John,
Yes, tar paper, minus the rock granules of roofing material of course.
It is available in various weights. 
Nice thing is, it will not soak up or hold any moisture.
Doug

  ----- Original Message ----- 
  From: Trvlr2001@aol.com 
  To: vanagon@astound.net 
  Sent: Wednesday, March 24, 2004 2:41 PM
  Subject: Re: Sound Deadener Installation Concerns

  Hi,
  When you say building felt,
  are we talking about the old tar paper?
  I have thought that, that stuff they put around the outside of the edge of roofs would work well.
  I think they call it snow seal, or such,
  and its used to keep ice from getting up under the shingles.
  thanks,
  John C.
  SLC, UT...

  In a message dated 3/24/04 3:24:29 PM Mountain Standard Time, vanagon@ASTOUND.NET writes:
    Anyway, if you want an alternative to Dynamat get some heavy construction
    felt like #30 or #60 and cut it into the shape and use a MILD contact
    adhesive that will keep it attached for then next 10 years or so like 3M #77
    spray on adhesive, it is not that strong of a bond use it sparingly and you
    will be able to remove the asphalt felt at a later time.