An Interview with Therm-a-Rest's Quilt Team
How much can you trust the temperature ratings that gear manufacturers give their backpacking quilts when there isn't any standardized way of measuring them, like there is with sleeping bags? Why do some people sleep colder in quits than others? What are some of the other variables that can affect your comfort level and warmth when you sleep with a quilt? These are important questions for backpackers who want to compare quilts from different manufacturers or want to improve their sleep systems so they can be more comfortable at night.
I recently spoke with Kyle Thackray and Greg Dean, both from Therm-a-Rest, about how they assign temperature ratings to their backpacking quilts, and the other aspects of quilt sleep systems that have the greatest impact on the user warmth. Therm-a-Rest is a engineering-driven company and they take a very scientific approach to this problem which I think you'll find instructive.
Here's a summary of our conversation. The questions were mine, but I've paraphrased their responses below for readability. Any errors in translation are my own.
There aren't any industry standards for measuring the temperature ratings of backpacking quilts, like there are with sleeping bags. How does Therm-a-Rest assign temperature ratings to their quilts and how reliable are they?
We test our quilts the same way we test our sleeping bags, by sending them to the testing lab at Kansas State University. They use the same test setup defined by the sleeping bag temperature rating standard to test our quilts, using a manikin dressed in long underwear and lying on a sleeping pad with an R-value=4. Since quilts have mummy hoods, we put an insulated hood on the manikin, because we assume that users will wear an insulated hat or hood when they sleep with a quilt. We've found that wearing a down hood will raise a quilt's temperature rating by 4 degrees, while wearing a fleece hat raises it by 2 degrees.
Are there any standards efforts under way or in discussion around standardizing the way that quilt temperature ratings are measured?
Not that we are aware of. Part of the reason is that most backpacking quilts are made by small companies that don't have the where-with-all to invest in a standards definition process. But we expect that a standard will evolve as larger companies and retailers, including REI, start making and selling more quilts.
It's well understood that men and women need different amounts of sleep insulation. How are gender differences reflected in the temperature ratings that Therm-a-Rest assigns their quilts?
We view it more as a function of body size than gender. The most important thing for men and women is to have a quilt that fits you properly. You don't want one that is too long or too narrow, so that the edges lift up and let in drafts when you roll around at night. We've found that the regular sized quilts sold by many of the cottage quilt brands aren't wide enough for most people, so we've significantly increased the width of our quilts, including the new Vespers.
Can back sleepers and side sleepers expect the same kind degree of warmth from a quilt or will one sleep colder than the other based on their sleep position?
As long as there are no side drafts and the quilt is tucked up around you, there shouldn't be any difference.
What are the most important features besides the quantity of insulation in a quilt that can affect its temperature rating?
Fit is the most important. You don't want a quilt that's too long for you. The pad attachment system is important to eliminate drafts from entering. Perimeter draft tubes are also effective, as well as draft collars around the upper chest and neck that can be fully closed with a snap or a draw cord.
What about baffle construction?
Box baffles are the warmest and that's what we use on the Therm-a-Rest Vesper Quilts (32 and 20), which are fully box baffled. There will always be cold spots with baffles that are sewn-through. We use a thermal imaging camera to study different design options and find where heat is escaping (See top photo).
Are Therm-a-Rest quilts designed for ground sleepers or can they also be used in hammocks.
They're primarily for ground sleepers, but the pad attachment system is removable, so there's no reason they can't be used in a hammock.
Is there a formula for estimating how warm you can expect to sleep by layering two quilts together. For example, if you stack a 40 degree quilt over a 20 degree quilt, what is the lowest external temperature you can expect to be comfortable in?
Sort of, but it's very hard to generalize because it depends on the specific quilts and how their insulation is distributed. It's also based on clo values and not temperature ratings, which makes it hard to translate for general use.
What is more effective in term of blocking drafts, a pad attachment system or an ultralight bivy sack?
A bivy sack would be better at blocking drafts, as long as you don't shift off your sleeping pad at night. Of course a bivy sack is also heavier, whereas the Synergy link system that comes with our Vesper quilt only weighs 3 grams.
At what temperature would you switch from a quilt to a mummy sleeping bag, in colder weather.
For me (Kyle) personally, I switch when it drops below 20 degrees (Fahrenheit). We also find that it gets increasingly difficult to convince people that they'll stay warmer in a quilt below 20 degrees, no matter how good the pad attachment system is. But a lot also depends on the kind of sleeper you are. If you move around a lot at night, there's a higher likelihood that you'll experience drafts. When it's colder than 20 degrees outside, drafts have a much higher impact on your comfort.
Philip: Thanks guys. I appreciate the background information and think my readers will find it informative.
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