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Ask WTA: How do Sleeping Bag Ratings Work?

Posted by cwakenshaw at Nov 11, 2021 03:42 PM |
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Few things are as comforting and important as a good sleeping bag when camping. But how are sleeping bags tested and what do temperature ratings really mean? Luckily Feathered Friends, a top manufacturer of down sleeping bags, is based right here in Washington, and they had some answers for us.

Few things are as comforting and important as a good sleeping bag when camping. But how are sleeping bags tested and what do temperature ratings really mean? Luckily Feathered Friends, a top manufacturer of down sleeping bags, is based right here in Washington, and they had some answers for us. We talked to Garrett Nixon, the chief production officer, who has worked at Feathered Friends for over 20 years and Juna Gates, the general manager whose parents founded the company back in 1972. Here’s what they had to say. 

A person in a sleeping bag sits up to look at the view
Photo by Grant Roush

WTA: What do temperature ratings really mean? Are they based on comfort or survival?

Juna:  Temperature ratings are pretty subjective. In general, the temperature rating should match the lower range where the “average” person would feel comfortable overnight. But what is the average person? Metabolism, body type, sleep system, environmental factors, hydration, even your suffer threshold - there are so many variables that can affect “comfort” that it's best used as a jumping off point. For comfort, we generally recommend that most people get a sleeping bag rated 10 to 20 degrees colder than the coldest temperatures they expect to encounter. 

WTA: How are temperature ratings established? Is there an industry standard or are temperature ratings established independently by each manufacturer?

Garrett: ISO (International Standards Organization) is the industry testing standard. That said, the test can be inconsistent between labs and fails to take into account a myriad of variables that can affect overall comfort. At Feathered Friends, we’ve been making sleeping bags for 50 years now so we tend to rely more on our decades of field testing and user feedback to determine a more realistic temperature rating.    

To get an ISO rating a heated mannequin is dressed in a base layer and placed in the sleeping bag on an insulated pad in a cold room. The temperature is lowered while data is gathered to determine the points at which the mannequin’s temperature stabilizes and then begins losing warmth. At some point the mannequin starts losing heat fast enough to determine the bag is no longer functional at that temperature. Results don't determine an absolute value but rather a range best expressed as “comfortable” to “extreme limit”.  

What temperature rating manufacturers assign along that scale is somewhat opaque. In reality, so many factors can affect “comfort rating” it's impossible to place an exact number on it.

WTA: What does a 3-season bag mean? 

Garrett: Primarily a summer bag that’s best for high altitudes and colder temperatures but warm enough to stretch from early spring into fall in most conditions. When the snow really starts to fall it's time to step up to a winter bag. 

WTA: What’s the difference between men’s sleeping bags and women’s sleeping bags? 

Garrett: In general, our women's bags are wider in the hips and taper faster toward the head, whereas men's bags have a wide point just below the shoulders, and they have a bit more fill overall with particular concentration in the foot area.  

WTA: What is the main difference between down and synthetic fill, and why is down so warm?

Garrett: The main difference is that down is a natural, biodegradable and highly efficient insulation material. It's warmth comes from the structure of the individual down plumes which are excellent at creating air space. 

WTA: What is the Responsible Down Standard and why do you follow it? 

Juna: The Responsible Down Standard (RDS) is a certification that aims to ensure down and feathers come from animals that have not been subjected to any unnecessary harm. We’ve always committed to sourcing the best materials available, and the increased transparency and third-party certification that the RDS brings make it easier for us to uphold that standard.  

WTA: Loft? Fill power? Fill weight? What do all these terms mean?

Garrett: Loft is a way of expressing or measuring the thickness of the insulation layer. The down clusters act as an insulating layer by trapping warm air. Larger down clusters (which will have a higher fill power) trap more warm air, making for a “puffier” and warmer sleeping bag. 

Fill power is a standardized measurement that refers to the size of a down cluster. It’s typically expressed as the volume, in cubic inches, that 1 ounce of down will occupy. Higher fill power equates to higher loft.

Fill weight refers to the total amount of down in a sleeping bag.  

WTA: Is a warmer bag always heavier? 

Garrett: Not necessarily. Factors such as fabric weight, insulation quality and design need to be considered but in general, all other things being equal, more insulation equates to greater warmth.

WTA: Any tips for maximizing the warmth of a sleeping bag? 

Juna: Start with a bag that fits well! Sleeping bags aren’t one-size-fits-all. Too much extra space inside your bag means you’re losing the thermal efficiency of a close fitting bag. Not enough space, which can happen when people get a bag that’s too narrow for their body in an attempt to shave some weight, can compress the down and create cold spots.    

Use a good sleeping pad. The exact type or shape doesn’t matter as much as having a good layer of insulation between you and the cold ground.   

Wear a dry, lightweight base layer to bed. Socks and a hat can help keep more heat in and are easy to kick off if you get too warm in the night. If it’s really cold, layer on fleece or a down jacket.   

Keep it clean. A buildup of body oil, dirt, and grime can impact the loft of your sleeping bag over time, which will negatively impact the warmth. People can be intimidated by washing down, but if done properly it’s actually pretty easy and will really extend the lifetime of your sleeping bag. This video walks through the steps of washing a down sleeping bag. 

WTA: What’s better, slipping into a sleeping bag to go to bed after a long day, or waking up in a warm sleeping bag to a beautiful view? 

Garrett: Waking up in a warm sleeping bag. Bags aren't that warm when you first get in. If I wake up warm that means I had a good night's sleep in the right bag. A good view is just an added bonus.

Juna: Getting in at the end of a long day. With two young kids I’m usually very ready to slip into my bag at the end of the day, and not as ready to get pulled out of my warm sleeping bag early in the morning. But a good view makes it easier!  

This article originally appeared in the Winter 2021 issue of Washington Trails magazine. Support trails as a member WTA to get your one-year subscription to the magazine.