Colin Snow
When it comes to sleeping bags, fill weight and fill power are two important factors that can affect warmth and comfort. Fill power measures the quality of down, while fill weight measures the amount of down in a bag, indicating how warm it will be. Sleeping bags with the same fill power but different fill weights will vary in warmth, with the bag containing more fill being warmer. Fill weight and power are especially important when hiking or camping in cold weather, as they can help insulate you from the cold environment by trapping body heat and preventing the exchange of warm and cold air.
| Characteristics | Values |
|---|---|
| Purpose of sleeping bag fill | Insulate the user from the cold environment by trapping body heat and preventing the exchange of warm and cold air |
| Factors affecting warmth | Volume of air, filling material, bag construction, user's metabolism, clothing, food intake, physical fatigue, hydration, etc. |
| Fill weight | Measures the amount of down in a bag; a greater weight of down will provide more warmth |
| Fill power | Measures the quality of down; greater fill power entails greater loft (volume of air between fibres) and warmth |
| Loft | A measure of the 'fluffiness' of down; the volume taken up by a given weight of down |
| Fill ratio | Proportion of down placed in the top and bottom of the bag; a common fill ratio is 55/45 |
| Types of fill | Down (natural) or synthetic |
| Down | More expensive, better warmth-to-weight ratio, compressibility, and durability; requires more care, performs poorly when wet |
| Synthetic | Less expensive, non-allergenic, provides some warmth when wet, easier to care for; lower warmth-to-weight ratio, less packable, shorter lifespan |
| EN/ISO ratings | Standardized ratings indicating the temperature range at which a bag will keep the user comfortable; includes comfort, limit, and extreme temperatures |
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What You'll Learn
Fill weight vs. fill power
When choosing a sleeping bag, it's essential to understand the difference between fill weight and fill power. These two factors determine the warmth and weight of a sleeping bag, which are crucial considerations for backpackers and campers.
Fill power refers to the quality of the down in a sleeping bag. It measures the loft and warmth provided by the down, indicating the overall quality of the bag. The higher the fill power, the more loft the down has, and the warmer the bag will be. Loft is the volume of air held between the fibres of down. As air is an insulator, a greater volume of air will keep you warmer. Fill power is often the more promoted metric used by manufacturers, but it doesn't always indicate how warm a sleeping bag will be.
Fill weight, on the other hand, measures the amount of down in a sleeping bag. It is a better indicator of warmth, as it takes into account the actual amount of insulating material in the bag. For example, a sleeping bag with a higher fill power but lower fill weight may not be as warm as a bag with a lower fill power and higher fill weight. This is because the latter bag has more insulating material.
When comparing sleeping bags, it's important to consider both fill weight and fill power, as well as other factors such as temperature ratings and reviews. For instance, a bag with a higher fill power may have better packability when compressed, taking up less space in your backpack. Additionally, the shell material and construction of the bag can also impact its overall weight and warmth.
It's worth noting that down sleeping bags are generally more expensive than synthetic bags, but they offer superior warmth-to-weight ratios and are highly compressible. Synthetic bags, on the other hand, are often more affordable, provide some warmth when wet, and are easier to care for. However, they may not provide the same level of warmth and packability as down bags.
In summary, fill power and fill weight are both important factors to consider when choosing a sleeping bag. Fill power indicates the quality of the down, while fill weight measures the amount of down in the bag. By understanding these metrics and considering your own needs and preferences, you can make an informed decision about which sleeping bag is right for you.
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Temperature ratings
Firstly, it's important to know that temperature ratings are estimates and not precise indicators of performance. Lab testing attempts to simulate real-world use by placing a test dummy inside the bag, dressed in base layers, and then measuring the amount of electrical energy needed to maintain the dummy's initial temperature as the air temperature is cooled. While this standardised testing procedure, known as the ISO Test, is designed to ensure consistency across brands, it cannot account for all variables that may affect your experience in the outdoors. These variables include factors such as your clothing and gear, body type, metabolism, weather conditions, and food intake.
The ISO Test provides three temperature thresholds: Comfort, Limit, and Extreme. The Comfort rating is based on the temperature at which the "average female" can sleep comfortably through the night without curling up for warmth. The Limit rating is for the "average man", indicating the temperature at which he can maintain his thermal equilibrium in a curled-up position without shivering or risking hypothermia. The Extreme rating, used in the EN (European Norm) standard, indicates a temperature at which a woman is unlikely to die of hypothermia within six hours. It's important to note that these ratings are based on averages, and individual experiences may vary.
When choosing a sleeping bag, it's generally recommended to select one that is rated for a slightly lower temperature than you expect to encounter. This is because the ratings may overestimate warmth, and sudden temperature drops can occur. Additionally, consider your personal preferences and whether you tend to sleep hot or cold. If you're a cold sleeper, opt for a bag with a higher Comfort rating, and if you're a warm sleeper, the Limit rating may be more relevant for you.
It's worth noting that not all sleeping bags have ISO or EN ratings. These standards do not apply to bags designed for extreme cold or children's bags. Additionally, some companies, especially smaller cottage companies, may use their own rating systems.
In summary, while temperature ratings provide a useful guide when choosing a sleeping bag, they should be considered alongside other factors such as fill weight, fill power, and your personal preferences and sleeping habits.
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Synthetic vs. down insulation
When choosing a sleeping bag, one of the most important decisions you'll have to make is between down and synthetic insulation. The type of insulation directly affects the warmth, weight, water resistance, compressibility, and price of sleeping bags.
Down is the lightweight undercoat of aquatic birds' plumage, insulating them in extreme cold. The plumes and clusters in good-quality down create hundreds of thousands of tiny air pockets, where air is trapped and warmed by body heat. This mechanism allows humans to trap heated air close to their bodies to maintain warmth. The quality of the down in a sleeping bag makes a huge difference in terms of warmth, weight, durability, and compactness. Loft, or the "fluffiness" of down, is a measure of the volume taken up by a given weight of down. The higher the loft (or fill power), the more room it takes up and the more efficiently it traps air. For a given weight of down, the higher the loft, the larger volume it will fill, meaning a smaller weight of down is required to fill each sleeping bag baffle. The advantage of higher-fill-power down is that sleeping bags require less down to fill space and achieve a certain temperature rating. Less down equals a lighter product. However, down is expensive and not easy to care for. When traditional down is wet, it loses most of its insulation value, and it can take a long time to dry. Some manufacturers have developed proprietary treatments to make down water-repellent, but even treated down will get wet if submerged in water or exposed to heavy rain.
Synthetic insulation, typically made of polyester, is quick-drying and insulates even if wet. It also tends to be less expensive than down insulation, and it is durable and hypoallergenic. Synthetic sleeping bags are a good alternative to down sleeping bags, especially for those on a tighter budget. However, there is a huge variation in quality in the synthetic sleeping bag market, and synthetic bags are generally heavier and bulkier than down bags. Some manufacturers make sleeping bags with a combination of water-resistant down and synthetic insulation. This hybrid construction can provide the benefits of both materials while limiting each material's imperfections.
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Loft
Over time, sleeping bags may lose their loft. This could be due to compression inside a backpack or stuff sack, or from the weight of your head on a down pillow. When a sleeping bag loses loft, it won't be as fluffy or warm as it once was. You can tell if your sleeping bag has lost its loft if it doesn't look as fluffy, doesn't keep you as warm, or if you can see light shining through when you hold it up to the light.
If your sleeping bag has lost its loft, there are several ways to restore it. Firstly, prevention is key. Keeping your sleeping bag clean and hanging it up loosely will prolong its lifespan and prevent clumps and decreased loft. If your bag has already lost its loft, you can try shaking it out and laying it flat for a couple of hours. You can also try throwing it in the dryer with no heat (or low heat, depending on the bag) and dryer balls or clean sneakers to loosen clumps and restore loft.
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Fill ratio
The bags with the highest down ratio are the highest quality and are used in high mountain materials. However, they are also the most expensive. For example, a 90/10 sack refers to a filling that contains 90% down and 10% feather, making it a high-quality sack. In the case of a 50/50 sack, the down and feather ratio is identical, and the quality is lower.
Synthetic sleeping bags are a good alternative to down sleeping bags. They are non-allergenic, provide some warmth when wet, and are budget-friendly. However, there is a huge variation in quality in the synthetic sleeping bag market. As a general rule, you get what you pay for. A good-quality synthetic sleeping bag will also be compact and last longer than an entry-level bag.
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Frequently asked questions
Fill weight measures the amount of down in a sleeping bag. It is a good indicator of how warm a sleeping bag will be.
Sleeping bag fill insulates you from the cold by trapping body heat and preventing the exchange of warm and cold air. The higher the fill weight, the more down is in the bag, and the warmer it will be.
Fill power measures the quality of down, whereas fill weight measures the amount of down in a bag. A sleeping bag with a higher fill power will be more fluffy and trap more air, increasing its warmth.
The right fill weight depends on your specific needs. If you are backpacking, weight and volume are important, and you should consider a synthetic bag. Synthetic bags are also a good choice if you are on a budget or plan to sleep in wet conditions. If you are car camping, you can prioritize warmth and comfort over weight and volume.
