Is it normal for my XPS foam to show up hotter than the radiant barrier on a thermal scan?

The reading you're seeing on that thermal camera is a classic case of emissivity error. IR cameras are essentially blinded by shiny, reflective surfaces becuase they don't emit heat energy efficiently. Instead, they act like a mirror and reflect the ambient temperature of the objects around them. Your foam board is an organic material with high emissivity, so the camera can actualy 'see' its temperature, whereas it's likely just showing you a reflection of the shed floor or walls when pointed at the radiant barrier.

To get a real reading, stick a piece of black electrical tape on both the foam and the radiant barrier. Wait about ten minutes for the tape to reach the temperature of the substrate, then aim the camera at the tape itself. That will give you a much more accurate comparison. But honestly, the fact that the barrier feels hotter to the touch tells you everything you need to know: it's absorbing heat from the roof deck and the foam is doing its job by staying cooler. You absolutely need an **air gap** of at least 3/4 of an inch to an inch between that radiant barrier and your XPS foam. Radiant barriers work by stopping radiant heat transfer across an open space. The second you sandwich that foam directly against the silver surface, you kill the radiant benefit and turn it into conductive heat transfer. Without the gap, the foil just becomes a conductor passing heat directly into your foam boards. Since you're in Florida with no venting, you're essentially building a hot roof. If you go the unvented route, make sure your seal with the spray foam is perfect. Any humid air that finds its way behind those XPS boards will hit that hot roof deck and cause major issues over time. I'd lean toward creating a small chimney effect if you can, but if you're committed to the sealed-box approach, just ensure that air gap exists so the radiant barrier can actually reflect the heat back toward the roof deck instead of soaking it into your insulation.

The emissivity issue is spot on, but since you are dealing with that Florida humidity and a hot roof, make sure you're using Owens Corning FOAMULAR NGX for that 2-inch layer. I’ve used it in similar shed setups and it holds up way better against moisture than the generic stuff. For that air gap the other guy mentioned, I usally just rip some 1x2 furring strips and screw them to the rafters first. It gives the radiant barrier room to actualy work before you slap the FOAMULAR on top. Also, grab a couple cans of Great Stuff Gaps & Cracks to hit the edges—you relly can't afford any leaks in a sealed shed.

Def agree with the furring strips idea. I did the same in my workshop last summer and it's a lifesaver. If you're worried about that 'hot roof' setup in FL, just keep an eye on your shingles. Since there's no venting, that heat's gotta go somewhere, and ussually it's baking the roof deck. I've seen guys wrap the XPS edges with some Great Stuff Pro just to be 100% sure no swampy air's sneaking back there. If you don't get that seal perfect, you're gonna have a literal rainstorm behind your foam boards by August. Good luck with the mini-split, it'll change your life once it's humming.

Back in the day, we didn't have fancy thermal cameras to confuse us; we just used our knuckles. If it's hot to the touch, it's holding heat. I've seen guys try to skip that air gap before, and it always ends with the adhesive failing or the boards warping becuase they're basially sitting on a griddle.

One thing to watch in that Florida soup is the 'sweat' on the roof nails. If you don't seal those XPS seams with a high-quality foil tape—not the cheap duct tape that dries out in two years—you'll get condensation dripping right behind your foam. I still prefer the old-school mastic for a permanent seal if you've got the patience for the mess.

Quick update — thanks for the help! You guys were spot on about the emissivity error; the camera was just getting a reflection off the silver. I ended up installing furring strips to create that air gap you suggested. It’s alredy feeling much cooler in here, and I'm alot more confident about the roof deck surviving the summer now. Problem solved!