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If the rated temp is 40 deg ..... how much can I lower it by using a silk bag liner? Any?

Thanks.

really depends on the liner quality. I would say that claims of 8-12 degrees are most common.
You could also make your own, Ive seen the big brands for around $65-
the cost of 2 yards of silk is $7, plus the cost of polyester thread (its recommended)
Im looking forward to sewing my own silk liner, and will post results if I ever get around to it.. :

It also can depend on the clothes you wear. Wearing a fleece cap to bed will increase your heat and so will other insulative layers, but being careful not to wear too much. (There wont be any air left surrounding your body in the bag)

In response to Abuelo's reply:

I use a silk liner most of the time. It helps, but probably only 5 degrees or so. I also wear long underwear or any other clothes I have when it gets cold, but you might be less sensitive to the cold. People vary quite a bit.

I would first spend a night in the back yard at the lowest expected temperature before push the rating in the field. To me, the only thing more miserable than being cold all night is being cold and wet. I've done both, hopefully for the last time.

Thanks for the responses guys. You'd think at 65 I'd have these things worked out by now but this hiking/camping thing had to wait until I retired.