According to Pete C. :
Chris Lewis wrote:

Eg: applied in the fall, doesn't show any severe effects until
full hot days in the summer, which have driven out the solvents
and the plastic rehardened.

[I should have said "which have _since_ driven out ..."]

The OP indicated spring '06 for the solvent and it's now summer '07.
Summer '06 apparently went by with no issues.

Yes, I know, but I'm not getting warm and fuzzies that the OP's
recollection is entirely accurate and/or complete.

The melt pattern on the Tyvek makes me think hot wiring between the
two indoor outlets. But the vinyl damage seems a bit too widespread
for that.

Vinyl will likely sag/distort at temperatures well below what would
singe/discolor OSB. So, discoloration/lack thereof isn't terribly
compelling.

Wiki torches? Citronella candles? Heck, a plate of just cooked
and still very hot burgers might be able to do something like that
in a short period of time.

The other comment about the shadow from the fixture is interesting.
Is it possible that the glass on the fixture can focus a hot spot
of light on the wall in that area at certain times of the day?
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.