Is there any upper temperature limit for a given sleeping bag?

Question

I'm actually shopping for a new sleeping bag and am a bit in doubt about a question that I do not find treated anywhere: By investing an arbitrarily huge amount of money I can buy a sleeping bag to sleep snuggly down to -30° C and even below. But what about the other side? Let's say, I buy a sleeping bag with a comfort rating of somewhere between 0 and -10° C, can I use it also in a warm summer night? Or is there some temperature where it will be just too warm to use it even with zipper fully opened and used more like a blanket?

I'm aware of the fact that I will carry more weight and volume than necessary for an insulation that I would not need. I'm just interested about the comfort at the warm end of the temperature scale. So if I buy a bag with some reserve for cold fall or winter nights, will it be unusable in summer or only not the best choice I could have had? If it is unusable above a certain temperature is there some rule of thumb that says something like "a bag that has a comfort rating of X won't be of use for temperatures roughly Y degrees above that X rating."

Answer 1

I have not encountered a situation where a sleeping bag was "too warm" and I was unable to do something about it.

I've gone backpacking in the mountains where it was 70 at night one day, and the next day, several thousand feet higher, it got below freezing. At night in the heat, I pretty much just used my sleeping bag as a blanket with the zipper all the way open.

However, you do risk the possibility of being in the sleeping bag, falling asleep comfortably, but being far too warm, and waking up later drenched in sweat.

Answer 2

There is an Upper Limit and Here's Why

There are in fact times when a sleeping bag can be too warm. Although leaving the bag unzipped allows heat to radiate away from your body upwards, the insulation between you and the ground significantly reduces the amount of heat that your body can dissipate. How far you sink into the insulation also has a significant impact, so thinner bags will do well at higher temperatures than thicker ones, even if they have a similar rating. Sleeping on a stack of wool blankets is comfortable at almost any temperature (assuming your skin doesn't react to the wool).

What is the Limit?

What too warm of a temperature would be depends on both how low your bag is rated and how far you sink into the insulation. My -20F bag gets uncomfortable above around 65F, whereas my 35F bag is good until around 75F, assuming low humidity. Higher humidity raises the range for the heavier bag and lowers it for the lighter one.

The Solution

Although the problem is real, the solution is extremely simple. Place a cotton sheet on the bag before laying down. The cotton sheet pushes down the insulation and creates gaps for increased airflow, which helps your body regulate your temperature.

Answer 3

I went out once with a bag that was too warm. Sometime in the night I woke up covered in sweat and freezing cold! My sweat had soaked the down, completely ruining its insulation properties.

If you want to go outdoors all year round then you just have to accept that you'll be buying twice the gear. A summer pack and a winter pack. A summer sleeping bag and a winter one. Summer boots, shoes or sandals and winter boots.

Answer 4

What hasn't been mentioned so far is that different materials have different ranges. A down sleeping bag is usually comfortable over quite a larger temperature range than a synthetic one. I have a flimsy synthetic liner bag (they are also available in silk, but this one claims to be microfiber) that at least avoids draft when the regular bag is better open because of warmth. My liner bag washes easily as opposed to the down bag, so it also helps keeping the main bag from getting grimy.

Answer 5

Just stating the obvious, you should consider where and when you are hiking. Death Valley and the Appalachin trail offer extremely different environments. I have multiple different sleeping bags for different occasions. In the summer, I generally use a 45 degree down bag. In spring and fall, I tend to carry a 30 degree bag, and in the winter I carry a 0 degree downfilled bag.

These hikes are generally in the Smoky Mountains where in addition to moisture from the environment, perspiration is also a factor. Unzipping the bag is an option but so is sleeping with multiple layers of clothing. I prefer to stay as dry as possible when sleeping.