Is it okay to put rigid foam board over Rockwool in a garage office?

Using 1-inch XPS over your studs is a solid way to kill that thermal bridging, but you have to be careful with the vapor profile. At one inch thickness, Foamular NGX acts as a Class II vapor retarder. If you tape the seams with a high-quality acrylic tape and seal the perimeter with canned foam, you've essentially created your air and vapor control layer. Adding a 6-mil poly sheet on top of that is usally a bad move. It creates a "moisture sandwich" where any humidity that finds its way into the stud bay gets trapped between the plastic and the exterior sheathing, which is a recipe for rot. The real heavy lifting for warmth in a garage conversion comes from air sealing, not just the R-value. Before you slap those boards up, make sure the Rockwool is sitting flush and not compressed. Use some fire-rated spray foam to seal the bottom plate to the slab and the top plate to the rafters. If air can't move, moisture can't hitch a ride into the wall. That 1-inch foam will do a decent job of keeping the studs from radiating cold, but it only works if the air behind it is dead still.

Also, think about your electrical. Cutting holes in the foam for boxes can compromise your air barrier. I'd suggest using shallow boxes or sealing around them meticulously with foam or fire-rated putty pads on the back of the boxes. And definitly skip the poly. You want the assembly to have some ability to dry toward the inside if things get damp, and 1-inch XPS is just permeable enugh to allow that, whereas poly is a total dead end.

Just double-check what's on the outside of your OSB. If you have a standard housewrap like Tyvek, you're fine becuase that's vapor-open. But if there’s already some kind of rigid foam or a foil-faced product on the exterior, you're essentially building a wall that can't dry in either direction. If the exterior is clear, the interior foam-and-tape method is a much better setup for a conditioned office than the old-school plastic-and-staples approach.

I'm planning a similar setup for my shed, but I'm stuck on how to handle the transition where the foam meets the floor. My garage floor is just bare concrete and it gets freezing in the winter. Does the foam board sit right on the slab, or should I leave a gap to keep it from soaking up moisture? Also, is one inch of XPS actually thick enough to stop the studs from feeling like ice when it hits zero degrees outside? I'd hate to do all this work and still have cold spots telegraphing trough the drywall.

To the guy asking about the floor transition—don't let the XPS sit directly on the raw concrete. I usally leave a half-inch gap and fill it with a good bead of closed-cell spray foam or use a PVC baseboard as a spacer. It keeps the foam from wicking any moisture if you ever have a spill or a damp slab. As for the R-value, that 1-inch break is usually enugh to stop the 'ghosting' effect where you see the stud lines on the drywall. Just make sure you aren't leaving the headers over your windows exposed. Those are the biggest heat leaks in a garage. If you can, skin those with foam too, or you'll see frost there when it's sub-zero.