I also use a piece of tyvek, about 2X1.5m (or 6.5X5 ft, for uncivilized non metric users :/).
I have doubled the fabric where I installed six small grommets, and ultra thin dyneema lines, and use this as sit pad, lunch pause or beach "blanket", emergency poncho, small kitchen /pause tarp, and eventually footprint on nasty ground (rarely!)
It stays in an external pocket of my pack, just in case.
I don't know what kind of tyvek it is (got it from a kite shop), it's waterproof, very thin and soft...but I have no idea of the weight!