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Rainscreens: a definitive checklist?

88Clayton | Posted in General Questions on

Is there a definitive, authoritative checklist one can use to determine if a rainscreen is needed?

Nobody uses rainscreens in fiber cement siding houses where I live. I’m in mixed-humid Climate Zone 4.   My area gets 55-60” of rain a year.  Half of that rain can come in just 3 months.  My area is humid and often wet…. like “I’m sick of this rain, where is the sun” wet.

Assuming roof overhangs are pretty standard, how necessary is a rainscreen? Yes, I know it can be a “good idea”, but is it enough of a good idea to insist on one?

Also, having a large front porch that will protect a large section of siding from 99% of typical rain, is it possible to do a rainscreen on more exposed walls, then turn the corner into the porch and skip the rain screen?

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Replies

  1. Jon_R | | #1

    Other than code or manufacture's instructions, not a black/white issue. I expect accurate quantification isn't possible.

    If you don't use a larger gap (furring or mesh), consider TYPAR Drainable Wrap (a good perm # and drainage).

  2. Expert Member
    MALCOLM TAYLOR | | #2

    Clayton,

    As Jon said it's a judgement call. Does it help every wall? Absolutely. Enough that it's worth doing in every climate? Probably not - although the judgement also needs to the into account how the walls are constructed. Rain-screens protect against bulk-water intrusion, but they also increase the drying capacity of walls (like double-walls) that may otherwise be vulnerable to concerns of wet sheathing.

    If the main concern is outside moisture, by all means pick and choose where you want to use it. The first adopters of rain-screens here in BC in the early '80s only used them on vulnerable north-facing walls and those subject to storm driven rain.

    I wouldn't build without a rain-screen even if my code allowed it, as I have adapted all my details to them, and I like the resulting robustness they yield.

  3. canada_deck | | #3

    Not a simple answer but some interesting reading here:
    https://www.probuilder.com/building-envelope-moisture-control-mixed-humid-climates

    One thing to keep in mind is that although it might be the "way it's always been done" where you are, other things are changing. No doubt your building will be more air-tight than buildings 5-10 years ago and that impacts the ability of the structure to dry out.

    It doesn't cost a lot more to build your wall with a rain-screen. Why not do it?

    1. 88Clayton | | #4

      What do you think of products like Coravent or Tyvek Rainvent? Are these better and/or cheaper than wood 1x furring?

      I’ll have board and batten too. Would it be needed for that?

      1. Expert Member
        MALCOLM TAYLOR | | #5

        In general the plastic rain-screens don't give you anything wood or plywood battens don't provide - except they cost a lot more. However, something like Cor-a-vent that can be mounted horizontally and doesn't impede air-movement, makes a lot of sense with a cladding like board & batten. And board & batten is a cladding that really benefits from a cavity behind it, because it typically lets a fair amount of moisture get through to the back of the boards at the joints.

        1. 88Clayton | | #8

          Do they cost more than the 1x3 and 1x4 that would be used over foam?

          1. Expert Member
            MALCOLM TAYLOR | | #10

            I suspect that their cost varies, as does the price contractor's charge to install them, based on location and how comm0n the practice is.

            Where I am, plywood strips are sold at all the lumberyards. They are a lot cheaper than both 1"x and plastic.

    2. 88Clayton | | #9

      Interesting link. Thank you. Looks like all walls were acceptable with or without rain screen ventilation approach. I wish it had mentioned something specific about fiber cement. I know it was included in the tests, but the article made no other specific comments about it.

  4. GBA Editor
    Martin Holladay | | #6

    Clayton,
    Considering how much rainfall you get, I would recommend a rainscreen gap. But as other commenters have noted, it's a judgment call. For more information, see "All About Rainscreens."

    1. 88Clayton | | #7

      Thanks!

      What kind of added cost can be expected regarding labor? Is this charged by the wall square footage minus window space?

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