Sleeping Bag Temperature Ratings

The temperature rating of a sleeping bag is the lowest temperature at which it can be used with comfort, at least in theory that is. Despite the efforts and good intentions of the manufacturer you can never find a sleeping bag with an accurate temperature rating unless you're really lucky.

A sleeping bag rated at 25 ºF can feel too cold at 30 ºF depending on a lot of factors. Putting all your faith on the temperature rating can be dangerous. For example, some beginners may assume that if the temperature ratings is low enough they won't need a camping mat for insulation.

The loftier the sleeping bag, the warmer it is. When you sleep, the bottom part of the sleeping bag becomes crushed by the weight of your body. This part of the sleeping bag does not effectively insulate you from the ground.

No matter how low the temperature rating, you won't be warm if you sleep on the cold ground without a camping mattress. Camping mats have something that's called an R-value, a number that indicates how effective it is as an insulator.

How much clothing, the type of clothing and if the clothing is dry will have a huge effect on how warm you'll sleep at night.

A sleeping bag that keeps you snugly warm in the open on a calm 10ºF night may be frigidly inadequate at 30ºF in a 20 mile/hour wind because of windchill factor. Bags with laminated and microfiber shells take the edge off the wind problem.

Dry air is a poor conductor of heat, damp air is a good one.  As a result, in wet weather, the air pockets insulates less effectively than in dry weather.

A sleeping bag may be adequate in dry weather at freezing and below but may be too cold at 40º after heavy rain has saturated the atmosphere, even though the bag is dry.

Believe it or not, a drop in temperature can even make you feel warmer if the moisture that has been ruining the insulation freezes.

The difference between air pockets that keep you warm and internal volume is that the air in the air pockets is dead while the one in the extra spaces between you and the walls of the sleeping bag is prone to convection, increasing heat loss.

In general, people who are bigger and more active tend to sleep warmer. There are other factors that as a whole determines how warm you sleep. An individual may even sleep different in similar conditions which may have to do with tiredness, emotional state, hunger or anything else than can affect your body chemistry. If you give your body enough time, it can also adapt to the cold well.

Despite being highly inaccurate, sleeping bag temperature ratings are still useful guidelines. Remember though that it can be dangerous if you uncritically accept as absolute statements.

Buying a sleeping bag is serious business. This is one camping gear that you need to get right. Making the wrong decision may even prove to be fatal if you're hiking in harshly cold conditions. When buying a sleeping bag, choose one on the warm side except when you're only going to be using it in the hot summer.

Don't just look at temperature ratings. To account for windchill factor, choose one with a laminated and microfiber shell. You should also buy one that fits you and make sure it's not too big or too small.

If you like your sleeping bag to turn with you as you roll, choose a sleeping bag that is form fitting and slender. Choose big bags if you want your sleeping bag to stay stationary as you turn.

Also take a close look at the hood and other accessories such as draft flaps and draft collars and make sure you know the advantages of any extras. See if flaps are sewn right through or it might create cold spots. The flap or baffle should lie snug along the zipper line.

Check for gaps where the heat might slip out by putting it in strong light and looking from the inside. This is mainly about workmanship which you should be able to judge if you've seen both a low quality bag and a high quality bag. Check the quality of the stitching especially in difficult places.

If you doubt your ability to judge workmanship then stick with reputable makers and look at the reviews. Since good quality bags are expensive, consider renting one that you think you like before buying it.