Fill in Extra Space Between Studs with XPS?
Is it okay to line my studs with XPS foam to fill-in the space so rockwool batts to fit properly? I’m concerned about causing moisture issues with the framing if I do so.
I’ve been renovating my home. As part of my renovations, I’m redoing my exterior insulation, including 16″ R13 Rockwool Batts (15.25″ actual width) going in the stud cavity. I’m using Pro Clima’s Intello as my vapour control layer.
The house is very old and not built well by modern standards. My latest predicament is that the framing isn’t quite 16″oc. Stud spacing is anywhere from 17″ to 18″oc, leaving me with (on average) 16″ gaps between studs. This is causing issues with the batts, as they are 1/2″ to 1″ too narrow for the space. Generally they just flop out, and even when they don’t, having that much of an air gap is unacceptable.
At first I tried cutting a batt into narrow strips to fill it in, but that is incredibly tedious. Something I considered, if sticking solely with rockwool, was to order a couple bags of 1″ thick Comfortboard, rip them into 3.5″ strips, and use them to fill the gap. However, this second option would cause downtime. Instead, my third option is to use XPS in the same way. I already have a sufficient amount of XPS foam, in various thicknesses, kicking around (from an interior insulation/vapour barrier scheme that Randy Williams talked me out of).
From an insulation standpoint, I don’t think I’m losing anything by lining the studs with XPS. Better R value per inch than the Rockwool, very easy to work with, etc. However my concern, as stated above, is about trapping moisture against/within the individual studs of my framing. Will the XPS cause issues trapping moisture within the framing; or, will moisture still be able to move through the wood to the inside/outside edge sufficiently to prevent issues?
I have attached a photo as an example of the stud spacing I am working with, and the solution as proposed. For this particular cavity, I would put a 1/2″ layer of XPS on the face of the left stud in addition to the 1″ piece of foam on the right stud. This would bring the space in the cavity down to 14.75″, which should be sufficiently tight for the Rockwool batts to then hold themselves in place as intended.
If anyone has thoughts/concerns about this idea that I haven’t brought up, I’d love to hear them.
Thank you for your time,
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