Flanged window placement with rainscreen and reverse board and batten siding
Hi all,
Building a 2-story garage with an ADU above in Nova Scotia. Construction is double stud wall with dense-pack cellulose and no exterior insulation. I’m planning on using strips of 1/2″ Ventgrid as the rainscreen. 1 1/2″ thick reverse board and batten siding nailed through the Ventgrid strips every 24″ vertically.
It’s time to install the windows. I’m going to make plywood window bucks to span the exterior and interior walls. The dilemma I’m running into is whether to install the window with the nailing fins flush with the sheathing, or to fir it out so that the brickmold sits flush with the boards.
I’d like to avoid window trim, if possible.
Questions:
– If the window sits inside the siding plane, how necessary is head flashing?
– Are there any concerns / issues that I’m creating if I have the window slightly recessed from the siding?
– Any better ideas that what I mentioned? I’m open to revising the plan entirely!
Many thanks
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Replies
I think it depends on a couple of things:
-Distance from nail flange to front of window
(Assuming it’s 1.25” but maybe different?)
- trim detail, how are you going to trim out the windows? No trim? Trim set at nail flange depth? batt depth?
-How will you WRB the buck? What does that air seal detail look like?
- how will you replace the window when it dies? Probably not huge issue no matter how you do your trim with reverse board and batt, but something to think about.
-When the water hits the window buck, where does it go?
No matter how you do it, you should draw it out in section, I would do a head flashing on both my window buck and my window, but maybe I’m just paranoid.
Also, your total reverse board and batten is only 3/4”? Or is board and batten both 3/4 for a total 1.5”. Lastly, one of the main benefits of reverse board and batten is you don’t really need a rainscreen…..
Thanks freyr. A few clarifications:
- nail flange to front of window is 1"
- was hoping to go without trim, but may go that route if I can't find a good solution. Would probably set it flush with battens?
- on the sides the wrb will wrap into the buck and get taped.
- I appreciate your consideration of future replacement. I suppose that is an argument against trim less.
- exterior end of buck is set flush with sheathing. Will slope and backdam the sill.
- sorry for the confusion. Just edited the question to clarify that total siding thickness is 1 1/2"
- I hear you about using the battens as the rain screen. Though, I'm also paranoid and I much prefer to have a continuous gap behind the siding that allows all of the hemlock to dry on both sides. Overkill? Perhaps.
Ry_Buc,
I'd suggest:
- Stop your window buck behind the sheathing.
- Picture frame the outside of your window openings with 1/2" plywood about 3" wide (so this exterior buck made of plywood and the Ventgrid will all end up in the same plane).
- Install a 12" piece of WRB on the exterior buck at the sill, and install a sill-pan.
- Install your flanged windows on this exterior buck. This will allow the 1" window frame outside the flange to cover the reverse battens on your cladding.
- Install a meal head-flashing with end dams.
- Cover the flanges, exterior buck and head-flashing with flashing-tape at the jambs and head back to extend under the Ventgrid several inches.
- Lap your WRB over the flashing-tape up to the window frame.
- Install your Ventgrid and cladding.
I think this will look funny with trim less… but I’m not sure the alternative… I think if you want to go trim less with reverse board and batt I would plane the outside of window with boards. Maybe rabbit batts at thicker exterior buck? Or maybe just use trim.
Edit: maybe use. 1/4” piece to match batts that you picture frame window with? Just spitballing, someone probably already has a detail for it…
freyr_design,
It's hard to say without seeing the drawings and knowing what aesthetic is being aimed at. Michael Maines has posted his version of an (almost) trim-less window. He uses a 1"x frame - but as I remember that was in the context of Innie windows. I'm not sure what that would look like without some depth.
edge-nailing into a plywood buck isn't a great solution if you aren't super careful with the install. the plywood can split and the holding capacity is reduced. you can pre-drill for truss head screws, but it's still challenging to get the fasteners to bit into the middle of the 3/4" wide material. i've had to fold over the outer edge of the nailing flange to get the window to seat into this recess -- [the flanges are about 1-1/8" wide and the full recess might not work out with the rest of the trim detailing.]
Especially with nails, it's better to have solid wood substrate. I've used 5/4 deckboards where I can't use a full 2x buck. faster if you pre-fab the buck into a box for install -- nailing in one unit instead of four pieces that you are trying to install in-plane, level and plumb.
Thanks for the tips. This definitely gives me a good starting point. A couple follow-up questions:
- If the window brick mold sits slightly past the battens but shy of the boards, should I then consider an exterior sill similar to the attached photo?
- running the vent grid plane into the 1/2" ply would leave no meaningful ventilation of the rainsceeen gap at the top and bottom of the window. Perhaps this does not matter as long as I have ventilated at the top & bottom of the wall?
If the window is inset a bit it would look strange without an exterior sill.
My $0.02. Skip the ventgrid and use drainable house wrap. This WRB will space the battens off the sheathing a bit and allow for a bit of drying. You can now mount the window at the sheathing plane and the brickmold will stick out enough for the siding.
One thing to watch with B&B is spacing around windows. You generally want the window edges to line up or at least be symmetrical between battens (at least on the more visible façade).
Anything trimless feels like the simpler install but it is not, generally you will spend a lot more time getting all the gaps and edges prefect for it to look good. Trim hides a lot of sins.
Thanks Akos. Mento wrb is installed but will definitely consider the drain able web for the house. Would simplify things and give some peace of mind for the battens.
Ry_Buc,
Rain-screens are interrupted by solid backing as strapping, at penetrations like vents - and at openings. They dry around them by diffusion just fine.
Edit: but if you were okay with using a thin surround - like Michael's Fin jambs - you could dispense with all the stuff I suggested above, and just install the windows like any other outies.
Got it. That reassures me about the rainsceeen having sufficient ventilation.
I'm partial to what I call fin jambs: https://www.greenbuildingadvisor.com/article/windows-in-thick-walls.
Interesting detail! In our case the fin jamb would be ~2" to be flush with the outside face of the boards. Might build a little model to see how that looks.
When / if you've done it with windows are at more outie than innie, where did you run the head flashing? Drip edge on top of the top "fin"? Sorry if that's an obvious question, just want to make sure I get all the water draining details correct.
Ry_Buc,
You can locate the head-flashing in either place. It needs to go back to the sheathing, so if you want it over the window rather than the trim, the trim needs be mounted on top of whatever rain-screen you use, so it doesn't interrupt the drainage plane.
Judging by what my Nova Scotia friends have told me about your weather this winter, it sounds like you might want to budget for a snow blower too.
Ha! No kidding. The snow and wind have been brutal. I guess that's what I get for framing in the winter here.
Sounds good re: head flashing. Should be easy to incorporate with the fin jambs if I go that route.