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How do I detail unfinished wood rainscreen siding?

slugboy6000 | Posted in Green Building Techniques on

I don’t like the idea of paint, stain, or wood preservatives. They’re expensive, a lot of work, and often toxic. Unshaped, wood boards are available, durable, relatively cheap, and easy to replace. Of all the siding options, unfinished wood benefits the most from a rainscreen, yet is proving to be very difficult to detail.

Two ways of installing board siding in a rainscreen fashion is vertically “board and batten” style, or horizontally as clap boards.

Board and batten rainscreen furring should be laid on the diagonal, or in a double layered cross-hatch pattern to allow for ventilation, support, and water drainage. Both are tough to detail, and irritating, I’ll just leave it at that.

The profile of clapboards is insect heaven, and I live in wasp-opolis. Painstaking attention to detail would be required to keep them out of the ventilation space. I don’t know how capillary gaps left above header flashings can be properly detailed to resist insect entry.

Going with shaped wood siding (channel, dolly varden, bevelled etc.) doesn’t have the issues of unshaped boards, but they’re more expensive, difficult to replace, and THINNER. Locally, shaped wood siding is 3/4″ thick, the thinnest parts being only 3/8″…. I question how long these profiles will hold up if left unfinished, even on a ventilated rainscreen. Does anyone know?

OPTIONS:
1) Forget about rainscreens, and rely on the durability of thick unfinished wood.
2) Have fun furring board and batten.
3) Go with clap boards and live with the wasps
4) Use a shaped siding and expect to replace it

Anyone have any advice? Sorry for introducing you to my madness,
-Richard

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Replies

  1. davidmeiland | | #1

    I don't see it as being that hard.

    First, for vertical siding, I don't agree that you need to install the furring diagonally. I would install it horizontally, and I would bevel the top of each strip outward. Nail the boards on vertically with the usual ~1/2" gap between, nail the battens over the gaps. There is enough space between the boards to allow air and water to move.

    Second, for horizontal siding, you need some type of filler material at the top and bottom of each bay to prevent bugs from nesting. I have seen folks using the 3d plastic mesh intended for eave venting for this purpose, and there are plenty of other ways.

  2. slugboy6000 | | #2

    Beveled horizontal board and batten furring is a great idea. It's too bad ventilation couldn't be improved. I guess the 3d mesh you spoke of could be used to stuff the gaps between boards at the tops and bottoms?

    I've been avoiding specialized plastic products to keep insects out, but they would solve some of the insect entry issues.
    I've seen designs using insect screen but have yet to see them applied with enough support to be effective...and for good reason, it's a difficult detail.

    Detailing clapboard ends is tough, even with 3d plastic meshes. At wall corners, and at gable slopes, clapboard ends would have to butt against something solid to stop insects from accessing the ventilation space via the profile of the siding itself. I'm not sure I can achieve this level of precision.

  3. davidmeiland | | #3

    Another option is to use something like Benjamin Obdyke Home Slicker, possibly the 10mm version. That goes on the wall without gaps, and then siding over it. There's nowhere for bugs to live. I've used it before and am about to use it again. It's not cheap but it's really fast to go up and avoids a lot of issues. I'm not a scientist but it seems to me like a vast improvement over nothing, although it's not the ~3/4" gap that some people are using.

  4. slugboy6000 | | #4

    Thanks for the recommendation. If I wasn't such a purist I'd be jumping all over some of these products. I'm wondering if bugs behind the siding is really an issue. As long as I have a tight air barrier on the interior, who cares if my wall assembly is crawling with critters. It would take a lot of bugs to impede the 1" of ventilation I have planned.

  5. Expert Member
    MALCOLM TAYLOR | | #5

    Richard, Both doubling the cavity furring for vertical siding and installing insect screens to block rain screen cavities are done all the time. They aren't very difficult or time consuming. I wouldn't dismiss either type of siding based on the objections you have set out.

  6. GBA Editor
    Martin Holladay | | #6

    Richard,
    Thousands of homes have lap siding (clapboards) installed over a rainscreen. Rainscreen vents are routinely screened with insect screen or this product from Cor-a-Vent.

    The traditional way to provide ventilation behind board-and-batten siding is called reverse board-and-batten. It's simple: first you install your vertical 1x3 or 1x4 battens on top of the asphalt felt, and then you install your 1x10 or 1x12 vertical rough boards. That way you have an air gap behind your boards. Of course, you don't have an air gap behind the battens, but so what? Everything stays dry. It's a good system.

  7. slugboy6000 | | #7

    It's comforting to know that these systems are commonly used, I shouldn't have to re-invent the wheel figuring out the details.

    Maybe I'll post one issue at a time and see if I can get some help.

    Although I've read a lot, I'm a beginner and don't have much practical experience to draw on. I'm dependent on, and greatly appreciate the advice of those with experience.

    Issue #1:
    At the top of a gable wall in a clapboard, or board-and-board rainscreen installation, neither insect screen, nor cor-a-vent will effectively seal the area between the wall sheathing, and the back profile of the siding.

  8. GBA Editor
    Martin Holladay | | #8

    Richard,
    The area you show in the detail is usually trimmed with a frieze board. If you shim the back of your frieze board with spaced 3/8-inch shims, you can get air flow behind the frieze board. The frieze board prevents insects from sneaking in the triangular gaps between clapboard courses.

  9. slugboy6000 | | #9

    I don't think I understand... Are you advising that I use a frieze/trim piece to cap the profile of the cut clapboard ends (please see drawing)?
    It could work, but wouldn't it be very difficult cutting the clapboard ends with enough precision to get a nice flush, clean edge which the frieze could press up against and create a decent seal?

  10. davidmeiland | | #10

    There are some siding installs where you have to make precise cuts at the gables. That would be one of them. Again, not hard, but it does take some careful work. You make a couple of short pieces with the gable cut in them, one left and one right, and use them as gauge to figure the length of each piece. Or, you develop a way of measuring (or calculating) the length of each successive piece. It's skilled work, but I would just go for it and figure it out. Based on your drawings you do not appear to be without some serious figure-it-out-ness.

  11. slugboy6000 | | #11

    *Thanks* Martin, and David. I've never had to be that precise before, but if that's how it's done, I'll give it a go. I'll try to get issue #2 posted later today.

  12. slugboy6000 | | #12

    Issue #2: Detailing around windows and doors.

    I want to apply the window and door trim on top of the siding. I know that's not how it's usually done, but it should be more water resistant than having the siding butt into the trim. Clapboard ends would have to butt against window and door jambs in order to keep bugs out.

    Above the end dam of every head flashing above all windows and doors, there is a space through which insects can access the ventilation space. I can't think of a good way to plug this hole, but it could be filled with a carefully shaped wedge, along with lots of trial and error. The good news is that this wedge could be duplicated and used for all windows and doors on that floor.

    There is also a space below all window & door head flashings. Clapboards could be precisely shaped, and the head trim could be beveled to accommodate it's odd shape.

    Is this crazy talk? Do real world builders actually pay this much attention to detail like this?

  13. Expert Member
    MALCOLM TAYLOR | | #13

    Richard, The most comprehensive guide to detailing rainscreen I have found is the Building Envelope Guide for Houses, which is put out by the Homeowner Protection Office of the BC government. It has diagrams and 3D drawings of all the conditions you are asking about, and is available here:
    http://www.hpo.bc.ca

  14. slugboy6000 | | #14

    Thanks Malcolm, that looks interesting,... and expensive. Can't I just keep harassing Martin?
    Government publications too often advise the use of carcinogenic goop. Is this one like that?

    I love that they not only charge for copies of the code, but for the guides on how to meet it.

    (Somethings funky with this website...posts aren't showing up until hours after submission.)

  15. Expert Member
    MALCOLM TAYLOR | | #15

    "Somethings funky with this website...posts aren't showing up until hours after submission"
    Hence my triple post to you...
    I agree completely. Why isn't the code free? You are expected to obey laws, but you have to pay to find out what they are. Absurd.
    The guide is expensive, but one of the few publications worth buying. It really is comprehensive in its coverage of all building envelope detailing from foundation to vents and electrical fixtures. It does use conventional materials - including caulking and membranes - which you may find aren't green enough for your tastes, but substituting alternatives shouldn't be difficult.
    Whether you chose to bug Martin or buy the guide, good luck with your project.

  16. slugboy6000 | | #16

    Maaaarrrtin! (just kidding)
    I've been trying to figure out how to best seal the head flashing area using a wooden wedge. The attachment shows it all. The shape of the wedge would change depending on the position of the clapboards. In summary, the clapboards on rainscreen approach requires a very high level of precision carpentry... everything would need to be within an 1/8" to keep bugs out, and honestly I don't think I'm skilled enough to do it.
    I'm going to investigate board and batten and see if it's any easier.
    (Is it ok if I keep posting this stuff?)

  17. slugboy6000 | | #17

    Attached below are some drawings of vertical board-on-board siding over a rainscreen.

    I would implement David's suggestion of using beveled horizontal furring strips.

    My method for screening the bottom of the wall, and above windows is the same, and I'm not happy with it. The horizontal trim/waterboard which holds and covers the insect screen will catch water running down the wall. I'm looking for a better solution.

    Although clapboards offer better ventilation, it's going to be easier to detail vertical edges with board and batten, and the profile is less insect prone overall.

  18. GBA Editor
    Martin Holladay | | #18

    Richard,
    Your analysis falls somewhere in the gray area between good planning and overthinking. If you have a basic understanding of flashing principles, odd little problems that crop up in the field are often (but not always) easily solved with a field decision and an improvised solution. (Note: by "improvised solution," I don't mean "gobs of caulk.")

    Cutting clapboard precisely isn't as hard as you think -- especially since the invention of the power miter saw.

    You detail shows a horizontal water table board under the bottom of your vertical battens. That's a bad idea. It's just a shelf to collect water -- designed to rot. If you want to put something there, install some metal flashing.

  19. slugboy6000 | | #19

    I do overthink, but these are basic rainscreen details for common siding applications which are not freely available online. Hopefully this stuff will help out someone else.

    I attached a drawing showing a metal flashing to replace the water table board , but it's not working out.

  20. slugboy6000 | | #20

    It's more complicated, but I think this might be it.
    The water table isn't necessary, but the vertical furring strips are there anyways - might as well use them.
    The same setup can be used above windows sans water table.

  21. Kopper37 | | #21

    Richard,

    I'm attaching some pictures of a rainscreen installation.

    You're right that it's not a common technique (at least not in my part of the country), so planning and forethought is required. Cor-A-Vent has some basic details on their website. Building Science Corporation has several articles about rainscreens---and several real world examples of using a rainscreen for new construction and renovation. You can also find an informative article in JLC March 2006, by Gary Katz & Bill Robinson, titled "Detailed Rain-Screen Siding."

    I found that the main siding field and large penetrations were fairly simple. You do need to think about the details: by that I mean ventilation grilles, electrical outlets, conduit penetrations, and transitions between stories.Where are you going to hang your meter base? Heat pump disconnect? Anyway, you'll work much of it out during the actual installation.

    As mentioned before, the eave and gable frieze boards overhang the ventilation gap, hiding that transition. I used 2X lumber underneath the frieze, to provide enough clearance for the siding (thickness) and to provide for the required ventilation gap. It was perfect for thickness of the fiber cement. See the attached pictures.

    For windows, I didn't get too complicated. I taped the window flange to the sheathing, applied felt to the wall as a counterflashing, installed the rainscreen, and then installed the trim head flashing (during the siding phase). Some people might frown on this, and say that the flashing should go behind the rainscreen. It depends on your climate, window type, window exposure, roof overhang size, etc. Remember that what little water gets behind the rainscreen has a drainage and evaporation path. That's the whole point, right?

    Depending on your window trim (if any), you will have to think about changing your furring strip size to accommodate both the trim and the siding. I've seen examples that used a "wing stud" for attachment (stud on its face, against the king stud and sheathing). If you want to tuck the siding behind the window trim, you will need a furring strip at least twice as thick as your main field furring strips (thickness would depend on the installed thickness of your siding).

    Back dams on the head flashing? Is it really necessary? For brick, maybe. For clapboard siding? I dunno. If you are really insterested in executing this detail, you will figure it out . . .

    I used the Cor-A-Vent product that Martin linked above (the SV-5 version) and 3/4" thick furring strips to match. Compared to the BO Home Slicker product, cost was roughly comparable. However, labor costs would be quite different . . .

    Hope this helps.

  22. slugboy6000 | | #22

    Hi Daniel, your name is very familiar, feels like I've read hundreds of your posts.
    I forgot about the BSC real world examples, but your photos sum up what they recommend nicely I think. I hope my project can look as slick as yours.
    I've read everything online regarding rainscreens, but the information available assumes you'll use product A, or window B. Trying to be non-toxic, I chose to deviate (minimal plastic, wood windows, & deep profile wood siding) and am paying the price for it by having to invent all the detailing. I think I have it all worked out now though.

    Here in Ontario, I believe end dams and back dams are required on head flashings.

    I have to get my butt in gear, otherwise I'll be building in the snow.
    -Thanks for the pictures-

  23. Expert Member
    MALCOLM TAYLOR | | #23

    Richard, here is another reference for basic rain screen details:
    http://www.mtcc1170.com/images/BCRainScreen.pdf

  24. EthanT | | #24

    Unless I'm reading it wrong, it looks like the Cor-A-Vent in the second image posted by Daniel Ernst isw instaleld flush agains the horizontal termination of the rainscreen. This seems to negate the very purpose of the Cor-A-Vent or Maybe I'm reading it wrong?

  25. GBA Editor
    Martin Holladay | | #25

    Ethan,
    I'm not sure I understand your point. Clearly, in addition to installing the Cor-A-Vent product as sceening, a builder has to leave a slot or gap somewhere -- one at the bottom for air intake, and usually one at the top for an air outlet.

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