Does “Never Tape the Bottom Flange!” Lead to Air Leaks through Headers?
The standard advice here, on FHB, Risinger, BSC, etc. is that (a) one should not put flashing tape on the bottom flange/fin of flanged windows and (b) one should not caulk behind said flange. This way, if your windows leak, the water can drain out from behind the flange (along your sloped rough-framing sill). So, the window is not fully air-sealed on the exterior, only on the interior.
This morning, while installing a flanged (aka “nail fin”) window, it occurred to me that this creates a major air leak as follows. Exterior air flows in from under the untaped bottom flange –> around the jambs (in the space between the window frame and the rough framing) –> above the window head –> *in between the plies in the header above the window* to end up inside the wall cavity.
Am I forgetting something about why this isn’t an issue? Sure, you could tape-seal the seams in the bottom edge of the header — and I think I’ll start doing that — but I just looked back at all of said sources and no one is doing that.
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Replies
jamesboris,
To get to the multi-ply lintel above the door, the air would need to get through the foam that fills the gap between the RO and the window frame at the jambs, and the 2"x6" under the lintel that frames the head of the RO. If the sheathing is taped to the RO around the exterior and corners, there should be no path available anyway.
Malcolm, I've never seen a 2x6 under a header in TX or Chicago, unless it was part of a window buck or a correction for a grossly oversized RO. So, a 3-2x10 header is just that... three 2x10s sandwiched together, directly above the window.
I don't tend to fill the entire gap-to-the-flange with canned foam, as I think it's not ideal water-drainage practice. Most videos or manufacturers tend to show the foam being a maybe 2" D bead starting from the interior. I do see a lot of people moving away from canned foam, as a less reliable air-seal than a good tape or caulk. A backer rod covered with Air Dam, or a gap spanned by Solido IQ (for example) is a better air-seal, but would do even less than a 2" D bead of canned foam for the problem.
The sheathing is always taped to the RO, but the leak I'm talking about is deeper inside the RO, and only applies at the header.
jamesboris,
Another interesting regional difference.
We include the 2"x6" header for a few reasons:
- You don't have to pad out the lintel to match the framing thickness.
- Less shrinkage, so less chance of drywall cracks.
- You don't have to put the lintel directly above the RO. It can be at the top of the wall, or in the rim-joist above.
- Easier to seal and weatherproof the window RO - as we are discussing.
Another regional difference is we still typically do our air-sealing on the interior with poly, so it isn't as important if air gets into the cavities from the exterior.
Without that top piece I agree, it would be a good idea to seal the plies of the lintel. I think it probably doesn't get done simply because no one thought it through the way you have.
I appreciate the idea of leaving a path for water to drain at the jambs, but the vinyl or fiberglass windows I use only have about 1 1/2" of frame inside the flange, so it isn't really feasible to leave a useful gap and still get any insulation in there.
Could you say more about your thoughts on the frame? This project is using Marvin Elevate windows, and the sill goes about 3" into the RO. That said, there's certainly not a ton of room under the sill... I plane a 5-degree slope into the rough sill, then shim the window at the front edge of the rough sill... practically speaking, you get a small wedge of air, 1/4" at the highest... I'd love to be convinced that maybe I can just tape some bottom flanges this time haha.
I try to do both exterior and interior air barriers when I can. I know that much of the advice here says "focus on getting one air barrier right," but in the field, stuff happens, so I like the redundancy.
Placing your header at the top plate is actually the only way to get full load-bearing capacity -- it's been a minute, but I think footnote F of the IRC's header span table says you must derate the span by a huge percentage if you don't! Reason being that the header must be laterally braced (e.g. by the top plate) to get full capacity. Who knew? Definitely not even most engineers haha. Hat tip to Mike Guertin for that one.
jamesboris,
I don't like to shim the bottom because it leaves you different depths for your interior casing. I set my saw at a little over 1/8" and hog out two grooves in the sheathing at the sill. The sill membrane conforms to them and leaves a drainage path. With the bottom flanges nailed tight, there may be a bit of drainage possible all along the rest of the sill, but I doubt much air leakage, except for those 2" grooves.
I think it is an interesting discussion, and I have also questioned this practice of leaving the bottom flange open, I find it counter intuitive. It is almost like we leave it open to drain the water that may be created by leaving it open.
For envelope related knowledge I tend to defer to the builders of the PNW as this environment is so challenging. But in very cold climates the risk of air leaks leading to condensation is greater than that of wind driven rain, and I can see an argument for fully sealing the window, both in and out.
FYI I install my windows to the bottom of the header like the OP.
I live/work in Zone 2A, where we have soup-like, moisture-laden, 100F+ air fairly often... I try to cool above the dew point, and advise others to do so as well, but it's still worrisome. That said, I think it's left open for something that matters way more than air leaks: rain. They say there are 2 kinds of windows/roofs -- those that leak, and those that will leak someday.
So to me, leaving the sill open seems like best practice. For now, I'm just wishing I'd thought earlier of the technique of taping the seams in the underside of the header. I've never seen it suggested anywhere, but at this moment it seems critical... truly hoping someone is gonna come to this thread and prove me wrong!
Here's one of the best Pac NW sources stating "NEVER sill the seal": https://hammerandhand.com/best-practices/manual/3-windows-doors/3-1-new-window-installation/.
Like I say, I defer to the experts on this one, I have just always had that feeling in the back of my mind that this detail does not quite make sense. But building science is not my expertise.
This is fascinating! I've never even considered the possibility that there not be a horizontal member below the header, but I also like to see headers tucked up higher anyway (which then requires it).
Regardless, this is a perfect argument for flashing the *whole* window opening, not just the sill and a bit up the legs. If the sheathing is the air barrier, and you tape from sheathing to the RO on all four sides past the plane of the interior surface of the window that you air seal, this is a non-issue. If you have a multi-ply header and nothing below it, you tape across the plys and break the air path contemplated in this discussion.
Patrick,
" If the sheathing is the air barrier, and you tape from sheathing to the RO on all four sides past the plane of the interior surface of the window that you air seal..."
That pretty much solves both the air-sealing and possible water infiltration issues.