I agree with Brian! A few ounces extra down weighs less and is warmer than a liner!

I tried using one of those silk liners (the kind advertised to add 9*F to the bag's warmth). I got so tangled up in the thing (I toss and turn a lot) that it took me a couple of minutes to get out of my bag in the morning. After a few nights of that nonsense, I gave up the liner. I noticed no difference in warmth with or without it.

For me, a 20*F bag with a full-length zipper (so I can ventilate it on warm nights) is far better!