What is R-value?
Author: Danae Baggs Date Posted: 4 June 2021
You've probably seen an R-value listed on sleeping mats from reputable outdoor brands. But what does an R-value mean?
What does R-value mean?
R-value stands for resistance to heat flow value. In essence, a sleeping mat's R-value measures its ability to resist the cold. The higher the R-value, the better its thermal resistance. It is similar to the EN/ISO ratings for sleeping bags.
A sleeping mat has two main functions:
- to provide a soft, comfy layer between your sleeping bag and the hard, bumpy ground
- to prevent the cold ground from leeching away your body heat (conductive heat loss).
Ground temperatures can be much colder than air temperatures. Without a mat, your body would have to expend lots of energy to keep you warm enough throughout the night.
How is R-value measured?
Testing and calculations!
The sleeping mat is put between two metal plates: one hot and one cold. The sleeping mat must be inflated to 0.5 psi; the hot plate must be applied to 2pa pressure, and be set a 35C to mimic body heat.
This hot plate is kept at a steady 35C with a measured electrical current. Electronic sensors measure how much electricity (energy) it takes to keep the hot plate at this temperature. The less energy required to keep the plate warm, the higher the mat's R-value, because the more insulation it is providing.
How is R-value standarised across brands?
A new industry standard ASTM F3340-18 was introduced in late 2019, which standardised the testing conditions described above to achieve accurate and comparable R-value metrics across brands.
The ASTM standard isn't a legal requirement for brands, but most reputable outdoor brands have adopted the ASTM. Only they can list their R-values as ASTM standardised, so look out for the ASTM label on a mat's packaging and online product description.
If you've noticed that a sleeping mat you purchased before ASTM F3340-18 standardisation has a slightly different R-value published on the same model now, don't worry. They perform exactly the same.
What R-value do I need?
Remember: the higher a sleeping pad's R-value, the more insulating (warmer) it will be.
Here is a Sea To Summit's guide to choosing your mat's R-value:
Move up the scale to a higher R-value if:
- You're a woman (most women sleep a few degrees colder than men)
- Your campsite has lots of cold wind
- You know the area's ground temperatures are much lower than the air temperatures.
Considering a sleeping mat's R-value can help you to build a sleep system with a better warmth to weight ratio. Factor in both your sleeping bag and sleeping mat's warmth ratings when prepping for a trip: you might not need to bring your heaviest bag once you consider the warmth your mat will add!
Comparing sleeping mats
R-value is a very important and useful stat to look into when comparing sleeping mats.
We recommend comparing the following points to find your best match:
- Packed size
- Inflation method