Overbag verses Bag liner.

Don't waste your money on a bag liner.

Paul,

I would definitely go with the new bag. A "bag liner" will disappoint you and freeze you, take it from one who tried the Thermolite liner (and froze) and took it back to REI.

For adding 20 F. warmth to my WM Megalite 30 F. down bag I now carry an old down "quilt" (really a sleeping bag TOP that used to zip onto a nylon covered foam pad). I sewed 6 female snaps on my Megalite bag and removed the zippers from the old down top and sewed 6 "male" snaps on the old down bag top. That combo is WARM and still very light. Plus, unsnapped, the two dry quickly and then stuff into their own dry sacks.

Your quilt/bag idea will be the envy of others B/C it is a dual duty setup that is very versatile when the parts are used alone as the temperatures dictate.

BTW, Check out Campmor for a very reasonably priced Campmor summer down bag before you buy a synthetic bag. If you do get a synthetic bag try to get Climashield as the insulator. It has a high CLO value per weight and is the most resilient syn insulation on the market. Shingled (overlapping bat) synthetic insulation seems the best internal design. TNF's "Cat's Meow" uses it, among others.

I dont have eric's experience, but from my trails on my back deck, I have to agree. I tried several times to take my 20 degree bag to zero (never really zero but single digits once) with wm clothes or liners, etc. I was never warm. I was OK, even cold, but never warm.

I finally got a zero quilt (and now of course its been 30+ every night) so I havent tried that out.

I seriously considered finding a 30-40 degree quilt to throw over my wm bag, but the sliding kept me from doing it.

Erics snap solution may be the answer.

I finally decided on a 0-10 quilt, and a 25-30 quilt for year round. I figure with my down clothes I can make both of these work in almost any temp. (that I would be out in anyway)