Remember your first backpacking sleeping bag? I got mine from an REI Garage Sale when I was 15—I didn't know anything about sleeping bag temperature ratings at the time. Nor did I know that two years later REI began requiring every bag that they sold to adhere to the
EN 13537 sleeping bag temperature rating standard. That was in 2010.
I still have it that bag (I should probably
wash it). That mummy bag did it all. I’ve tossed it out on forest floors, friends’ floors, and airport floors. It went backpacking with me, went to college with me, it's been canoeing and kayaking, and quite honestly has seen more car camping trips than backcountry trips.
The reality for many of us is that we need a sleeping bag that delivers warmth and comfort when we backpack, yes, but also anytime we are sleeping outside or away from home. We need one reliable bag that is versatile enough to go anywhere and be comfortable, so when we get there we get a good night's sleep.
The
Boost 650 is
that bag. With WarmZip technology, it's a compact chameleon of a bag that can shift between a spacious semi-rectangular shape and a mummy. As it zips up to be a mummy bag, WarmZip boosts the bag's warmth by up to 10°F/6°C.
Given what WarmZip does, and how well it works, you may be inclined to look at the Boost and think of it as two bags in one: a semi-rectangular and a mummy; a 20°F/-6°C and a 10°F/-12°C.
Is the Boost 650 Two Sleeping Bags in One?
The short answer is no, particularly when it comes to temperature ratings. If you want to think of it as a both semi-rectangular and a mummy bag, have at it. But keep in mind, WarmZip is a two-way zipper, meaning you can go mummy in the torso and semi-rectangular in the foot box or vice-versa. It's both shapes and everything in between.
But the Boost 650 only has ONE official temperature rating.
This is important, as an /ISO sleeping bag temperature rating is an
independently verified standard that enables customers to objectively compare sleeping bags from different brands. Calling temperature data an EN/ISO rating when it was not determined by the official test is disingenuous, misleads customers and undermines the system. Not our style at all.
Yes, WarmZip can add
up to 10°F/6°C of warmth to the sleeping bag when it closes the bag into a mummy shape. We thoroughly tested this effect ourselves and are confident in that number. However, that test methodology is not included in the very specific procedures of the
official EN/ISO temperature rating standard, nor was it tested by an independent group.
Therefore, the data we collected on the WarmZip effect cannot be represented as an EN/ISO rating, and we want to be 100% transparent about that.
To understand why, we’ll take a closer look at what WarmZip is and what it does, and we’ll take a semi-technical dive into the EN/ISO temperature rating test. In the end, you’ll understand how WarmZip adds warmth to a sleeping bag and why it can’t be tested by the current EN/ISO test.
We’ll be getting into some details, data and technical concepts here. Read until the end and we promise you’ll have a more holistic understanding of sleeping bag warmth and temperature ratings, and become a more knowledgeable camper.
The WarmZip Effect
Sleeping Bag Shape & Thermal Efficiency
WarmZip helps you regulate the Boost 650 bag's warmth by giving you the ability to manipulate its shape and the distribution of its
insulation. Changing the sleeping bag’s shape changes its thermal efficiency, or its ability to retain body heat.
Mummy bags sleep warmer than semi-rectangular bags because they are more thermally efficient. The narrower profile of the mummy shape holds the insulation closer to your body resulting in less internal space (or ‘dead air’) that needs to be heated.
Testing WarmZip
We exhaustively tested this action in our
cold chamber, using the same equipment the
independent test sites use for EN/ISO testing. Our testing measured a meaningful and consistent improvement in thermal retention when the Boost transitioned from a semi-rectangular to a mummy shape with WarmZip.
The graph below shows the data collected for both the Boost 20°F/-6°C and 32°F/0°C averaged together. The y-axis indicates whether the WarmZip is zipped (mummy) or unzipped (semi-rectangular). The x-axis indicates the
heat flux of the sensor-laden test manikin (expressed as watts per square meter), or the energy output required to remain 93.2°F/4°C (roughly skin temperature) over the course of the test.
With WarmZip unzipped, or open, the test manikin had to put out more energy to stay at skin temperature than it did when the WarmZip was closed, meaning the bag retained warmth better in its mummy shape.
At this point, you might be wondering, “If they used the same equipment, then why can’t the EN/ISO test measure WarmZip for both its opened and closed ratings?” We're glad you asked.
EN/ISO Testing
We wrote a previous blog post to help you
understand EN/ISO sleeping bag temperature ratings, but we’ll review some more technical aspects of the testing process here.
Getting into the technical details shows us precisely what the test is measuring, how it goes about doing it, and why it does it in a very specific way to generate an accurate and reliable temperature rating.
Measuring Sleeping Bag Warmth
A sleeping bag’s warmth is a function of its
thermal resistance or
R-value,
just like a sleeping pad. The air inside of the bag is heated by your body.
Per physics, the warm space inside and the colder space outside the bag will seek to establish thermal equilibrium with one another. The sleeping bag is a barrier to that heat exchange. That’s how it keeps you warm.
There is an established equation that converts the sleeping bag’s R-value to a temperature rating. Determining a sleeping bag’s R-value involves testing its ability to insulate a body over time. This is done with an array of tools and equipment. Some of it is highly engineered and expensive, while some is normal stuff the average camper has. All of it is put to use in a very thorough and rigid test methodology.
The Test Equipment
The Manikin: A high-tech manikin is used to simulate a real user in the sleeping bag. Our manikin is named Hugh. He was made by
Thermetrics and cost us about $200,000 when we bought him over 2 decades ago. We don’t know if other manikins have names, but we hope they do.
What does a $200,000 manikin do? Well, it can measure the ambient conditions of the room, measure its own "skin" temperature in 16 different locations, heat itself using internal heaters, and measure the power output of those heaters.
The Cold Chamber: The cold chamber is the small room in which the test is conducted. It is climate-controlled and can be precisely held at a specific temperature and humidity level for the duration of the test.
Other Equipment
- A 20mm-thick wooden board that is at least 100mm off the floor to allow air circulation underneath.
- A foam pad with an R-value of 4.0.
- Top and bottom base layers, socks, and a cold-protective mask for the manikin.
Conducting the Test
The manikin is dressed in the mask, socks and base layers and put into the sleeping bag, which lies atop the 4.0 R-value sleeping pad, all of which is set up on the “artificial ground” wood board.
The manikin is fully zipped up in the bag with the hood cinched as much as possible around its head. (Yes, there are diameter specifications for how much the hood should close.)
The temperature of a human’s skin surface is typically between
33.5 and 36.9°C or 92.3 and 98.4°F (it varies depending on location). To mimic a real sleeper, the manikin is programmed to hold its temperature at 34°C/93.2°F. The temperature of the room is set at 10°C/50°F.
The power consumption required to keep the manikin at its programmed temperature is recorded. This is done three times per sleeping bag to validate consistency. Each round lasts several hours.
Calculating a Temperature Rating
Dividing the amount of power needed to heat the manikin by its surface area provides the
heat flux value, which is expressed as
watts per square meter (W/m
2). A higher heat flux value correlates to a higher sleeping bag temperature rating. The more power that's needed to keep the manikin warm, the less the sleeping bag is insulating.
With the known starting temperature difference of the manikin and the air, and the amount of energy required to maintain that difference, the thermal resistance of the sleeping bag can be calculated into an R-value.
Going from an R-value to a temperature rating that uses the Fahrenheit and Celsius scales involves another, more complex, equation. This equation uses physiological data standards to take into account heat generated by the user, and heat loss due to physical contact, breathing and sweating.
Three different calculations are performed, one each to determine the Comfort Range, Transition Range and Risk Range.
The WarmZip & EN/ISO Conundrum
Steady-State Systems
The ISO standard states that the test is designed to assess the “performance in steady-state conditions of a sleeping bag with regard to the protection against cold.”
Steady-state conditions. In systems theory, a system is in a
steady state when the variables present do not change over time—they no longer evolve, they are constant.
In our case, the EN/ISO test environment is the system. The key variables are the temperature of the manikin and the ambient temperature in the cold chamber. The data that determines your sleeping bag’s temperature rating is collected from a steady state.
You’re in your sleeping bag, in your tent, 10 miles from the trailhead. Are you in a
steady-state system? Absolutely not.
The Real World ‘System’
For one, the climate conditions (temperature, humidity, wind speed) are not constant throughout the night. Secondly, your body temperature and the thermal conditions within your sleeping bag vary throughout the night.
Merely moving in your sleep impacts the thermal conditions in your bag. When you move, you move into air space that is not as warm and feels colder. You also open new air space, causing your sleeping bag to act like a bellows and draw outside air in through the hood to fill up the new space, lowering the temperature inside. In a semi-rectangular bag, there is more space inside than in a mummy-style bag, resulting in more air being transferred in when you move.
This is a huge reason why semi-rectangular bags ‘sleep colder.’ Say you have a semi-rectangular bag and a mummy bag with the same EN/ISO temperature rating. The semi-rectangular bag sleeps colder because of its greater air transfer when you move around. The manikin didn’t move around in the test and that’s why the two bags may have the same rating, but the mummy bag will feel warmer in the real world.
The EN/ISO test deliberately needs to measure sleeping bag warmth in ‘steady-state’ conditions (explained in a moment). As a result, it is unable to measure variables that are not fixed. WarmZip, by design, is not a ‘steady-state’ feature. Its effect cannot be measured by the EN/ISO test and therefore the data showing the added warmth as a result of WarmZip cannot be labeled as an EN/ISO temperature rating.
Limitations of the EN/ISO Test
Don’t worry, the EN/ISO standard fully recognizes its own limitations. From page 19 of the ISO standard:
The insulation of a sleeping bag varies widely with the conditions of use (wind, radiative ambience, posture and clothing of the sleeping bag user, ground insulation, eventual humidity in the sleeping bag etc.). The perception of cold of the user is also individually different (influence of acclimatization, physical and psychological state, food etc.).
Broadly put, there is a limit to how accurately the test can replicate real-world conditions. We already talked about moving around in one’s sleep; another example is how each temperature range is calculated with a presumed physiological “standard” of a man or woman. Here’s how they break down:
| |
Age |
Weight |
Height |
Body Surface Area |
| Standard Woman |
25 |
60 kg (132 lbs) |
1.6 m (5’3”) |
1.62 m2
|
| Standard Man |
25 |
70 kg (154 lbs) |
1.73 m (5’8”) |
1.83 m2
|
Looking at that data, it’s pretty clear most of us are not the so-called ‘standard.’ Different body types have different metabolic functioning levels, i.e. the amount of heat they create varies. According to the ISO standard, “while sleeping, a 50 kg person produces around 60 W heat, while a 110 kg person produces about 100 W”.
We hardly need data to tell us that some people sleep cold while others are heaters at night. But the test requires a standard and fixed human model, and the test needs to be a steady-state analysis.
The goal of the test is to provide a baseline for customers to make reliable one-to-one comparisons of sleeping bags from different manufacturers. Each unique design must be tested in identical conditions for an objective standard to be established.
Interpreting EN/ISO Temperature Ratings & Why They’re Useful
The EN/ISO standard calls for sleeping bags to be labeled with a graph showing three temperature ranges. By compiling its findings into three ranges, the EN/ISO test makes the ratings more useful for interpreting a sleeping bag's warmth against varying real-world conditions.
Here are the definitions of each range:
Comfort Range: The temperature range where a ‘standard woman’ is not feeling cold (no shivering) in a relaxed posture.
Transition Range: The temperature range a ‘standard man’ is fighting against cold (aka posture is curled up inside the sleeping bag) but is in thermal equilibrium and not feeling cold (no shivering).
Risk Range: The temperature range a ‘standard woman’ is in a situation of high cold stress, is curled up inside the sleeping bag and shivering to increase basic heat production. Risk of health damage by hypothermia is possible.
The Value of EN/ISO Temperature Ratings
It's important to note that EN and ISO tests are entirely
optional for manufacturers. There are brands that sell non-EN/ISO-tested sleeping bags.
While it’s unlikely that non-EN/ISO temperature ratings are dishonest and not informative, you have to take that brand’s word for it. An EN/ISO rating lets you know that a bag’s temperature rating has been established by an independent and impartial organization (independent testing is a requirement for a true EN/ISO rating). It adds transparency to temperature ratings and helps eliminate disingenuous marketing.
The EN/ISO sleeping bag temperature rating system enables consumers to
reliably compare various bags one-to-one. A 0°F/-18°C rating for a sleeping bag from Brand A is the same as the 0°F/-18°C rating from Brand B. Always.
Most brands today use the EN/ISO tests. In the USA at least, again, credit is due to REI for making that happen. Previously, it was a wild west of warmth ratings. Folks would sleep in a 20°F/-6°C bag from one brand and then try out a 20°F/-6°C from another and have wildly different experiences. REI clearly listened and made the right call for consumers.
Final Thoughts
The Boost 650 sleeping bag is all about making you comfortable. Whether you are using it as a backpacking sleeping bag miles from the trailhead or rolling it out in the back of your car, thanks to WarmZip, the Boost will deliver a good night’s sleep in a variety of conditions.
Sleep comfort is subjective, and no temperature rating test can objectively capture what’s comfortable. But there is at least one universal sleep comfort truth: The Goldilocks Principle. Not too hot, not too cold, just right! With the Boost 650 and WarmZip, being just right is within your (literal) reach.
If you have any questions about WarmZip, the Boost 650, sleeping bag temperature ratings or about sleeping comfortably under the stars, don't hesitate to leave a comment, hit up our social channels, or reach out to our customer service team! Happy sleeping!
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