read one or two weeks ago, a post in another thread on this subject.
the poster recommended adding drops of silicone seam sealant/grip to either the bottom of the pad or the floor of the bivy (i forget which). this cured the problem for him.
Standard disclaimer req'd by my legal dept:
if you can't find that post, you might start with the pad - it's cheaper to replace if it gets messed up w/the sealant/grip. you wouldn't want to "mess up" or damage the bivy in any fashion. if it works on the pad, fine, problem solved. if not, then add some drops/dabs/lines to the bivy. i'm thiinking, not having tried this myself mind you, that silicone sealant/grip should be 'ok' for the silnylon floor of your bivy. it's used to seam seal silnylon, isn't it?!!
Also, if you like Dane's idea (good idea Dane, wish i had thought of it) of using the 1/8" thick (not the 3/8") GossamerGear ThinLight pad, you might be able to get by with cutting 2 or 3 strips the width of your thermarest & maybe 3"-6" tall(? long?) & afixing/gluing them to the underside of the thermarest. this would save a small amt of wt not using the full length of the 1/8" ThinLight pad. you could start by afixing them using a thin double-sided tape for testing purposes, then if the tape is no good as a long term sol'n, use an adhesive to permanently afix the strips to the thermarest. obviously, you could also, if you prefer, afix the entire Thinlight pad to the underside of the thermarest.
hope this info helps.