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XPS cut-and-cobble: Layering beyond the cavity?

RetiredSingleMommy | Posted in General Questions on


-Zone 4, above grade exterior walls -gutted from interior, 2×4 framing is old but not true.
-No plumbing nor electrical to consider, all windows will be replaced.
-Insulation on hand (from Craigslist, shipping containers worth of both types)  …is 2.5” XPS  & 2” felt-lined PIR.
-The exterior sheathing is old-school, sufficient, and her VB was 7mil poly against the warm side; immediately behind the gyproc.
-I may or may not replace the siding next year; R-value isn’t my main concern at this point since there’s many ways to skin that cat (so I’ll tweak the details in due time).

I prefer to use the PIR elsewhere; I mention it only in hopes to get your 2-bits-worth about whether it’s possible to layer XPS with PIR? If so, any thoughts on how to execute that install? (Let’s refer to this as Q.#2)

Currently struggling with how to expertly finish these exterior walls with my XPS. After the air gap/rainscreen spacers, the (2.5”) XPS is cobbled in … I have 3/4” to spare in the stud bay, and a shameful thermal bridge to contend with.

Q: if I install a second layer of blueboard (the aim being to attain C.I.) could I do it by notching our it’s backside to accommodate for the 3/4” exposed stud (followed by expanding foam-air seal, taped joints, furring strip for drywall).
Please advise me how to think about air-sealing a double layer of rigid insulation, in general.

Thanks in advance.

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  1. Expert Member
    Zephyr7 | | #1

    You can easily layer XPS and polyiso, no problem. I’d put the XPS on the exterior side of the polyiso since the XPS has better cold weather performance. XPS on the outside helps to protect the polyiso by keeping it a bit warmer for maximum performance. You will want to tape all the seams.

    I’m a little confused by your wall description. Are you intending to have the insulation on the outside of the rain screen gap? You need the insulation tight to the wall — a big air gap will negate much or all of the benefit of the insulation. The usually practice is to have the rigid insulation tight to the outside of the sheathing and then your rainscreen gap followed by siding.


  2. RetiredSingleMommy | | #2

    Hi Bill,

    The exterior walls are gutted from the interior.
    And going forward, I think the stud bays are to be filled in this order, (yes?)
    existing sheathing>airgap aka rainscreen>2.5” XPS +2.5”XPS>airseal>Furring>drywall>Paint>Picture Frames *she smirks a little as she goes too far. It’s the ovaries, lol*

    Do I foam/seal/aka great stuff both layers of (rigid insulation)? The first layer is cobbled in SNUG (so IF it requires sealing, I’ll use flexible caulk) I can’t figure the “if”.

    Since I’m “making due” with recycled material; 2x 2.5” XPS it shall be.
    The 1st layer is cobbled in and there’s room to spare before (the drywall). The 2nd layer overfills the stud bay. Too much. Hence, I’m considering notching out one side of the board (the side that faces the 1st layer, in the cavity) to fit snugly over the framing.
    I happen to be in a position to fart around & finesse a jigsaw. Unconventional as it may be, it will eliminate the framing’s thermal bridge & exceed code. (Yes?)


  3. Expert Member
    Zephyr7 | | #3

    I’m still not entirely clear, so let’s try the typical stackup from the inside wall to the exterior siding which would be in this order:
    Drywall - studs/framing - sheathing - exterior foam insulation - furring strips (rain screen gap) - siding.

    It sounds like you’re maybe putting the foam on the inside face of the framed wall, or maybe trying to fit it in between the studs in the wall cavities?

    It is possible to use rigid foam in wall cavities, but it is a LOT of work and you’d be better off with batts or dense pack cellulose (blown insulation, basically). It is both very labor intensive to install, and very difficult to completely seal rigid foam in wall cavities, but it is possible.

    Personally, id use mineral wool batts in the wall cavities and save the rigid insulation for the outside of the wall when you do a reside. You’ll get better overall performance that way for a lot less work. You could also use cellulose or even fiberglass in those wall cavities, I just happen to like mineral wool myself due to some of its other properties.

    Regardless of what you do for insulation, it’s worth carefully airsealing the perimeter of each wall between the top/bottom plates and the sheathing, and the outside studs (probably corners) of each wall. Try to seal any holes too where wires or pipes penetrate vertically, and that part goes for interior walls too. The orange “firestop” greatstuff is the best way to seal those holes.


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