Greetings. My 45F Mountain Hardwear sleeping bag is usually warm enough in summer months. However, I'd like to be prepared in case the temps unexpectedly drop to 32F, without carrying more weight than necessary of course.

My first thought is to get a Sea to Summit silk sleeping bag liner, which should add 10F of warmth but costs $69, and weighs 4.8 ounces.

My second thought is to get an SOL Emergency Bivy for $16, and weighs only 3.8 ounces, but I've been unable to find an advertised temperature rating. I know this piece of gear isn't really intended for multiple uses, but I'd be happy if it lasted for a week-long trip since it's inexpensive. I have heard that this can cause condensation though which doesn't seem right to me -- wouldn't that cause hypothermia and defeat the purpose of an emergency bivy?

Does anyone have advice on which path to take?

Thank you.

I don't think the silk liner would add 10F, but I have no experience.

I have that bivy and have used it some as a vapor barrier liner in winter. I wouldn't want to use it over insulation except as a last resort to protect my bag from rain.

Are you warm at 45F in that bag? Would you be warm at 30F with all extra clothing? What is your sleeping pad R-value? Maybe you could bring more clothing or a warmer or extra foam torso pad for lower temps, and that would get you down to 30F? Do some backyard testing to find out. :)

Otherwise, the best solution is to get a new bag, preferably down with good DWR. I use my MontBell Super Stretch UL #3 down bag (manufacturer rating 30F) in temperatures from 30F upward in fairly humid and wet areas. Once it hits 50F or so, the bag is used as a quilt with some ventilation.

I've used a silk liner bag extensively indoors and I suggest about another 3-5 degrees of warmth. It's a nice thing to have to keep a bag clean or for couch surfing but an emergency blanket will be lighter and warmer.

The condensation will be impressive and definitely get something wet.... you - your bag - depending on if it's inside or outside the bag...... On a week-long trip no telling how many pounds, seriously... of condensation the bag would absorb.

It also doesn't give like fabric and is surprisingly kind of clingy so you probably will get all tangled in whichever position you use it.

I learned this, as usual, the hard way one trip where I packed one as an emergency bivi to use under a tarp in case of REALLY nasty weather and giving it a try as a vapor barrier liner for the heck of it, as an experiment, the last night of the trip. I had pre-made a 4 foot deep collar-vent-seam with velcro dot closings in mine to help make it more open, accessible and better venting and it was still completely covered inside with condensation. I got so seriously snarled up at one point in the night that I had to get up and "re-make" the bed and start over.

The thing would be however; exactly as advertised, great in an emergency.

use the non breathabe bivy as a VBL inside the bag and youll gain 5-10F