GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Rain Screen Recommendation Under Vertical Metal Siding?

Madcodger | Posted in Green Products and Materials on

We’re building a home and garage barn in Zone 6 (Maine). Framing is 2×8 with a 1.5″ layer of foam on the outside and we’re still debating either dense pack cellulose or mineral wool for the cavities, and whether to use Membrain, Intello Plus, or Majrex as the interior vapor barrier. Exterior is covered by 1/2″ WeatherLogic taped at seems and over fasteners as the exterior WRB, and we plan to use vertical metal siding with a board and batten look as the exterior cladding, to keep it low maintenance. If the siding was corrugated I wouldn’t be concerned, but this board and batten-look material has a flat piece that sits against the wall that’s about 8-10″ wide, and then the “fake batten” that’s about 2″ wide by 3/4″-1″ deep. That’s where my problem comes in. Local builder and materials supplier are both discouraging a rain screen because they’re afraid the metal siding will look wavy, but I’m not totally comfortable having that 8-10″ piece of flat metal against the WRB, and relying on those “fake battens” for drainage. Thoughts on that? Will those “fake battens” be enough to drain the cladding properly over Weather Logic? If not, can anybody recommend a good rain screen material to use over Weather logic panels, and under vertical metal siding? Thanks in advance for your help.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member
    Michael Maines | | #1

    What type of foam are you using? Hopefully not foil-faced polyiso, and preferably EPS or GPS, for permeability reasons. XPS would be ok at that thickness but I would only use it with either Majrex or Intello, because they have lower perm ratings and are more durable than Membrain.

    Your siding profile will provide a small amount of ventilation, but a risk when using highly insulated walls like yours is that even with a well-detailed vapor retarder on the interior, some amount of moisture will still make its way to the sheathing, with little heat energy remaining to push the moisture through the sheathing. The more air flow you can provide behind the cladding, the easier it will be for the sheathing to dry. Ideally you would cross-strap the wall with vertical and horizontal strapping (typically 1x3 spruce in Maine) but since the climate impact of the materials you are using doesn't seem to be a priority for you, you might consider using Coravent rain screen battens, oriented horizontally.

    1. Madcodger | | #4

      Hi Michael,

      Well, of course it's foil-faced polyiso, because that's what gets recommended by the builder supply folks, and what local builders are familiar with using (I live not far from you). My original request was for 4" of exterior foam or rock wool, but the builder and supplier were hesitant to go with that due to concerns about fastener length. So, putting the original rain screen question aside for a moment, what I'm now thinking of doing to prevent condensation on the inner side of that exterior sheathing is having a layer of 2" of closed cell foam sprayed onto the inside of each cavity, right onto the inner side of that polyiso, to give me additional insulation and, more importantly, a vapor barrier. That should give me about R-26 or so between the foam and the polyiso - warm enough to prevent condensation inside the wall while also protecting the sheathing from the back side (or so goes my theory). Then, I'm thinking dense pack cellulose to fill the rest of the cavity, putting us over R40. If I do that I now think I should be able to avoid the smart vapor barrier altogether, although at an increased cost from the CCSF. Thoughts on that would be appreciated.

      Back to the rain screen, thanks for the ideas. As I look at it, the Weatherlogic has a bit of dimpling to it that will help it dry, but not as much as I might prefer..

      Finally, re: climate impact of the materials... What I want, and what I can find in terms of people locally to build it, are two different things. Frankly, this is the first of two buildings, and I need to get this first one done, fast. We wanted to start on it last year but couldn't get it onto a builder's schedule. So, we're doing what we can to make a more energy efficient building but we're making more "non-green" compromises than I'd like to make, just to get the building done on the timeline we need. We plan to do the house (other building) next year, and I'd start on it this fall if the builder could get us weathertight before winter. BTW, happy to talk about that project over a beer if you have an interest in a somewhat small local project!

      1. Expert Member
        Michael Maines | | #7

        If you have R-30ish in the stud bays and R-8ish impermeable insulation on the exterior, you WILL have condensation issues. The problem may take up to ten years to show itself, but you can't violate the laws of physics forever and get away with it.

        Spray foaming the inside of the sheathing will work. The usual advice is to follow the ratio of impermeable to cavity insulation implied in R702.7.1:, which for us works out to at least 33% of the R-value to be impermeable. A higher ratio is safer.

        Because this is a long-term moisture issue, I use the long-term R-values for foam, not what is published, which is after 6 months of aging in carefully controlled conditions. I use about R-5.6/in for closed cell spray foam and depending on the situation, about the same for polyiso. So your impermeable layer would really be R-11 in the spray foam and R-8 in the polyiso, for a total of R-19. Dense-packed cellulose is about R-3.6/in so you'll have about R-19 of that as well. A 50:50 ratio is safe, almost safe enough to eliminate the required Class 3 vapor retarder on the interior, but not quite, so you'll need painted drywall. With that ratio you shouldn't need a variable permeance membrane, though adding one is always good insurance.

        Rain screens do three things: Allow liquid water that gets past the cladding to drain, which even LP's tiny dimples and the ribs in your siding should provide; allow the cladding to dry on the back side, which isn't an issue with metal siding; allow the wall assembly to dry to the exterior, for which you need at least 1/2" and more is better. With your double layer of foam, virtually zero moisture should be getting through from the interior, so I don't think you need the deep rain screen, though like a variable permeance membrane, it's almost always good insurance against something going wrong.

        I think it's worth trying to reduce the climate impact of the materials we choose, but the decisions made on a single house aren't going to make or break the environment. That's why I mention it here often, where many eyes can see it, and maybe on your next project you'll be able to plan ahead for a lower climate impact. But building is inherently a destructive act and I make compromises on every project, so I understand your viewpoint.

        I'd be happy to meet up for a beer sometime, but I'm fully booked with projects for at least a year.

  2. BirchwoodBill | | #2

    What does the metal profile look like? The vertical metal siding from Luxe or Vesta have a 3/8” gap - so no rain screen is required. Can you pour water down the back of the siding?

    1. Expert Member
      Deleted | | #3


    2. Madcodger | | #5

      Well, it doesn't look like that (rain gap on back). It looks a lot like flat, standing seam metal roofing except that it has a "fake batten" about every 10", that gives it a large (3/4"-1" deep x about 1.5" wide channel. It's the flat part of that metal that worries me a little.

  3. iwatson | | #6

    Maybe take a look at Ventgrid and see if it's available in your area. The builders of my ADU used it under the metal siding and I have no complaints about the quality of the finish.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |