Thoughts on furring out 2x4 walls vs. using rigid foam for a storage crawlspace?

I went through this exact debate when I was finishing my attached garage workshop last winter after some rodent issues. I ended up furring out the 2x4s with 2x2 strips to fit the R23 Rockwool Comfortbatt. Honestly, the labor was a massive pain, especially around the outlet boxes and the window returns. If you aren't planning on heating the space 24/7, the jump from R15 to R23 might feel like diminishing returns for a storage area. What made the biggest difference for me wasn't the extra thickness, but using Siga Wigluv tape on every single seam and gap before the insulation went in. In a Zone 5 winter, that draft is what kills you, not the stud conduction. If I had to do it over for a basement-adjacent space, I’d probably stick to the 2x4 depth but add a layer of 1-inch foil-faced polyiso over the studs. It breaks the thermal bridge much better than just adding more wood, which still conducts heat. It saves you from ripped 2x2s splitting and keeps the footprint of the room a bit larger.

I actually think the labor of furring out with wood is a waste of time, but not for the reason the previous guy said. If you're already ripping things out, don't just add more wood. I dealt with a similar damp corner in my daylight basement in New Haven. Instead of 2x2s, I slapped 2-inch XPS foam boards (the pink stuff from Owens Corning) directly over the 2x4 studs.

By doing this, you're getting a continuous thermal break. Wood is a terrible insulator, and if you fur out with 2x2s, you still have studs conducting cold air from the outside. With the R-10 rigid foam over your R-15 Rockwool, you’re hitting R-25 total. Plus, it acts as a secondary drainage plane if you ever get another leak. It’s way easier to screw through the foam into the original studs than it is to shim every single 2x2 to get a flat wall. For a storage area in CT, keeping that wall assembly warm prevents the condensation that leads to the mold you just fought off.

Have you ruled out the floor and ceiling as the main culprits? I'm skeptical that jumping to R23 in the walls is going to move the needle much if the sunroom floor above it isn't airtight. In my expirience with those 'tricky areas,' the cold usally dumps in from the rim joists or the transition to the upper floor, not through the studs. Also, since you mentioned mold, are you sure about adding that much depth? You're creating a thicker cavity that takes longer to dry out if a leak happens again. Have you looked into just staying with 2x4 and using a higher density batt instead of the labor of furring?

Forget the 2x2 wood furring; it's a thermal bridge nightmare. If you realy want to kill that R23 target, look at Rockwool Comfortboard 80. I did a similar Zone 5 crawlspace project and instead of thick walls, I stuffed the 2x4 cavities with R15 Comfortbatt and then outboarded 1.5 inches of the 80-series board. It creates a continuous wrap that stops the studs from bleeding heat. Since you're alredy doing windows, just make sure you use DuPont Flashing Tape or some 3M 8067 to seal those rough openings. Are you planning on a smart vapor retarder like Intello Plus? In CT, you really need that assembly to breathe so you don't end up with a mold sequel.

I skipped the furring strips on my last crawlspace project and just used InSoFast panels right over the existing studs. Ripping 2x2s is a nightmare and they always twist on you anyway. I found that just focusing on the rim joists with closed-cell spray cans made a way bigger difference in how the room felt than the extra two inches of Rockwool ever did. If it's just for storage, I'd save the floor space, stick with the 2x4 Rockwool, and spend that extra cash on a high-end dehumidifier. That's what actually stopped my mold from coming back in Zone 5.

Honestly, if you're worried about the R-value but don't want the headache of furring, don't overlook the windows you mentioned. I'm in a similar climate and I've seen people go crazy on wall insulation only to have cheap single-pane glass dump all the heat anyway. Since you're replacing them, spend that extra 'furring money' on triple-pane or at least high-end gas-filled units. If the windows are solid and you've got R15 with decent air sealing, you're gonna be fine for a storage spot. No sense losing floor space to 2x6 walls if it's just for holiday bins and gear.

Update: Thanks for the help. I decided to skip the 2x2 furring after hearing about the labor headache. Instead, I focused on the rim joists and floor transition like one of you suggested—found some massive air leaks there. I ended up going with rigid foam board over the 2x4s to break that thermal bridge. defintely the right call for this space!