Selecting a sleeping bag is a fairly straightforward process. You need to make a decision about two things—the shape of the bag and the insulation of the bag.

The shape of the bag is important in terms of how well it will keep you warm and how comfortable it will be, in some ways. The warmest bag you can get (and the lightest) is the mummy shape, which looks a bit like a sarcophagus. It is fitted to the body, so that there is little space for air pockets, which require body heat to warm up, and the hood around the head helps keep heat trapped, especially important in winter conditions.

Mummy bags are the most popular for several reasons. As I already said, they are the lightest and most efficient, so backpackers like them. They are warmer than non-mummy bags of the same weight, and they pack small. All in all, a good set-up. They are not, however, without flaws. The downside of a mummy bag is that you might feel like you’re constricted. If you move around a lot at night, you’re likely to feel a little claustrophobic. You can spend some time in the bag at home and get used to it, which I have heard helps some people.

Another shape that is growing in popularity is the semi-rectangular bag. It shares some of the features of the mummy bag, just not as extreme. It is tapered to fit closer to the body, but it is not nearly as tapered as a mummy bag. It lacks a hood, but compensates for that by being somewhat longer so you can pull the top of the bag around your head, still breathing outside to control vapor. It is not as efficient as a mummy bag, as it is a looser fit, but some people (myself included) find them more comfortable, especially stomach sleepers.

Semi-rectangular bags are not as efficient, especially in extremely cold weather, as air can move in and out of a bag more easily when it doesn’t have a hood. For that reason, you’ll find few semi-rectangular bags with a rating below 15 degrees F. The good news is that winter bags, even mummy bags, are cut with a little more room so you can wear clothes in your bag and still move around a little. Semi-rectangular bags can also be unzipped and used as a quilt in warmer conditions. Not a perfectly square quilt, but close enough.

The final shape which we will discuss is the rectangular bag. Rectangular bags are, for the most part, relegated to slumber parties and car camping, where size and bulk (and material) are mostly irrelevant. They are inefficient, requiring massive body heat to warm up, and are usually made of materials that do not handle moisture well.

This doesn’t mean there isn’t a place for them, it’s just not outside. I keep one in the rear of my truck for emergency use as a quilt or a sleeping bag, and if the dogs curl up on it, that’s fine. It’s nostalgic to see ducks and shotguns on the flannel interior, and I think I paid twenty-five dollars for it. It weighs about five pounds, and is pretty huge.

Now on to insulation…

There are two basic types of insulation; down or synthetic. Each have advantages and disadvantages, and each can vary in quality from excellent to poor.

Down sleeping bags are held in a premium as goose down is one of the lightest, most compressible materials that can achieve incredible R-values. Down is extremely light, as little as 800 cubic inches per ounce. To give you a perspective, one pound of down of this quality would fill a two-foot cube. Down usually ranks from 650 to 800, with 650 being a very common quality of down. You have a little less insulating power with 650, but as you’ll see later, that’s not very relevant except in extreme climates.

Given its lightweight, its compressibility, and its insulating properties, why then aren’t all bags made of down? Simple economics. Down is quite expensive when compared to synthetics, and has one nasty habit—if it gets wet, it loses all of its insulating properties.

That said, I’ve never had my bag wet, and I like the ability to stuff a down bag down small and store it in the rear hatch of my kayak. It is the best material if you can afford it and don’t spend all your time in extremely wet climates.

Synthetic insulation has come a long way, and I mean a loooong way in the past decade. When I was a kid the synthetic bags were filled basically with quilt batting. Not compressible in the least, and not all that warm unless the shell was windproof, it was at least a step in the right direction. Then came along Thinsulate, Qualofill, Hollowfill, Polarguard, and a host of other synthetic materials that progressively approached down. Funny thing was, they all compared themselves to down—“We’re almost as compressible as down”… “We’re almost as warm as down…” etc. Until recently, nothing was really close to down.

One of the latecomers to the synthetic insulation world is Polarguard 3D. It’s a very common synthetic material used in sleeping bags, and it claims the same loft and insulation qualities as 650-fill down. That remains to be proven, at least in my eyes, but it is a very good material, and it is significantly less expensive than down.

So how do you make a decision? Good question. Here are some questions to ask yourself:

To answer that question, as yourself where you are going to sleep and when. People who camp from Memorial Day to Labor Day have a good thing going—they can get by with less insulation and hence a smaller, lighter bag. Unless you’re travelling in the far north or you sleep cooler than most, I’d say a 25- to 35-degree bag should suffice anywhere you want to go. Colder sleepers might want a little more insulation, perhaps a 5- to 15-degree bag. Winter campers can get a full-on winter bag, or perhaps an overbag that converts your summer bag to a winter bag, good for temperatures between 0 and 25-below zero.

Yes.

Oh…what kind. Got it. Simply put, synthetics do not perform as well as a good down bag. So the only reasons to get a good synthetic bag are if you are not in the position to shell out the extra fifty to seventy-five bucks for a down bag, or if you spend a lot of time in a very wet climate, where the chances of having a wet bag are high. That said, synthetics have come a long way, and there’s nothing wrong with them. If you only use the bag three times a year, then there’s little reason to spend the extra money. Also, if you have kids who are not completely done with potty training, synthetic bags are easier to launder after an accident. So the simple answer is “choose the material that works best in your environment.”

This is a religious discussion that can only end with hard feelings. There are zealots for mummy bags and hard-core semi-rectangular fans. The only way to tell is to get into a bag on the floor and move around in it. To be safe, if you are a warm sleeper, I’d usually recommend erring on the side of semi-rectangular, as they are more comfortable for most people. And in the applications I am accustomed to, like canoeing and kayaking, weight is not a huge issue, and a down semi-rectangular can pack down pretty small.

I should add that I am not a zealot, though my wife is, having stolen my Marmot Wasatch semi-rectangular 25-degree down bag. I have been busted back to my old mummy.

Ah. Good question. The answer is “it depends.” If you want maximum warmth in very cold conditions, ironically it is better to sleep alone, as moving around pushes air out of the bag and there is a very large gap created between the sleepers which can be hard to keep warm. Also, mummy bags usually do not have full-length zippers, with most of them ending near the knees, so your feet will never be together unless you do the "foot together" maneuver.

That said, in warmer climates and with summer weight bags, it can be downright pleasant to share a bed. If you do, be sure to make certain your sleeping pads are attached to each other so you don’t end up on the cold, cold ground.

Not much else to say, other than this—be certain to check out the tech article on sleeping pads. The insulation under you in a sleeping bag is almost always compressed and pretty worthless as an insulator. What’s under you is as important as what’s on top of you. Pleasant dreams.