Pluses and Minuses of Overbags and Other “Bag Warming” Alternatives
There are several ways to extend/reduce the temperature range of your “basic” or “core” sleeping system:
In Hot Weather:
Pull the hood off your head if you are using a mummy bag; unzip the bag partially; stick your feet out with designs that permit this; unzip the bag completely and use it as a quilt. OR just use a quilt which allows a lot of these variations on the fly (by wiggling around and shrugging). Skip a tent, use a bug net over your head (tents are notoriously hot and muggy in the summer). Use a tarp if rain is a possibility. Adjust the amount of clothing based on the temperature (take off clothing and sleep in base layer - to keep the bag clean). Once again a quilt is useful because you can go to sleep in your trail clothing (or switch to your alternate set, if your trail clothing needs to dry out. I like to have the stuff in my pockets and feel naked if – I am, well, naked or just in base layer.
In Cold Weather:
Add a tent. Nothing keeps a secondary envelope of warmer air around you like a tent.
Add a bivvy sack. The secondary envelope of warmer air isn’t as great as with a tent, but it’s better than just the sleeping bag.
Add vapor barrier layers. This is especially important if you are going to be in a tent or bivvy sack, since otherwise moisture will accumulate in your sleeping system and reduce its efficiency. Oware used to sell a very light, impermeable emergency bivvy sack that you could wear as an “inner bag liner.” I prefer using my “light” rain gear – Houdini jacket and pants (the original Dragonfly version is REALLY a vapor barrier, but the Houdini builds up a “comfort zone” around your skin while keeping your sleeping system relatively dry). Wear the vapor barrier right over your base layer, which means your clothing will be “inside out” with the jacket and rain pants closer to your skin, and extra clothing on top of them.
Add clothing. Before thinking about an over-bag, think about puffy jackets and puffy pants (I like the Patagonia Micropuff with hood.and their pants). You should be thinking survival – if it’s really nasty, you will probably never have enough layers and overbags to just lie around waiting for rescue, so you have to count on building up therms by moving (hiking out!) and figure out the minimum you need for the coldest, wettest projected or historical weather (or somewhere in that vicinity, depending on your taste for risk). Naturally Primaloft, Thermalite, Polaguard all all superior to down, which can wet out. (If you hike out in worst case scenario, the “light rain” gear goes OVER the puffy layers. There will be some wetting out but probably not enough to impair the insulating layers.)
If you are STILL pushing the limits on your basic bag, ONLY NOW should you think about an overbag. OVERBAGS ARE INHERENTLY INEFFICIENT. You are trading a single, high loft bag with only TWO layers of shell fabric for an EXTRA TWO LAYERS OF SHELL FABRIC, not to mention potential weight additions like zippers.
If you are not a believer in vapor barrier/slow down clothing layers (the ultralight “light rain” clothing), the extra fabric layers are somewhat helpful in preventing humidity soak through/rain soak through, but the only advantage of using two bags is to save money.
My personal preference for the overbag option is to combine a summer weight quilt (which I usually spec in Jardine’s “extra insulation” configuration and a little oversize, since I sleep cold) with an ultralight mummy bag like a Montbell or Western Mountaineering – something around 16-20 ounces. It’s important to spec the quilt oversize so you can tuck it under you (and during the summer it’s adequate for couples camping). It’s also critical to use a bivvy bag (like the Bibler Winter Bivvy, except it runs a little small) to make sure air doesn’t seem under the edges of the quilt, or else to wrap a tarp around you (which traps air, another reason why you need vapor barriers as your first defense). As an alternative, you can purchase elastic bands (an inch or so wide) and sew them into “hoops” to wrap around your quilt over mummy sack.
Finally, check the stats and reports. You need a MASSIVELY thick (relative to ultralight hiking standards) ground pad to keep from losing heat through the ground, and it needs to be long enough to cover your legs – now is not the time to let your legs dangle on cold ground off the pad, or to drape them over “just” your backpack.