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Rainscreen siding with a very large gap

user-2510005 | Posted in Green Building Techniques on

Reading about rainscreen siding systems it seems that the recommended gaps are typically 1/2″ to 3/4″ between the housewrap and and the back of the siding. Is there any reason why one couldn’t do a very large gap using thicker furring material? Maybe around 3″ or 4″ between the siding and the sheathing for architectural reasons on a remodel? Would there be problems with having that large of a gap?

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  1. Expert Member

    Kevin, Taking it as a given that you have a reason for leaving such a large space empty and you have thought out how you will support the thicker furring, the only thing to watch for is providing fire blocking as may be required at floors, concealed spaces and the roof.
    There are practical issues with flashing such a deep cavity but nothing insurmountable.

    My own preference if I wanted to increase the wall's depth for architectural reasons would be to incorporate the new framing into the existing wall and build a conventional rain screen gap on the new built out exterior.

  2. GBA Editor
    Martin Holladay | | #2

    In addition to the challenges raised by Malcolm, there is another disadvantage: very thick walls restrict the amount of natural light that reaches the building's interior. There are ways to address this problem -- for example, by splaying the rough openings of your windows -- but in my mind, you need to have a very compelling reason to thicken your walls to make up for this disadvantage.

  3. user-659915 | | #3

    I don't share Martin's distaste for thick walls - the quality of light obtainable from a deep interior reveal is matchless. But an oversize rainscreen gap is probably not the best way to achieve it. I don't know why you wouldn't put that extra dimension inside the sheathing/WRB and pump it full of cellulose.

  4. user-2510005 | | #4

    Thanks for the reply's. To clarify there only a few sections where the wall will be furred out. The reason is to provide articulation where there are changes in the cladding material simply to add architectural interest to what is currently a very plain facade. Usually I would just build the wall thicker or build out the framing, however since this a remodel it would be easier to leave the current sheathing in place and just have a larger gap under the rain screen. It also seems like adding a second layer of sheathing and insulation over the current insulation and sheathing would be a bad idea potentially trap moisture? Project is located in a transitional area between climate zones 4C and 5B.

  5. Expert Member

    You would need a second layer of sheathing, but it's a lot easier to build out walls with, say, 2"x4" framing which can be supported by a bottom plate / ledger and avoid all the long fasteners necessary to attach unsupported studs and the blocking for trim if you treat them as deep furring. Rain screens need to be open at the bottom for drainage and air flow. The flashing has to extend from the WRB to the face of the cladding. Even if you do choose to leave the new cavities empty, I still think treating the bump outs as wall, not rain screen, would be a lot easier.

  6. gusfhb | | #6

    It would seem like more insulation is the easy answer, rather than bigger gap.

    If one was clever it could be use to advantage. For instance if minisplits were used as the heat source, the thicker wall section could be located in an area far away or in a bathroom that might run cold. Probably a tiny difference but wouldn't hurt

  7. wjrobinson | | #7

    You are building a critter condo

  8. user-2510005 | | #8

    Yes, though about the critter condo problem too. Well as long as they pay their HOA dues ... but thinking about all the responses will just remove the sheathing and build the bumped out areas with 2x4's, new sheathing and a normal 3/4" rainscreen gap.

  9. fitchplate | | #9

    I would find the challenge in the detailing of the joinery/connections of the siding to the windows, doors, bottom sills and soffits. How do you carry the profile to the foundation?

    But even if you can solve the fussy detailing problems, there is something no one mentioned, yet: convection. You can’t really consider your siding in your R value calculations: a 3-4" air space will strip off heat in winter and increase cooling loads in summer. A fundamental principle of using dense and solid insulation in today building technologies is to diminish and hopefully eliminate convection paths and cycles in walls and roofs.

    Too, AJ's point about critters should not be dismissed.

  10. Expert Member
    MALCOLM TAYLOR | | #10

    Kevin, Why remove the sheathing? I'd just build over it.

  11. user-2510005 | | #11

    Update: I removed all the sheathing and built out the walls with 2x4's. A lot of work, but came out great and hopefully no problems with critters. Well except for a pair of angry giant house spiders I found living in the walls after pulling off a piece of the old T1-11. They must have been 2" across. Malcom to answer your question about why not build over the sheathing is that it's badly deteriorated T1-11 from the 1970's.

  12. Expert Member
    MALCOLM TAYLOR | | #12

    Kevin, Glad it turned out well.

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