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Community and Q&A

Flashing mounting blocks in a rainscreen

tjones1014 | Posted in General Questions on

Climate zone 3A/4A
2001 home gut reno
Exterior wall assembly – OSB w/ taped seams, Tyvek commercial WRB, Benjamin Obdyke slicker max rain screen, Hardie

My GC and siding team seem confused about how to incorporate mounting blocks with a rain screen system. All wires, ducts, etc. are flashed to the Tyvek using flex tape and quickflash boots. They don’t know whether to put the blocks directly on the tyvek and butt the rain screen up to it, or rain screen over everything and mount the blocks on top of the rain screen.

Any advice would be greatly appreciated!

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Replies

  1. Expert Member
    Michael Maines | | #1

    Blocks should be treated just like a window or door-flashed into the WRB (aka housewrap) and detailed to drain and be airtight, except for the hole the wire or pipe goes through. They need to be thick enough to span the rainscreen gap and the cladding depth. They can then be flashed again into the cladding, or sealant may be enough, depending on the rain screen and cladding systems.

    A good rule of thumb for detailing exteriors is that they should be 100% watertight BEFORE any cladding is installed.

    1. tjones1014 | | #2

      The wires and ducts that would be going through these blocks have already been integrated with and flashed to the WRB. To clarify, both the block AND the duct/wire/whatever need to be integrated with the WRB?

      I watched this video where Risinger puts the block on top of his rain screen of choice (furring strips). Does that not translate to this scenario as well?

      https://www.youtube.com/watch?v=sV3zzrRKr7A

      1. Malcolm_Taylor | | #3

        tjones,

        Risinger has this one wrong. The rule for flashing on rain-screens is that horizontal changes in material that occur on top of the furring (so things like belly-bands, or changes in types of siding) have the flashing applied to the furring, which keeps the cavity behind continuous. All penetrations (windows, doors, blocks, vents) get flashed back to the WRB.

        If you don't, the penetration is unprotected from water in the cavity - which is the case with that vent where Risinger mounted the block on the strapping.

        1. jollygreenshortguy | | #4

          I'm not sure I understand how Risinger got it wrong. The penetration is sealed with Proseco back to the WRB, which is the face of the Zip sheathing. With no cladding this penetration would still be watertight. The block just basically provides a neat edge for the siding to butt against.

          1. Malcolm_Taylor | | #5

            JGSG,

            The drainage plane for any water in the rain-screen cavity is interrupted by the round duct sandwiched in between two pieces of furring. Like windows and doors, when the cavity is interrupted the flashing needs to divert water to the exterior (something explicitly called out in our code).

            Suppose this was a small arched window. Would you mount your head-flashing on the furring and rely on liquid flashing to keep the frame behind that dry and divert any water to the jambs?

      2. Expert Member
        Michael Maines | | #6

        I don't watch videos unless absolutely necessary and generally don't put a stock in what sponsor-driven influencers have to say. That aside, integrating the block with the WRB has always been my preference as I think it's safe and simple to do. In general I agree with Malcolm's advice, and appreciate that Canadian building codes are significantly ahead of ours in the US when it comes to building science. But I can also see that considering the block to be part of the cladding instead of part of the WRB could work, as long as the wires or pipes are completely watertight and airtight in their holes--a difficult feat, but possible. I would not use that approach for anything larger than a light block though.

        1. Malcolm_Taylor | | #7

          Mike,

          Exterior insulation over the WRB can definitely complicate things. With a typical rain-screen on sheathing I don't see any downsides to running the blocking back to the WRB.

          I'm not sure Canadian building codes are ahead of American ones in all areas of building science any more. They still don't make the distinction between vapour-barriers and vapour-retarders you see in the IRC

          1. Expert Member
            Michael Maines | | #8

            Malcolm, good point--I would be more concerned in that case. I re-read the post and noticed that the rain screen is only 1/4" plastic matrix so I think flashing the block in would actually be easier as well. If it were a deep rain screen requiring thick light block stock, that might make a difference. Same for exterior insulation.

  2. tjones1014 | | #9

    My contractor did not wait for me to get clarification on this before he gave the order to add the mounting blocks on top of the rain screen. So now the blocks are on top of the rain screen. Any tips for making the most of this situation? Anything we can do from the interior to help with what we already have in place (flashing boots, flex wrap, etc.)?
    Note: Please refrain from doom and gloom comments, as I'm already stressed enough about this whole project as it is. I just want to make the most of the current situation. Thank you!

    1. Malcolm_Taylor | | #10

      tjones,

      I was commenting on best practices, not the possibility of disaster. What you have will be fine - especially as like Michael I missed that the rain-screen gap is just Slicker.

      1. tjones1014 | | #11

        Sorry, I wasn't insinuating that you were being doom & gloom. I just didn't want any of those messages now that things didn't go the best practice route like I wanted. Things keep going wrong, so the stress and frustration is high, and I don't want anything else adding to it. Definitely wasn't a comment directed toward anyone specifically! :) Thank you!

        1. Malcolm_Taylor | | #12

          tjones,

          I didn't take it that way at all. I know exactly where you are right now. It's hard to keep perspective when a lot is being thrown at you at once. The mounting block is fine. I can say from experience, pretty soon you forget all these problems and get to just enjoy your house. If possible remember that's coming soon when things feel like they are piling up.

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