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What is an R-value?

The R-value is a measure used to rate how well your sleeping mat will stop heat loss. Thermal resistance is the measurement that indicates a materials ability to insulate - there are a number of different ways to express this but, as they all measure the same factor, they are all related. The starting point is the R-value.

In the past, some brands quoted the R-value in SI units, others in US units and some using a TOG rating. Additionally, there was no standard method for testing so it was hard to make accurate comparisons between brands.

In real terms, the R-value tells you how well your sleeping mat will help to keep you warm as part of your sleep system.

New R-value Standard

From 2020, a new industry standard has been adopted by most brands for the measurement of R-value, developed by ASTM International – the ASTM FF3340-18 R-value standard.

This new standard has defined how the majority sleeping mats on the market will have their R-values measured and quoted in the future.

Some brands didn’t quote R-values in the past. Instead, they quoted a recommended temperature range but now almost all brands be working to the ASTM F3340-18 R-value. All measured in the same way allowing direct comparison for the first time. This standardised R-value is more like the EN rating on sleeping bags, giving more confidence when selecting such a key item of your backpacking kit.

What temperature does the ASTM F3340-18 relate to?

Simply put, the higher the R-value figure, the more insulating and therefore warm, that the mat will be.

Every person sleeps differently, so it’s impossible to give exact temperature estimates. Additionally, the conditions such as ground temperature, the shelter you are using and other factors can have an impact on the performance of the mat.

There are also other personal factors to consider including physiological differences (women tend to sleep colder at night than men), tiredness, sleeping bag selection, diet etc all influence how warm the sleeper will feel.

As a rule of thumb mats can be broken down into 3 categories – Summer, 3 Season and Winter:

• Summer – roughly ASTM F3340-18 R-value 0.1 to 2.0 - mats aimed at temperatures no lower than 5C. These mats will tend to be air mattresses with little to no insulation inside them.
• 3 Season – R-value 2.0 to 4.5 – mats containing moderate levels of insulation and suitable for typical temperature ranges of +10C to -10C.
• Winter – ASTM R-value 4.5 + (the colder the temperature the higher the R-value you will require) - suitable for use in Winter temperatures of -10C and below (often combined with a Closed Cell Foam mat in extreme temperatures).

These are just rough guides so we would recommend speaking to a member of our team if you would like help choosing a mat appropriate for your activities.

Sea To Summit have produced an excellent guide to the new R-value standard which is shown below.