Getting a good sleeping bag is essential to having a great camping trip. There’s nothing worse than overheating or shivering through the night and waking up tired in the morning. Investing in a good bag will help you stay at a comfortable temperature so you can get a good night’s rest and be ready for adventure the next day.
When it comes to choosing a sleeping bag, there are so many things to consider to make sure you get the best one for your needs. You may have seen the term GSM come up when searching for sleeping bags.
So what does GSM mean on sleeping bags?
GSM means grams per square meter and is a measurement of the density of the insulation or filling inside a sleeping bag. If you have a high GSM number on your sleeping bag, this means there is a higher density of insulation or fill in your sleeping bag. A high density of insulation or high GSM means that your sleeping bag will hold heat better than a bag with a low GSM rating.
Many factors go into the warmth of a sleeping bag and GSM is one of them. The higher the number of GSM, the denser the filling is.
While a higher GSM rating usually means the bag will be warmer, it’s important to understand how a sleeping bag works. The insulation (sometimes called fill) itself doesn’t generate warmth but instead prevents body heat from escaping while you’re sleeping.
Some bags also include a double layer or may have baffling to prevent the filling from shifting around or getting bunched up in the corners. It’s important to have the insulation evenly distributed throughout the entire bag.
Of course, more filling means more insulation throughout the sleeping bag so there should be an adequate amount of coverage and no bare spots for body heat to escape through.
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Sleeping Bag Rating Systems and GSM
Some companies use a numbered rating on their bags or may refer to them using seasons or numbers, similar to the way tents are categorized. Common categorizations may include:
- One season (summer) – Perfect for warm summer nights or for kids to use indoors during sleepovers.
- Two seasons (from late spring to early autumn) – Ideal for weather that may be warmer during the day but become cooler at night or for those who get cold when sleeping even during the summer months.
- Three seasons (spring to autumn) – Designed for cooler temperatures but not so cold that there will be frost overnight.
- Four seasons (winter) – These should be used when there is snow or frost during cold winter nights.
Other manufacturers will use GSM to indicate how much the bag has been filled and how warm the bag should be.
Many bags also have a temperature rating which outlines the range of temperatures that the bag is recommended for. The EN 13537 testing originated in Europe in 2005 and has since been adopted by many manufacturers. This system also has a range of ratings, from upper limit to extreme:
- Upper limit – This is the temperature that a man should feel comfortable inside the bag without sweating or overheating when the bag is unzipped.
- Comfort – This rating refers to the temperature for which a woman would feel most comfortable because most women are cooler when they sleep than men.
- Lower Limit – This range is the temperature at which a man can sleep in the bag for up to 8 hours and be comfortable without waking up.
- Extreme – The lowest temperature recommendation for a woman to use the bag for up to 6 hours without getting hypothermia and for the bag to be used as a last resort.
Of course, these ratings are based on a “standard” man or woman who is a specific age, height, and weight, so they won’t apply to everyone. They also aren’t the best guide when choosing a sleeping bag for children because kids often get colder or hotter faster compared to adults.
If you know that you tend to get colder at night, err on the side of warmth and choose a sleeping bag that will allow you to sleep comfortably at lower temperatures. It’s much easier to take off layers or unzip the bag than it is to get warm once you’re already cold.
There’s More to Warmth Than GSM
However, there are factors other than GSM that should also be considered when determining the warmth of a sleeping bag. One factor is the materials that were used to construct the bag. Cheap materials or zippers that break easily will render even the best bags useless no matter how high the GSM rating is.
The size and shape of the bag can also affect warmth, with mummy-shaped bags being some of the warmest styles. An attached hood or draft collar allows you to snuggle into the bag and have an entire layer of coverage over your neck and shoulders.
Having the shell fabric treated with DWR to make it water-resistant will also ensure you stay dry which will, in turn, promote warmth. A draft tube is a bit of fabric that covers the zipper and adds some extra protection and warmth from coming through the holes in the zipper.
Sleeping bags have either a synthetic or a down fill and this will also affect the warmth as well as the bulk of a bag. Even if the GSM rating is the same, the bag could be heavier or bulkier depending on what it is filled with.
Synthetic bags are known for retaining more insulation when they get wet compared to down. So a sleeping bag with a lower GSM rating with a synthetic fill may be warmer than a down-filled bag with a higher rating when camping in damp areas.
Down, on the other hand, is incredibly lightweight and compressible and is known for its warmth. So a down sleeping bag with a high GSM rating might feel lighter and easier to carry compared to a synthetic fill bag of the same GSM or lower.
Although a high GSM number usually indicates more warmth, it’s always a good idea to check the temperature rating of a particular bag to get an idea of how cold you can camp in it.
Getting a bag that fits is essential to have a lot of warmth because there will be less open space for airflow. A tight-fitting bag will be much warmer than one that is too big or too long for your body.
Increasing the Warmth of a Low GSM-rated Sleeping Bag
If you’ve already purchased a sleeping bag but are now concerned that the GSM rating isn’t high enough, there are ways to add additional warmth to the bag.
The first thing is to make sure you have a quality sleeping pad. A pad should go beneath the sleeping bag to insulate the area between the bag and the ground. While a pad by itself doesn’t provide warmth, it prevents your body heat from leaching out.
Get an air-filled pad instead of a thin foam one because it provides more insulation and will be much warmer. They are also much more comfortable and not as bulky as self-inflating pads, making them perfect for backpacking.
Next, you can add a sleeping bag liner inside the bag. Depending on the material, you can get liners that will add quite a bit of warmth. Some great materials are microfleece, wool, thermolite, or silk.
Another tip is to wear layers inside the sleeping bag. Bring warm pajamas made from a warm fabric like fleece or some thermal underwear. Avoid cotton because it absorbs water, doesn’t insulate when wet and isn’t breathable. Always wear dry clothes to bed too and put on plenty of layers.
If you’re still cold after all that, get a cap and gloves that aren’t too bulky to wear while sleeping. A balaclava, bandanna or handkerchief can provide additional warmth over your face for particularly cold nights.
The type of tent you are using will also play a factor in warmth. Even if you have the best sleeping bag in the world, don’t expect to be able to sleep comfortably in winter using a flimsy 3-season tent that is meant for warm spring and summer temperatures.
Tent size is equally important because it takes a lot more to warm up a large tent if only one or two people are camping in it. A good rain flap is essential for trapping warmth and keeping the tent secured. For maximum warmth, solo campers should opt for a small tent, even if they are car camping and can manage to bring something bigger and heavier.
Final Thoughts
Although GSM ratings can tell you a lot about the density of a sleeping bag’s filling, they aren’t enough on their own to let you know how warm a bag will be. There are many aspects of a sleeping bag’s design that can affect the warmth, so it’s important to look at different ratings and information to get a broader perspective.
If you can’t afford a high GSM filled bag, there are ways to find a warm bag by choosing one that fits your body well and is properly shaped. Accessories can also be added to increase the warmth of your existing bag so you don’t have to purchase a new one.
So keep GSM in mind when choosing a sleeping bag but don’t make it the final deciding factor. Instead, feel the bag, read the label, check the temperature rating, and crawl inside to make sure it’s a good fit.
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