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Rainscreen Under Trim – Skip It?

jamesboris | Posted in General Questions on

I’m in Zone 2A, installing 1” thick cedar lap siding on a ventilated ½” rainscreen. My trim stock is 2 3/4″ thick cedar — enough to cover the laps even without a rainscreen (the thickness is partly happenstance, partly an effort to use local materials as much as I can). Fiberglass windows. No exterior foam.

I’d rather omit the rainscreen behind the trim… but how much do you think it matters? How substantially does it change the risk calculation? 

Every detail I’ve seen yields very little air flow behind the jamb trim (which is caulked on both edges), just dead air space behind the head trim (which is sandwiched between metal flashing (back to the sheathing) and caulk (to the window frame), and maybe a little flow behind the sill (detail here: Bottom of Window (

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  1. Expert Member


    You are fine. Any attentional moisture at the trim will diffuse into the cavity adjacent to it. The increased risk is minimal.

    The only change in detailing it occasions is the location of the head-flashing, which moves from immediately above the window or door to over the trim.

    1. jamesboris | | #2

      Thanks Malcolm. As usual I think I wrote an unclear question (edited now) -- I meant to say, I'm leaning toward *omitting* the rainscreen behind the trim. So, trim would be on top of WRB, siding would be on rainscreen battens.

      "You are fine. Any additional moisture at the trim will diffuse into the cavity adjacent to it. The increased risk is minimal."

      But if I don't put any rainscreen battens under the trim, how will there be a cavity adjacent to it? I could put the adjacent siding batten 1/4"+ away from the trim, which, in combination with my non-beveled lap siding, would create some air space.

      I should also add... with the bugs here, I feel I have to caulk the siding-trim joints. I've read good arguments against it here with respect to water infiltration (e.g. from Peter Yost), but yeah, I'll have a cathedral of mud daubers behind my siding if I'm not careful...

      1. Expert Member
        MALCOLM TAYLOR | | #3


        No I understood what you were proposing. I do it regularly. The trim as lots of drying paths - including into the cavity it butts up against. I typically leave a 1 1/2'" gap from the trim to the first batten - especially around windows where it allows some air movement up the wall.

        I understand the rationale for not caulking cladding because it can trap moisture, but that all changes when you include a rain-screen, which provides an alternate path to dry. I like the keep the cladding (the primary layer of protection) as water-tight as possible.

        1. jamesboris | | #4

          Got it, thanks Malcolm. I really appreciate your combination of technical knowledge and field experience. Trying to get there myself. I've loved building for a long time, but this project has been the first where I'm getting to try everything I've learned here on GBA. I read about rainscreens all the way back to the 2010 Riversong wildfire debate ha. I think I've almost got it now. Well, stay tuned :)

          1. Expert Member
            MALCOLM TAYLOR | | #5


            When rain-screens requirements were brought into our code here in Coastal BC I wasn't thrilled. I felt they were compensation for builders shoddy work. But once I started using them the benefits were so obvious I was converted. I bet you will enjoy this build!

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