Is this a new technique? I was driving past this two story house which was being re-roofed. After removing old shingles, crew installed a blue tarp looking thing. It was applied similar to Tyvek prior to siding a house. At first I thought they were protecting against possible rain; but being that we’re in a mild drought in our area, it didn’t seem logical. They installed new shingles on top of liner and were done in no time at all. Are you familiar with this Procedure?

i know next to nothing about roofing, but it seems to me that if heat rises and roof is lined (sealed), one could create the undesirable heat pockets.

I dont know the people not did I feel comfortable stopping to ask. But I’m sure curious.