When people talk about some kind of roofing material for sound deadening, are they talking about rain and snow layer self adhesive that goes under the first rows of shingles?

Probably Peel & Seal. I've used it some, but have never used the quality stuff to even compare the two.

There is the peel and stick Butyl with the aluminum face for roofs, and there is peel and stick Butyl with a white face for the perimeter of residential windows. I'll assume the aluminum faced can handle higher temps, and the white facewd stuff does seem to be a bit softer, and stretchier.

I will use either, but I cover the exposed edges with Nashua Flexfix 555 Tape. This is not the regular aluminum tape, this stuff is flexible and does not tear as easily. It's been on the inside of my engine cover closest to exhaust manifolds for 14 years. No durability issues.

I recommend wiping the surface with Isopropyl alcohol before bonding either the peal and seal or the flexfix tape.

I too have no basis for comparison to a $$$ product like Dynamat. The peal and seal stops large sections of flat panels from ringing like a bell when struck with a knuckle, but sometimes a few different size patches have to be stacked to stop it ringing.

Sound deadening can be taken to extremes.

If Ram4Ever reads this, well he has gone there and redefined 'extremes'

I just use a knuckle, and any improvement is appreciated.

Same here. I put it in the roof of my wife's 2000 Cherokee. Did the trick for about a year, then we noticed the entire floor pan on both sides is rotten, so I basically wasted a bunch of the Peel & Seal as we are going to part that one out to fix our other 2000 XJ. DOH!

I was at a guy's house yesterday and he was getting a new roof. I saw this stuff laying there that was asphalt with a peel off back. The roof guy said it was rain and snow barrier,required on all roofs now. It's a 3ft wide roll, they put one roll across each edge. Anyway the guy gave me a piece probably enough to do a whole van.

Nobody there ever heard of it for that.
I bought some heavy pads called Q pads at O'REILLY that worked for a while but it got up to 110 and they landed in the bottom of the door.
I cleaned them good, but I think the tape is the answer.
I think I'll stick some in my doors and see what happens.
I'm not trying for an isolation booth either.