What is R-value?
Technically, R-value is a measure of thermal resistance; the higher the R-value, the more thermally resistant the material or structure is. This isn’t just used to measure sleeping pads. Engineers and scientists also use R-value to measure everything from windows to fiberglass. Our team believes that it’s also the best way to measure the insulation of our pads.
How Do We Measure R-value in Sleeping Pads?
We measure every pad design in our on-site cold chamber, kept at a cool 4C (39F). Inside this thermally-sealed container, a sleeping pad is placed between two metal plates. One section of the bottom plate has been turned into a large sensor and is kept at a steady temperature with a measured electrical current. A pad that provides a lot of insulation will help keep the bottom plate warm, so the sensor will require less electricity (energy) to maintain its temperature. A pad with less insulation will require more energy. The amount of energy required to maintain the plate’s temperature plugs into an equation to provide an R-value for each pad. The less energy required to keep the plate warm, the higher the R-value and vice versa.
Why does Sleeping Pad R-Value Matter?
If you were to roll out your sleeping bag and laying directly on the ground, you would quickly get cold due to conductive heat loss between your warm body and the cold ground. The compressed down would fail to create an insulative layer between the two. By selecting an appropriately insulated sleeping pad, you’ll stay warm throughout the night, and wake up prepared for the trail. This process is what makes a sleeping pad so important.
How to Use it When Choosing a Sleeping Pad
R-value can be very informative while building your sleep system for a specific adventure. Let’s say you’re going into the High Sierra in September. You’ll be expecting brisk nights, but there’s real potential for frigid ones too. Do you bring the giant 0F winter sleeping bag to be safe or your three-season bag and risk a few cold nights? Most people will make a decision based on sleeping bag warmth alone, but by factoring in sleeping pad warmth, you suddenly have more options. For instance, on a NeoAir® XLite® (R-value:4.2, Weight: 12 oz.), you should get the comfort your bag is rated for. However, jump up to NeoAir® XTherm™; (R-value: 6.9, Weight: 15 oz.) and you could get by on those coldest nights with a greater comfort margin for only an extra 3 ounces – which is much lighter than hauling that heavy winter bag.
Where does the term "R-Value" come from?
ASTM F3340-18 (American Society for Testing and Materials) is a new testing standard that provides R-value ratings for sleeping pads. While Therm-a-Rest has always used R-value to rate our sleeping pads, this new standardization will allow campers to make “like-for-like” comparisons with our competitors pad.
For years, R-value–a measure of a material’s resistance to let heat through–has been the standard that most pad manufacturers have used to rate their warmth. However, with a little research, it became readily apparent to us (and lots of people in the field) that not everyone was determining R-value with the same test, and some companies were trying to use temperature ranges and other subjective measures to rate their pads. This resulted in some wild claims about sleeping pad warmth and a lot of confusion and disappointment with users in the backcountry–something that was bad for everyone involved.
To fix that problem, in 2016, Therm-a-Rest, along with a group of other sleeping pad manufacturers, began developing a standard methodology for rating the insulative properties of sleeping pads. that would become known as the ASTM F3340-18 standard. This standard allows campers to look at two or more sleeping pads and directly compare the R-value of each.