areas. The exterior sheathing is 1/2″ cdx with Zip tape at all seams, followed by astm #15 asphalt felt, with screw down vertical rib exterior steel siding. Windows are triple pane with a U=.19 rating. The interior finished wall surface is 7/16″ acx plywood that would be only nailed to the framing members. No caulk or tape to air seal.

I’ve been interviewing insulation companies and have finally reached a company that really seems to understand energy efficient construction methods and insulating methods. His company has a track record of several homes testing at a .65 ACH or better without spray foam, and his previous clients seem extremely satisfied. He is proposing to install a blown fiberglass bib system with an approximate full depth cavity r-value of R-34.

However, he is adamantly recommending that an interior vapor retarder being installed. His recommendation is to install continuous poly from bottom plate to top plate, sealed at the edges with Tremco acoustic sealant. The poly rolls would be wide enough to cover the entire wall without a horizontal seam. Any vertical seams in the field would be taped. His concern is moisture from the interior reaching the cold exterior sheathing and condensing. I was planning on no interior vapor retarder, under the assumption that the interior plywood will function as a smart vapor retarder. It concerns me that the exterior steel siding is likely a vapor barrier. I’ve been trying to figure out how much benefit (if any) that the ribs help the siding function as a vented rainscreen. Google searches have been fruitless, but I don’t think I can count on much drying to the exterior.

Where do you think that moisture would condense in this assembly? Does it condense on the nearest cold surface (i.e. interior side of the exterior sheathing) or do you think it would condense on the interior side of the steel siding? Condensation on the siding would probably be harmless, as it would be able to follow the asphalt felt and flow out the bottom of the ribbed siding. If it condenses on the backside of the plywood sheathing, I could potentially have rotten, wet sheathing.

Because the space is a shop (no cooking, no showers, etc) there are few sources of interior moisture. I still want my walls to be as resilient as possible. Thoughts?


  1. Expert Member

    Just a comment on the steel siding. If it is a corrugated profile it will create its own rain screen cavity and will allow drying to the exterior - as long as you don't seal the bottom with the usual L flashing. Instead use perforated stock to provide ventilation.

  2. rocket190 | | #2

    Malcolm, the picture attached is a decent representation of what my bottom trim looks like. The bottom is not perforated. I never even thought of the bottom being perforated. Is that a custom made product?

    Mine is not perforated, but I think it will allow some air movement since there is a small gap at the bottom and top.

  3. rocket190 | | #3

    siding profile

  4. Expert Member

    I agree. With that profile I don't think you can rely on much drying to the exterior, so poly on the interior probably isn't a very good idea at all. Without the poly it sounds like just the kind of resilient, energy efficient assembly you were aiming at.

  5. user-1140531 | | #5


    Poly on the interior side is said to be unnecessary because it stops diffusion and diffusion is said to be insignificant. However, airflow will transfer water vapor out of the heated space and into the wall. Then if the insulation is air permeable, the vapor will travel out to your plywood sheathing inner surface and condense there if the surface is below the dew point.

    If I understand it, your contractor recommends interior poly, and you want to omit it. You also say that you will not air seal the interior plywood. Why do you choose to not air seal it? I would air seal it to prevent air from transporting vapor into the wall. Maybe it will be sufficiently air sealed without tape or sealant if it is just nailed tight, the framing faces are true enough, and the plywood edges are butted tight. But that is a lot to count on as being reliable.

    Without the air sealing, the only factor that would make the wall resilient is the minimization of vapor sources in the heated space. That may be successful if you have no cooking, no washing with water, and no running water, no vehicles with snow or ice on them, or any other significant source of water. Otherwise, I would call it risky. Interior vapor will pass through the wall and reach the cold sheathing where it is likely to form ice. If the process continues, the ice will move into the insulation as well.

    It will dry to the interior eventually, but it may take a long time. I would keep the vapor out of the wall in the first place rather than let it into the wall and wait for it to dry out later.

  6. Expert Member

    He has an air barrier at the exterior taped sheathing. There shouldn't be much air movement into the walls.

  7. user-4524083 | | #7

    Rick - I'm guessing that you don't want to seal the interior plywood for aesthetic reasons. Fair enough. But you could certainly caulk the plywood carefully to the framing as you put it up, and keep it mostly invisible, and similarly caulk the plates to the floor and ceiling. That would make it considerably tighter to any interior moisture traveling through the assembly and condensing on your sheathing.If you end up using a film inside, MemBrain would be preferable,as it wouldn't trap any moisture inside the wall and would allow drying to the interior if necessary.

  8. GBA Editor
    Martin Holladay | | #8

    You wrote, "He [the insulation contractor] is adamantly recommending that an interior vapor retarder being installed. ... I was planning on no interior vapor retarder."

    You should tell the contractor that you already have an interior vapor retarder -- a Class II retarder at that. The interior plywood layer is rated at 0.5 perm. (For a list of the permeance values of a variety of materials, see All About Vapor Diffusion.)

    If I were building this wall, (a) I wouldn't install any interior poly; (b) I would probably install the interior plywood in an airtight manner, while recognizing that this step is unnecessary; and (c) I wouldn't worry.

    Next time, install your steel panel siding on furring strips.

  9. charlie_sullivan | | #9

    If the contractor is more comfortable and experienced with using poly to achieve air tightness, it may make sense to buy MemBrain and have him install that since it fits in his flow, rather than asking him make the plywood air tight, even though the air tight plywood approach would be fine.

    As for whether the air tightness is necessary there, probably not, but
    1) there will be some convection currents into the wall from the interior at the top, and back from the wall to the interior at the bottom if it is not sealed. That's minor compared to what you'd have if the exterior was leaky too, but it would only help to reduce that.
    2) The exterior is sealed well, but surely not perfectly. Again, not a major concern, but if you've got a contractor who actually wants to do good air sealing, you might was well let him do it!

    Be warned that Tremco sealant is smelly and smells some for months. I prefer Contega HF from 475 to eliminate that problem.

  10. rocket190 | | #10

    The reason air tight plywood is difficult is that the walls are 14' tall. If I install the plywood horizontally, there would be no studs to seal to at the ends of the sheets. I don't want to air seal the plywood with wood because the plywood is my interior finish. I thought of applying tape on all seams and covering it with pine 1x4s, but that seems like a time consuming and wasteful endeavor.

  11. rocket190 | | #11

    Thanks all for the recommendations. This site has been a major help.

    I like the idea of a secondary interior air barrier, so I went ahead and purchased Membrain for the inside. I had a chance to inspect the product at Menards, and it does seem tone as tear resistant as 6mil poly. I was worried that is would be fragile. On a side note, menards is discontinuing membrain, so it was marked down 35% and there is currently an 11% off sale. The price per square foot with discounts is about $0.07 per square foot, which is comparable to clear 6mil poly. Anyways, my plan is to air seal the membrain as well.

    Martin, how would u vent steel siding on a furred out wall?

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