Air sealing windows w/ jamb extensions
GBA won’t let me see previously asked questions for some reason, so please excuse the double post, as I had another thread about air sealing windows.
For our fiberglass Pella windows, we planned to air seal on the interior by inserting a backer rod before adding low expansion spray foam (to ensure a drainage gap was maintained). After foam, we planned to use backer rod and caulk to get a good seal.
They are windows with factory applied jamb extensions. We didn’t know this before they were installed, so we didn’t seal the seam between the window frame and jam extensions. We were hoping the spray foam would help with that.
But now I’m not so sure I’ve been thinking about all this correctly. Pella’s instructions specifically say NOT to use any foam between the jamb extensions and opening. It’s only to be used between the window frame and the opening.
Does the air sealing need to happen back at the window frame itself?
If so, how can I effectively get back there to ensure it’s air sealed? And what do I do with the rest of that empty space in the opening (from jamb extensions inward)?
It’s a pretty deep cavity, and some of the openings are very narrow.
Would love some guidance here!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
We have jamb extensions on our windows. The previous owner neglected to insulate or air seal them. After removing the trim, I foamed using window and door foam back at the window with a nozzle that could reach. Where it was super tight, I used a plastic nozzle that tapers. A few windows, I had to pull out the extension as it was too tight. I didn't bother insulating the space next to the jamp extension.
For clarification, you filled the entire gap between the window frame and rough opening with foam and left the space between the rough opening and jamb extensions empty (so the trim is covering dead space back there). Is that right?
I can reach back to the nail flange, but I want to leave a small gap toward the flange for drainage. Based on what we are seeing, the jamb extensions are ~2.5 inches deep while the window frame itself is only ~1.75 inches deep. Seems like an impossible task!
Here is your other thread: https://www.greenbuildingadvisor.com/question/backer-rod-spray-foam-windows.
The reason they don't want you spray-foaming behind the extension jambs is due to the risk they will get pushed away from the framing. If you fasten the extension jambs to the rough framing first, preferably with something that resists pull-out such as trim-head screws, you should be able to fill the gap with foam. I don't know of another way to make the window airtight otherwise. I would still try to leave the outer portion of the gap unfilled so it can drain a bit if necessary.
Does your photo show the back of the nailing flange? If so, that's a huge construction gap. North American window manufacturers usually call for 1/4" construction gaps.
Yes, that's the back of the nail flange. They just used the previous openings and called it "good enough" apparently. Some of the gaps get up to 3/4" wide. Just one of the many things we aren't happy about with this install.
Do you have an image of what you're referring to with fastening the jambs to the framing with trim-head screws?
And with the gaps being as large as ours are, do you still recommend using that method?
I second Michael Maines concern with the wide gapping between the rough opening and the window.
The limited width of the flanges may not allow for secure nailing into structure (if the gaps are too wide for the given width of the flange).
Agreed, definitely not ideal by any means.
They used the pre-existing rough openings (this is a remodel), and they were wonky to start. Instead of fixing them before the install, they just rolled on through.
We made sure all fasteners were completely in the studs (none are poking through into the rough opening). About all we could do after the fact.