Window jamb extensions
Hello,
I have 2″ RockWool exterior insulation, vertical furring strips, and horizontal furring strips and I will be using Hardie Panel on top of that. My windows are flanged and taped against the WRB so they should be protected from water intrusion.
I’m struggling to figure out the best way to build jamb extensions. So far I am considering:
1) 1×4 PVC trim for top, sides and bottom
2) 1×4 Hardie trim for top and sides with sloped 2×6 cedar with drip kerf on bottom
Would Hardie be a good thing to use there? And should I do anything at the top, e.g., flashing above? If I use 1×4 PVC for the sill, should I do anything for a drip edge.
Also, there’s already a drip edge on the window, so should I caulk the seam between that and the jamb extension?
Thanks for any suggestions.
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Replies
The window should be fully watertight before adding exterior extension jambs and sill extensions. The window should have a sill pan that preferably drains over your exterior insulation, but with mineral wool it's probably ok to have it drain behind the insulation, as there shouldn't be a lot of water. Don't seal the bottom flange to the wall/sill pan, in fact it's a good idea to add thin shims so it can drain readily.
I have used a similar detail many times, but never with Hardie trim, only wood or PVC. Boral TrueExterior would be my next choice, and in fact I would probably choose it over PVC.
The drip kerf is to keep water from getting back into the wall, so it's a good idea to include one regardless of the material. I also like to provide a rabbet to tuck the cladding up into, as many traditional sills have.
You should leave small gaps on all sides between the window frame and the extension sill and jambs. Window manufacturers usually call for 1/4" but I find that to be ugly; I leave about 1/8" and seal it with a good caulk, such as Big Stretch by Sashco. Don't apply the caulking too deeply; it should be no deeper than it is wide to allow for movement.
This article explains a few ways I do exterior extension jambs, though it doesn't go into much detail: https://www.greenbuildingadvisor.com/article/windows-in-thick-walls.
Thank you.