Jamb sizing in window installation when using exterior rigid board insulation
jostrick
| Posted in Green Building Techniques on
Thank you for taking my question. I am planning a renovation/addition to my home and am using 2×4 wall framing with 2″ of rigid foam board on the exterior beneath vinyl siding. My question is: can i use 2×6 window jambs in this construction so that i won’t need to do the plywood box approach in the “innie/outie” debate i read earlier on the site (https://www.greenbuildingadvisor.com/blogs/dept/musings/innie-windows-or-outie-windows) or do i need to use 2×4 window jambs and construct the plywood box?
thanks for your assistance,
jbo
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Replies
Jack,
Will you be installing vertical furring strips, or do you plan to install your siding with long nails to the studs? Will you be installing plywood or OSB sheathing under the foam, or will you be bracing your walls with metal strapping?
If you are planning to order windows with jambs designed for a 2x6 wall, be sure you add up all the layers of your wall thickness to be sure they work -- otherwise you'll still need to install jamb extensions.
Thank you for the prompt response. I can follow any prescription at this point as i am taking this chance to revise my basic "code designed" architectural drawings with only cavity insulation before i break ground and would welcome the team's suggestions. I believe it's currently slated as 1/2" plywood and i think my builder may not be familiar with metal strapping techniques. As for long nails vs vertical furring strips i was leaning toward furring strips but again am happy to take best practice advice. As a first time renovator/owner i am open to and welcome all thoughts on this topic.
The entire wall assembly should be designed before you go much further. Once you know the total wall thickness you can determine the best window jamb detail. If I'm ordering wood windows I sometimes order factory jamb extensions and I sometimes order windows for 2x4 walls and make my own jamb extensions. It depends on the cost of the factory option, and what their detail is. Your window salesperson should be able to give you enough specifics for you to make a decision.
Is there a view that nailing the foam + siding to the assembly with long nails would compromise the cladding? My guess is this is less expensive so more likely how my builder would want to go.
Martin:
My proposed wall assembly will be from inside to out: 1/2" GWB, 2x4, 1/2" plywood, and 2" XPS and then vinyl siding. my builder intends to nail the siding directly without furring strips. Will 2x6 window jambs with the window casings built out be the appropriate sized jambs (they are 6 9/16"). thanks for your assistance.
Jack,
I wouldn't do it that way -- I'd install furring strips -- but your math is correct.
3 1/2 inches + 2 inches = 5 1/2 inches.
Martin, thank you for your timely post. I just want to be absolutely clear - the standard 2x6 window jambs are 6 9/16" and you indicated 5 1/2" was correct. are you saying 6 9/16" is correct b/c it includes the 2 separate 1/2" plywood sheathings in my math or should I be using 5 1/2" jambs? I will discuss the furring strips with my builder - generally 1x2's are ok?
Jack,
You are changing your layers now. You just mentioned two layers of 1/2-inch plywood sheathing; however, your earlier question mentioned only one layer of 1/2-inch plywood sheathing. Which is it?
I wouldn't use 1x2 for furring strips because they can easily split. The standard recommendation for those who want to avoid the frustration of split furring strips is to use 1x4s, although some builders use 1x3s.
The wall assembly i posted is correct. 1/2" GWB drywall, 2x4, 1/2" plywood, 2" XPS. Thank you for the recommendation on the furring strips. Given that assembly, can you please confirm the proper jamb size? thx
Martin:
I just hung up with my siding manufacturer and they require that the siding be nailed directly to minimum 3/8" plywood. If my wall assembly from outside to inside were: siding, housewrap, 3/8" plywood, 2" XPS, 1/2" structural plywood, 2x4 wall, GWB would that be a concern for you? thanks again.
Jack,
Q. "If my wall assembly from outside to inside were: siding, housewrap, 3/8" plywood, 2" XPS, 1/2" structural plywood, 2x4 wall, GWB would that be a concern for you?"
A. Well, 3/8-inch plywood isn't thick enough to hold your siding nails, so you'll end up using 3-inch nails, which are expensive and a pain to install.
Martin: So if i thickened the exterior plywood with the housewrap from 3/8" to 1/2" and had the rest of the wall assembly remained - would you still recommend furring strips or could i nail directly to the plywood then? i am sorry for the tediousness of my questions - i am a complete novice to homebuilding, i'm just a consumer trying to get educated.
Jack,
According to the Vinyl Siding Installation Manual published by the Vinyl Siding Institute -- a publication that you can download for free by clicking here -- "Fasten nails or other fasteners in the center of the nailing slot and make sure the fastener penetrates a minimum of 3/4” (19mm) into a nailable surface."
That means you need 3/4-inch thick furring strips or 3/4-inch plywood. The alternative, as I said, is to use nails that are at least 3 inches long.