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Community and Q&A

Flashing windows in a “vent” screen

wwLuke | Posted in Green Building Techniques on

Seems like every new project these days requires re-inventing the wheel on flashing systems. Here’s my new one. 10″ double stud construction with celluose insulation. Even though I’m in a dry climate (rocky mountain west) I want this wall to dry to both sides so airtight drywall with no vapor barrier. Plywood, (not osb) exterior sheathing. Since we’re using vertical FS siding I want a vent cavity between the sheathing and the siding, plus, I don’t like the idea of nailing FS to just sheathing so horizontal 1X4s provide both a gap and solid backing. I don’t need a proper rainscreen, but this detail will allow everything to stay dry. The devil is in the details though. flanged windows are installed on the outside of the strapping. Tyvek has to go against the sheathing, under the strapping. So how do I tie the flexible flashing over the window flanges back to the tyvek? Current proposal is to install tyvek, then a continuous bead of sealant around each window opening, then the furring strips, then install a window , continous,flex flashing, etc… (see the detail attached). The problem is that the flex flashing basically has to wrap around the 1x furring and stick to the face of the tyvek. I may be over thinking this but any thoughts would be appreciated.



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  1. 2tePuaao2B | | #1

    What does the line outside of the hardi plank represent?

  2. Riversong | | #2

    The problem is that the flex flashing basically has to wrap around the 1x furring and stick to the face of the tyvek.

    Yup. That's how I've typically seen it done, and it's one of several reasons that I think a rain screen is excessive and counter-productive except in high rain/high wind zones.


    Your window extrusion stands proud of the flange by 1" so go ahead and install it on the sheathing and housewrap and tape it as usual then install the 3/4" strapping as you have shown over the tape and it will sit 1/4" shy of the window, install and caulk the 5/4 x 4 composite window trim over the strapping and caulk the 1/4" overlap between the back corner of the trim and the window extrusion (won't need any backer rod) install your fiber cement siding over the strapping and all is well, Not that big a deal after all. (assuming your extrusion does in fact stand proud 1" from the flange, the ones I use do)

  4. Lucas Dupuis | | #4

    Thanks guys. First to Robert. Believe me, I'd rather not detail rain screens, but I have concerns about vertical FS siding applied directly to plywood sheathing for a couple reasons. Maybe you have some thoughts. 1. with a 10" cellulose wall the inside of the sheathing is going to be below dewpoint. We'll do everything possible to avoid water getting to that point from the inside. The house will be well sealed and have an HRV. However, with vertical hardiplank applied directly over the plywood the possibility for bulk water intrusion inherent in vertical siding and inward vapor drive from the FS will not have much of an opportunity to dry out. I've seen this create rot with SIP construction and hardipanel even in this dry climate. If the house had lap siding I probably wouldn't sweat it as much. Michael- by the time you nail 1x and 5/16" hardipanel around the window the face of the siding is actuallly slightly proud of the extrusion making the caulk joint kind of messy. If I butt the siding to the 5/4 the 3/4 battens sit slightly proud of the face of the 5/4 window trim. The real bugger on this one is the vertical siding.


  5. Riversong | | #5

    I think you mean FC (fiber cement) siding.

    And that's the reason I never use vertical siding. It's simply not as weather tight as shingled siding (lap or other).

    We should work with gravity, not fight it.

  6. lucas dupuis | | #6

    yes, I meant Fiber Cement... that was a typo. Unfortunately I find that building science and "architecture" butt heads sometimes. I was just hoping someone out there had run into this and had a magic bullet solution since vertical siding is fairly common.


    From what I'm looking at in your drawing the siding is separated from the window flange by the 5/4 window trim and stands proud of the window trim in either case. I was assuming that you had side and bottom trim on the window as well and would be having this issue in any case at all sides. I think the solution is to use thinner battens or possibly a reverse board and batten detail, sometime called board-on-batten. Another solution would be to use a thicker window casing. we just did a reverse board and batten project with local rough sawn 1x8 poplar over local rough sawn 1x4 poplar battens and trimmed our windows out with PT 2x4s applied over the battens with the window trim and the extrusions overlapping about 1/8" which is where the caulk went. The poplar for this project was $1,300 including delivery for the boards and battens for the entire house.

  8. Riversong | | #8

    I find that building science and "architecture" butt heads sometimes.

    And they can both butt up against common sense.

    Reverse B&B is classier than regular B&B, but I maintain that either is a good siding for barns and outbuildings, not houses (it's kind of like shingling a roof sideways - I actually had a professional roofing crew insist on installing felt vertically, and then only after I insisted they use a shingle underlayment).

  9. mike eliason | | #9

    despite robert's thinking - rain screens don't 'fight gravity' and they're also not 'excessive'.

    also, not everyone views shingles as an aesthetically pleasing finish - after all, it's hardly the 1880s.

  10. Riversong | | #10


    You could respond more appropriately if you actually read what I posted.

    Of course I never said that rainscreens fight gravity, only that any cladding which is not shingled in a top over bottom manner is not working with the laws of gravity - and that proper cladding is not limited to sidewall shingles, but (as I wrote) includes lap siding of all types, as well as roofing tiles, slates and shingles and traditional thatched roofs.

    That vented rainscreens or unvented drainscreens are excessive in all but the wettest and windiest climates is not only my thinking but also that of the HUD Partnership for Advanced Technology in Housing (PATH), as delineated in their extensive 125-page Best Practices manual called Moisture Resistant Homes.

    It's also, by the way, common sense - a quality in very short supply among today's designers and builders, but was actually far more common in the 1880s.

  11. mike eliason | | #11

    even a rainscreen (not shingled) works with the laws of gravity, robert - it's not as if rainscreens create some gravitational field anomaly. non-shingled siding (esp. as a rainscreen) works as effectively as shingled siding - and tends to look significantly better as well.

    HUD isn't anti-rainscreen... far from it - their best practices manual calls for their use:
    "Consider a drained cavity weather-resistant envelope (WRE) system for most non-severe climates and building exposures"

    a drained cavity WRE is what we architects call a rear ventilated rainscreen.

  12. Lucas Dupuis | | #12

    Hey Guys. please don't hijack this thread over your disagreement about siding options. I think I have a reasonable argument for using a vent screen in this application. I asked the GBA community for help with a unique detail and I'll bet that I'm neither the first or last person to run up against this. Thanks to all who provided constructive advice. As far as design advice goes, I would be happy to participate in a constructive conversation about the lack of "common sense" in this industry on a different thread.

  13. Riversong | | #13

    Apologies Lucas, but these discussions are always organic and may diverge into tangents that others might find useful. As a regular participant here committed to sharing the best of what both building science and common sense has to offer, I will not allow disinformation to go unaddressed.


    If you read the HUD report more carefully (or don't deliberately misuse it) you will discover that a drained cavity WRE is NOT a rear ventilated rainscreen. HUD - and the entire building science community - acknowledges four types of WRB cladding systems: the sealed face, the concealed barrier, the drained cavity and the vented rainscreen.

    The drained cavity relies on a capillary break and gravity drainage, while the vented rainscreen adds evaporate drying enhanced by convection.

    And the entire sentence which you failed to fully quote reads: ""Consider a drained cavity weather-resistant envelope (WRE) system for most non-severe climates and building exposures, or select alternative WRE approach based on climate, site condition and target performance level."

    In other words, consider a drained cavity approach unless you're willing to do the work to select the appropriate WRE approach for your specific climate severity, site exposure, overhang ratio and moisture index.

    And - once again - yes, a rainscreen works with gravity, but also adds unnecessary and potentially problematic complexity, including WRB and flashing continuity (as discussed here), extra materials and cost, an exterior fire chase in wildfire country, and the aesthetic challenges of integrating windows with trim and siding. It also decouples the cladding from the thermal envelope and thereby has the unintended consequence of reducing the solar radiant drive which helps drying to the interior within the structural and thermal cavities.

    So you would better serve both your clients and this GBA community if you would read carefully and better educate yourself before responding.

    "Leading building scientists recommend rainscreen systems be used in areas prone to wind-driven rain and/or areas that have an average annual rainfall of 40 inches or more".
    - Benjamin Obdyke, manufacturer of a variety of drainscreen products.

    "In areas like the Southwest that receive low rainfall (less than 20 inches annually), a housewrap or building paper should offer sufficient water resistance protection, according to most building experts. In areas that experience moderate amounts of rainfall (20 to 40 inches annually), protection against rain penetration should include an enhanced housewrap. And for wet and/or humid climates, coastal areas and hilltop exposures receiving high (40 to 60 inches annually) or extreme (60 inches or more annually) rainfall, a ventilated rainscreen assembly is recommended; a rainscreen system is also advised for areas that receive high winds in addition to rain. Rainscreen systems are recognized by leading building trade associations for their effectiveness in controlling rain water intrusion into wall assemblies in areas of high and extreme rainfall."
    - “To Build a Better Home,” published by the APA-Engineered Wood Association, 2002

    And Building Science Corps supports the fact that the overwhelming majority of the North American continent is in the low to moderate exposure zones.

  14. Lucas Dupuis | | #14

    I know you're a fan of thick cellulose walls, so I'm curious on your thoughts about this. I'm sold on the idea that you need to allow any wall system the opportunity to breath to both sides of the assembly. So, no vapor barrier, and a higher perm exterior sheathing (no OSB). In the case that a wall is clad with a fairly open exterior cladding such as lap siding or shingles, and has overhangs and low to moderate exposure I can see a traditional drained cavity WRE as prescribed by the HUD report being sufficient. However, enter the inevitable variables such as vertical siding or panel siding, higher exposure walls, etc... and I start to get concerned because the inherent capillary break between siding and the house wrap just isn't there. A second concern with FC siding in particular is its hygroscopic qualities and the potential for inward vapor drive from solar exposure. Again, even a small capillary break mitigates this. I ask this because the building community in general doesn't often think about these kind of potential incompatibilities. Sorry if I'm one of those design snobs who refuses to use vinyl siding because it solves this problem. I know there's a better solution out there.

  15. mike eliason | | #15


    per the HUD's own flier, " A drained cavity WRE relies on deflection, drainage, and drying to protect the wall from moisture damage." - oops, try again robert.

    "So you would better serve both your clients and this GBA community if you would read carefully and better educate yourself before responding." - likewise. not everyone wants to live in the cheapest, ugliest, soul-less box 'common sense' can build.

  16. Riversong | | #16


    You're quite right. Any deviation from common sense best practices requires additional methods of protection from the consequences. But then those additional methods of protection, as you realize, require more complex "solutions" for proper integration of all elements and the increased complexity creates more opportunity for unintended consequences.

    The KISS principle is still the best security.

  17. 2tePuaao2B | | #17

    Lucas, If you install the1x4 furring cont. over header first, then install Tyvek continuous to wrap around the 1x4 and into the window opening.
    Next Tyvek tape a 6"~ 8" flap above the wrapped 1x4 firring that metal head flashing will tuck under.
    Install window flange overTyvek and then caulk and flash window flange w/flexible flashing tape adhered to Tyvek.

  18. Lucas Dupuis | | #18

    KISS is antithetical to the designer's mandate. You're right, your solutions work, but clients come to me to design personalized, aesthetically driven homes that also perform and last. I greatly respect the KISS principal, but if that wins, I'm out of a job. I'm trying to merge the disciplines of architecture and building science here, and that's not allways easy. Thanks to everyone who contributed ideas. If i find the magic bullet you can be sure I'll share it on this forum.

  19. jbmoyer | | #19


    So let me get this straight...

    If I don't have vertical siding on my home, I live in cheap, ugly, soul-less box?

    Give me a break. Your pretensions are obvious and sad.

  20. mike eliason | | #20


    oy. please DO get it straight - i never stated if you didn't have vertical siding you lived in a cheap, ugly, soul-less box.

    mr. riversong advocates for buildings sans detail, 'luxury' upgrades, interior spatial variation and rainscreens (obviously) - deeming anything beyond what he believes to be acceptable as 'excessive' - damn the owners and their intent/desire to live in something they enjoy and appreciate. not everyone wants to live in a soul-less box like that - i certainly don't, none of my friends, co-workers, collaborators, clients want to live that way. go ahead and put whatever siding you want on your house - it doesn't bother me, hell even shingles are fine given the right context. i'm just trying to expand the discussion beyond the continual "oh noez!! a little extra work and thought for a building that performs well and looks stunning is 'excessive'" meme.

  21. homedesign | | #21

    can you post a fresh drawing (with instructions)after you make the revisions
    I am having trouble visualizing this detail

  22. Riversong | | #22


    Sounds more like you're pimping for architectural vanity than for true soul. The soul of a house (or any other human artifact) is more a result of proportion, materials, texture, "fit" into it's environment, "fit" with its essential function, and the craft and care that is imbued into it than by architectural embellishments.

    A simple log cabin typically has lots of soul, while very few modern homes have any at all - they are mostly an expression of the ego of either the architect or the owner.

  23. mike eliason | | #23

    actually, robert - you seem to be confusing what architects do:
    proportion (9' ceilings)
    materials (what you term 'luxury' items)
    well sited
    taking advantage of views, light, shadow
    functions well
    well crafted, etc.

    with what you do - architectural embellishment (e.g. the gables on the walkway and facade of the warren house)

    any act of building is an expression of ego, even the smallest hermitage.
    and yes, lots of modern homes lack soul. i never stated that they didn't. if you wish to continue proving you know nothing about architecture, proportion, the play of light - please move it to another thread.

  24. Riversong | | #24

    any act of building is an expression of ego

    You've proved my point.

    And, if you don't understand that entry gables primarily serve the essential function of diverting rain and snow from those who pass through those portals, then you have failed architecture 101. If you don't further understand the importance of "welcoming entry" as part of the essential pattern language of a soulful house, then you have failed advanced architecture.

    I've taught hundreds of architects the elements of soulful and functional design and construction. You really need to go back to school. You obviously missed your most important lessons.

  25. Lucas Dupuis | | #25

    for craps sake people. I asked a question about flashing a window. You've turned this thread into a pissing match over architectural semantics. I'm an architect. I happen to be an architect who cares more about building science than most in my profession. By virtue of my chosen trade I find myself challenged by details such as flashing a window on a drain screen wall with vertical siding. I don't believe that lap siding= a soulless box, but I also believe that lap siding is not the only answer to building durable, soulful home. I joined GBA because I think I know what I'm doing and I want to learn more. If anybody wants to have a productive conversation about BUILDING SCIENCE. I welcome their input on a different thread. Please invite me in, but I'm done with this one.

  26. 2tePuaao2B | | #26

    For craps sake~ nice term...

  27. homedesign | | #27

    sorry to hear that you are "Done with this One"
    maybe someone will start a spin-off thread
    1880's vs Today's Fashion Statement?

  28. Kopper37 | | #28


    I'm probably missing something here, but why does the window frame have to sit perfectly flush with the window trim? Why can't the frame be recessed inside the trim? What's wrong with a small reveal between the frame and trim?

    You could follow Michael Chandler's suggestion---keep the window flange tight against your plywood sheathing---and keep your metal flashing in the location shown in your drawing---then the water management details are simple.

    What problem does this create? Is this an aesthetic issue? Aren't reveals used to avoid problems with trying to mesh and flush various surfaces? Why can't a trim reveal work in both directions (in OR out compared to the window frame)?

    I hope you haven't fully disengaged from this thread!


  29. Riversong | | #29

    The problem is the joint between the bottom of the window frame and the bottom exterior casing, which would have to be sealed with caulk (back to face sealed cladding!).

    One of the things that worked in old buildings was that wooden or masonry window sills were sloped and projected out from the cladding surface to allow drainage and protection. Today we use flat exterior casings, which are fine as long as they don't project beyond the sloping "sill" of the window frame but are problematic if proud.

  30. T7sX5ebany | | #30

    "Leading building scientists recommend rainscreen systems be used in areas prone to wind-driven rain and/or areas that have an average annual rainfall of 40 inches or more".

    a rainscreen works with gravity, but also adds unnecessary and potentially problematic complexity, including ... an exterior fire chase in wildfire country,

    What siding details do you recommend for those areas in which there is both a wildfire hazard and an annual rainfall greater than 40 inches?

  31. Riversong | | #31

    With high annual rainfall there is generally little wildfire risk.

  32. T7sX5ebany | | #32

    Thank you Robert, for a prompt and interesting reply. It seemed obvious that your objection to rainscreen siding on the basis of wildfire risk was spurious and you have just confirmed it.

  33. 2tePuaao2B | | #33

    Timmy, You are picking... to the tavern with you!

  34. T7sX5ebany | | #34

    Roy, I would agree but for the fact that the contradiction could usefully be addressed to improve the quality of the advice offered. I fully expect the response to this post to be sufficiently off-topic for it to be consigned to the Tavern. If you're over there before me - what are you having?

  35. Riversong | | #35

    It seemed obvious that your objection to rainscreen siding on the basis of wildfire risk was spurious

    Obvious? Only to those who have either not thought through the issues or are ignorant about wildfire risk and precautions.

    1) Wildfire risk is highest in dry woodlands with poor overstories and lots of underbrush, not in temperate rainforests with 40+ inches of rain per year.

    2) In wildfire country, it's illegal to have vented soffits or roofs, for the same reason that vented cladding is problematic as an exterior fire chase that can quickly move ground fire into the roof.

  36. T7sX5ebany | | #36

    I couldn't agree more with your most recent post. I'm sure that you will accept that you have thought through the issues and are not ignorant about wildfire risk and precautions. It would therefore be obvious to you that your objection to the use of rainscreen siding on the basis of wildfire risk flatly contradicted your own (and HUD's) recommendation that rainscreen siding was only suitable for regions with an annual rainfall of 40 inches or more. Even the worthy Homer sometimes nods. And your homes are surely worthy.

  37. MICHAEL CHANDLER | | #37

    To the tavern with you!

    The San Diego wildfires were much exacerbated by flames pulling into the attics through vented soffitts, and we have both wind-driven rain and droughts with wildfire risk here in North Carolina and it is not illegal to have vented roofs.

    Here's a sketch of what Lucas' situation might look like with the flange taped to the house wrap as usual and 1 1/2" casing and an angled 2X sill with a drip kerf.

  38. T7sX5ebany | | #38

    Sorry if I'm one of those design snobs who refuses to use vinyl siding

    If you're still here, please be assured that if there's one thing the disparate voices of GBA all sing in harmony it is "We Don't Like Vinyl Siding".

    I joined GBA because I think I know what I'm doing and I want to learn more.

    It's why the vast majority of us are here. That and wanting to learn the technical why as well as the technical what.

  39. T7sX5ebany | | #39

    Since the previous person I asked dodged the question, what siding details do you recommend for those areas in which there is both a wildfire hazard and an annual rainfall greater than 40 inches?
    This is a genuine question - I'm planning a house in a region which is currently soggy but getting slowly drier year on year.

  40. HDendy | | #40

    To add to Michael Chandler's detail (which I'm in agreement on), reducing the thickness of the furring strips would help too, 1/2" should work fine (instead of 3/4), possibly even less (or "cedar breather" or eq.). It makes the detail much simpler to keep all flashing/WRB in one plane, work out the trim/siding details from there.

    Lucas, I hope you haven't completely abandoned ship. I agree it can be difficult to get straight answers at times...but I also enjoy some of the deviation (as long as it remains civil).

  41. T7sX5ebany | | #41

    Thank you for such a detailed and helpful reply. It has given me a great deal to think about - exactly what I expect from the technical advice given on the GBA forum; exactly what I was seeking.

  42. MICHAEL CHANDLER | | #42

    Fire resistant rain screen siding is tough.

    I think stucco works pretty well and the drip screed gives you vented back drainage with pretty robust flame protection in that the air entry is low and there is a lot of mass in the stucco, assuming that we're talking about a fire that doesn't have a lot of fuel on the ground close in to the house where the entry to the back of the siding is. The soffit is much more vulnerable due to the intensity of the heat and wind at that level. We used stucco on the Bryant residence for this reason, (the wife was concerned that her scientist husband would blow up the work shop building experimental aircraft) It's a double wall structure with regular house wrap detailing and screwed on fiber cement window trim with two layers of felt, galvanized weep screed and two coats of stucco on SpiderLath gasketed fiberglass stucco lath.

    I think Lucas' detail would work well if you just worked out the bottom of the vent screen carefully and used fiber cement for the battens as well at the boards. I found a picture on my website which shows how we did it on a pole barn dance studio but the concept is the same. Here we do it for termites as much as fire but you have your regular termite flashing that runs under the bottom plate and is folded down about an inch and caulked to the exterior of the foundation using the sill seal for a backer rod. With vertical siding we also place an over-sized Z-flashing that we fold up out of 6" galvanized roll stock. It will generally have a two or three inch horizontal leg depending on how much exterior foam and the size of the horizontal blocking and the depth of the first layer of vertical siding. In Lucas' situation we have no foam, 3/4" strapping and 3/16 boards so the leg is only an inch and you could use regular window style Z-flashing. When we do reverse board and batten the spaces between the battens get strips of aluminum mosquito wire folded into loops (I'll add a photo or drawing later) for a fire proof detail you would want that to be able to absorb more heat so you would upgrade it to 1/4" galvanized rat wire. the air passes through it twice as it enters the vented area. In Lucas' situation the air passes over the strapping only in the 2" x 3/16" gaps between the fiber cement boards and behind the 1x4 battens so I would think that so long as you hold the battens 3/16" above the base flashing and allow a 3/16" gap at the top for exit air it would hardly be worthwhile worrying about screen wire. I would hold the bottom piece of strapping up off the fold in the Z-flash to create a larger volume in that intake area as well as to hold the top edge of the flashing onto the house wrap. (in this situation the house wrap is taped to the bottom header wrap and not lapped over the Z-flash as that is being used as a fire barrier.)

    In my rough-sawn sided houses the header wrap is folded up onto the sheathing and the termite flashing is bent down and caulked to the foundation the Z-flashing is then run down and inch or more below the caulked termite flashing and taped to the house wrap and the whole house gets wrapped with 15# felt prior to siding.

  43. GBA Editor
    Martin Holladay | | #43

    [I received the following message by e-mail from Doug, who asked me to post his message here:]

    “I love that the brake metal goes all the way back to the sheathing and out to the outside.

    To me, a window flange should be in plane with the drainage plane.

    Here are two ways to do that: 1) push the windows in 3/4" (which is what I would do; the minor aesthetic issue at the window frame perimeter will be long forgotten before you leave the job), or 2) try to use peel & stick to wrap the drainage plane out over the furring around the window perimeter.

    If you make the top 1x piece a bit taller and sloped at the top, the key area (the top) will be pretty easy to do a good job with. Flex wrap may not be the right stuff as it tends to pull itself off complex shapes unless you fasten the crap out of it.

    I learned from another contractor to kerf the back of horizontal strapping to allow incidental water to run down behind it rather than pooling on top, I think I'd do that next job I have.

    While I love the horizontal strapping for strength, a way out of all these issues is to use cedar breather-type mesh to create a drainage space, instead of strapping. It's thin enough that all your flashing details already work.”


  44. MICHAEL CHANDLER | | #44

    the issue with Cedar Breather is really the cost. At $140.00 for a 3'3" x 61'6" roll it's $1.50 a sq ft. 7/16" OSB at $6.47 / 4x8 sheet is $0.20 per sq ft. I can rip 3" strips of OSB and strap the exterior of my house wrap for a very small fraction of a full wrap of cedar breather. In most cases I have scrap OSB that I can divert from the landfill by using as rain screen.

    If I'm running horizontal siding it is even cheaper to buy 5 1/2" x 50' sill seal for $7.45, rip it into four 1 3/8" strips and tack it to the wall over the studs. I get 200' of foam ribbon for less than four cents a foot. So to strap an 8' x 30' wall section @ 24" OC it will cost me $358.45 for cedar breather, $12.94 for OSB strips, or $9.54 for Sill seal strips I can carry up the ladder in a trash bag, rip to length by hand and put up with a slap tacker. I just don't see the advantage of the cedar breather when I run the numbers. (Plus, if I run short it's not a special order. and if I have extra left over I can use it for drywall gaskets.) I've attached a sketch of the sill seal ripper we make out of a scrap of 2x6 framing, a saw horse, four 16D nails and three sharp utility knife blades. pull the stuff down the board, through the nails into the blades and feed it into a trash bag until you need it.

    For the bottom and top course I can cut strips of aluminum mosquito screen and staple them up as loops as shown in the sketch above. It helps me deliver a durable home at a competitive price if I don't buy into the fancy stuff.

  45. MICHAEL CHANDLER | | #45

    Posting this for Lucas Dupuis who is having frustration with the spam filter:

    Thanks for the constructive comments all. for the most part I would agree with Michael's detail using 1 1/2" trim. However, I've yet to see a composite wood trim (Miratec, etc..) that comes in 1 1/2"- probably because it would weigh so much it would sag and look like a noodle... but that's not for this discussion. I'm kind of stuck with composite on this project too keep it all looking consistent, Plus, short of a very high end engineered trim such as Windsor One or Duraprime, I haven't had good luck with wood trim holding paint in high exposure areas such as this. So, this leaves me with the choice of either Hardi trim or composite trim. Thinner battens would certainly solve this, and James Hardi makes a scaled down "1x4 batten" for this exact application. However, My client is particular about the look and just isn't sold by the thinner battens. So, All good suggestions. How about this?

    I hold the FC vertical siding off of the window flange to allow for the extra thickness of the
    flange+flashing tape, and to allow for future servicing of the windows flange + if they ever had to be replaced. At the head a shim would be required, maybe a scrap of Hardi? However, at the jambs and sills the siding could just be cut right up to the flange. The 5/4" window trim and 3/4" battens meet in plane at the outside of the FC siding. The remaining weak point, as I see it, is that the flex flashing doesn't get taped back to the tyvek at the jambs and sills. However, this shouldn't be such a big deal as long as the furring at the windows is sealed to the face of the plywood as shown right?

    The battens are outside the tyvek on this house anyway, so why should this matter at the windows? Thoughts, opinions? I know you all have some.

    Also, to clarify. This isn't a fully vented rainscreen. Though the idea has certainly sparked some interesting debates. The battens are horizontal on this application. They provide 2 critical functions. 1, a solid backing for the FC to nail to. I realize this works with SIPS, and that James Hardi has instructions for nailing directly to sheathing, but I would prefer not to go that way. 2, more important to the building science end of this. I've gone to great lengths to ensure that this wall can breath to the outside. That, as I've said before, means plywood sheathing rather than OSB, The relatively vapor open tyvek or typar instead of something cheaper, etc... This 'Vent Screen" does in fact act more like a 'cedar breather' type of system. It provides a capillary break between the inherently leaky vertical siding, and also a gap to prevent inward vapor drive from the FC.

    I believe these are all critical details when it comes to building high R value cellulose walls. I realize this detail may be overkill, but better safe than sorry. I'm hoping that discussions generate the knowledge to bring down the cost and complexity of these types of systems for all of our future projects.

  46. MICHAEL CHANDLER | | #46

    Lucas what if you strapped the house with 2x4 strapping 2' 3 1/2" OC with 2' wide strips of 1 1/2" foam between 2x4's and wrapped that with house wrap and installed the windows, then tar paper, then 1/2" (or less) strapping with a double (or triple) 3/16" x 3 1/2" FC outline around the windows to kick the window trim 1/8" (or 5/16") proud of the vertical siding?

    In the image we actually used 2" polyiso and shimmed the 2x4's out at the studs with 3x3x1/2 scraps of OSB and then shot spray foam on the inside of it.

  47. MICHAEL CHANDLER | | #47

    Can you show us a front elevation? I don't see how the siding will work running right up to the window flange, seems like you need trim on all four sides.

  48. Riversong | | #48


    I think what Lucas is suggesting is that since the 3/4" vertical battens and the 5/4 head and sill casing will be in the same plane, the battens can become the jamb casing.


    While I'm not crazy about the vertical board and batten or the fiber cement and composite materials, it seems that Lucas has thought this through pretty well.


    I think your proposal would work fine, I like the idea about keeping access to the window flanges for serviceability, and I don't see any red flags or obvious improvements other than possibly cutting kerfs in the back of each horizontal furring strip for drainage.

  49. homedesign | | #49

    Thoughts, opinions?

    Lucas Dupuis,
    since you asked...these are my thoughts & opinions

    I think that your detail is overly complex and likely to fail

    I think you are "attempting" to do the Wrong thing Right

    just my opinion

    reason for edit..poster's remorse

  50. wwLuke | | #50

    Here's the window elevation per request. Hopefully this clears things up.
    By holding the FC clear of the flange the back of the 5/4 window trim (on all four sides of the window) and the back of the 3/4" battens are in plane on the face of the FC, thus fixing the alignment problem. By notching the FC around the flange the window can be removed if it ever has to be replaced without tearing siding off- BTW I've seen plenty of cheap siding details where the siding was applied right to the extrusion of the window and caulked -yuck-.

    John. - You're partially right, I'm unsure of this detail because I've never seen it done this way. I know its complex, but not that much more than many of the exterior foam window details I've seen. Why do you think it will fail? As for the argument that my client is driving the design- I'm not sure how exactly to respond to that. I certainly can and do lay down the law when a client's request is impossible, but I don't think that recreating a proportionally correct looking board and batten vertical siding detail with "modern" materials is that out of line. I agree with my client that the strong shadow lines created by a 12" o.c. with 1x4 battens is the right look for this building. I, personally would compromise with thinner battens, but it's not my house. This is custom design as I live it.

    Michael- I've done that exact detail before. But in trying to get away from using so much foam in favor of cellulose I find myself with this solution. You do understand that putting foam over a "vapor open" cavity wouldn't work in this case right? I bet I would need 4" of external polyiso to make the dewpoint calc work out with 10" of cellulose inside the wall. Short of passivehouse detailing I don't see why I would do that.... Thanks though.

  51. wwLuke | | #51

    and here's the elevation, again

  52. homedesign | | #52

    I cleaned up my previous post a little
    you seem to recognize that water is more likely to get past the vertical siding
    and that your superinsulated wall may be less forgiving
    and then there is the horizontal strapping that traps water
    sorry it just makes me squemish

  53. Riversong | | #53

    it just makes me squemish

    Is that kinda like a Sasquatch who speaks Flemish?

  54. homedesign | | #54

    sorry, Squeamish

    just so you know .. I am not picking on you
    I have questioned BSC and GBA details for the same kind of issues
    I tend to be OVER cautious

  55. wwLuke | | #55

    me too John. That's why I'm putting it out there for the pickin' No offense taken.

  56. Riversong | | #56


    One potential problem would be the joint between the bottom window casing and the battens below. That's a flush horizontal joint with no flashing - really should have Z drip cap over each batten.

  57. homedesign | | #57

    Is it possible to show a drawing before the window casing is installed?
    I can not exactly visualize how this all fits together.

  58. wwLuke | | #58

    I agree Robert. Adding a z-drip cap below the sill trim would be pretty straightforward.
    John- the window casing is nailed over the furring, I just hold the FC siding back from the flange. So, from in to out, plywood, tyvek, 1x4 furring, window flange, butyl flashing, FC siding, window casing. (a shim will be required at the head to keep the head casing in plane with the jamb- see the head detail.
    What do people think of the mastic between the furring and the plywood. An alternate solution would be to wrap the tyvek into the opening and caulk between the furring and tyvek. I like this less for the same reason I spec windows to be installed before tyvek on traditional construction- that is, that I think the flange to the sheathing is a better seal than the flange to the tyvek.

  59. MICHAEL CHANDLER | | #59

    Lucas I like the flange to the housewrap taping because the house wrap makes a slip sheet when the house settles, the give puts less stress on the tape than if it were taped to the sheathing. But I too have opened walls and found serious degradation of the Tyvek so that's part of why we run tap paper over the Tyvek on our jobs.

    I would think that the fact that the battens are a composite material (FC?) might help with the end grain to casing flat joint.

    Great discussion, I've really enjoyed the give and take here.

  60. Riversong | | #60

    This crazy SPAM filter won't let me post this so I'll use a jpg file to fool the damn thing.

  61. MICHAEL CHANDLER | | #61

    Just to be clear we do lap the tyvek over the top flange but we tape the top flange rather than taping the flange to the sheathing and then lapping the Tyvek over the taped flange. Physics trump chemistry, but this is also a place where water runs uphill. When wind creates a higher pressure out than in suction trumpet gravity.

  62. Riversong | | #62

    SPAM filter won't let me post anything here.

  63. bdrfab | | #63

    I realize this post is more than a little late, however, in response to a comment about not knowing of any composite materials available in 1 1/2" thickness, Extira (same thing as MiraTec, only in sheets) is available in 2'x16' sheets at 1 1/4". You may have problems with availabilty, IIRC we had to buy a bunk at a time. In the past, we have used 2x Trex material for window sills. In theory it needs to be aged prior to painting, although we haven't had problems painting it quicker. What was the final solution for this detail?

  64. Natur Haus | | #64

    Great stuff on this thread... Was there a final detail decision Lucas?

  65. wwLuke | | #65

    We have proceeded with the detail as I proposed it last in the forum. Seems to be going well so far. Thanks for the tip on the Extira. You're right, I would have to buy a truckload to get it here. Here's a photo of the application of the windows.

  66. wwLuke | | #66

    And here's the photo.


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