Hot stripes on a wall in the afternoon - missing batts or just sun load?

Same thing happened to us a couple of years back on the west wall of our bedroom, down here in Perth. Warm vertical stripes spaced exactly like the studs, wall reading a good 5 or 6 degrees hotter than the rest of the room in the late afternoon. I was convinced the batts had fallen down. Turned out the wall was just copping the sun and the timber framing was conducting the heat through, which is pretty normal for a sun-baked wall even when the insulation is fine. The stripes showed up on the east wall too, but only in the morning, and never on the south side. The thing that settled it for me was comparing the same wall at 6am. If the stripes were missing or slumped batts, you'd often see cooler patches where insulation is absent, because the cavity air differs. With thermal bridging, the whole wall goes quiet overnight and the stripes disappear too. On the wall plate idea, be careful. I started unscrewing one and then realised I had no idea what was behind it besides wiring. If you do go that route, kill the power at the switchboard first. Honestly a torch through the gap behind an outside tap or light fitting tells you a fair bit without opening anything live. If the stripes only show up in the afternoon sun and the room is fine on cloudy days, I'd lean towards framing conduction and shading the wall rather than assuming the insulation is gone.

I'd push back a bit on blaming the studs alone. When we renovated our rumpus room the west wall showed the same banding on the camera and the builder's mate told me the same story, framing conducting heat through. When we pulled the plasterboard a year later for a rewire, two of the batts had slumped halfway down the cavity and one was never installed at all. The stripe over the missing section ran a few degrees hotter than the stud lines did, so there was more going on than timber alone. On your numbers, 40 degrees at the surface with 34 outside during peak sun on a west wall isn't shocking by itself, but the fact that the bands run floor to ceiling with no break is what would bug me. Thermal bridging through studs shows up strongest mid-wall, not right down to the floor plate in my expirience.

Rather than the torch behind a power point, which only tells you about one stud bay anyway, consider hiring a borescope for an afternoon. Drill a small hole in an inconspicuous spot near the skirting, have a look inside the cavity, and patch it with a dab of filler. Cost me about forty bucks and settled the arguement in ten minutes.

Also worth checking whether there's external cladding or a diferent wall lining behind the patio side, since different materials heat up at very different rates.

Before you pull anything apart, try shading the wall for an afternoon. We hung a shade sail on the fence side of our west wall one stinking hot weekend and re-scanned after lunch. Most of the banding faded back with the shade off it cooled again. That told us the wall was basially fine and it was just copping the full arvo sun.

Also worth walking outside and shooting the wall from the yard. If the outside face shows the same stripes, heat's coming straight through the cladding. If the outside looks even but the inside is striped, that's when I'd start suspecting the cavity itself.

Beginner question from me, so bear with me. Could the camera itself be getting fooled by the sun heating the outside surface? Like the wall skin being hot to touch regardless of what's behind it, and the camera just reading that? I say this because I borrowed one of these things for our garage last summer and the dark panels looked freezing while the metal frame looked boiling, which turned out to have nothing to do with insulation at all, just different materials warming differently. Does the colour of the outside cladding mess with these readings too, or does that not really matter?

No real advice from me since I'm still learning all this myself, but your post reminded me of our place. We had one warm stripe on an internal wall that had nothing to do with the sun at all. Turned out there was a hot water pipe running up through that cavity. Took me ages to figure out because I kept blaming the insulation like everyone does. So maybe check whether the stripes line up with any plumbing or flue routes before you go poking around, expecially if one of them is in an odd spot like near a bathroom.