R-Value
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R-Value

Or – how Australian Product Developers and Swiss Laboratory Technicians are likely to put you to sleep.

A sleeping pad or sleeping mat has two primary functions: to provide a comfortable layer for you to sleep on, and to insulate you from the ground.

In summer, the differential between your temperature and the ground temperature is not something you will notice. By late Fall, however, the ground is getting pretty cold and may already have frozen in some latitudes/at higher altitudes. In winter, it will likely be colder still.

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Basic physics means that you and the planet are trying to even out each other’s temperature. Basic physics means that the planet will win – unless you put something between you and the ground which will slow down the transfer of precious warmth. You, therefore, need an insulating medium underneath you, one which provides adequate resistance to heat loss.

Fortunately, there is a way of determining resistance to heat loss; it is called an R-Value.

Sea to Summit makes every effort to provide an independent measurement for the performance of all of its products. For instance, the down in our sleeping bags is evaluated at the International Down and Feather Laboratory and a detailed analysis is provided to the consumer. The temperature rating of those sleeping bags is tested in independent laboratories according to the EN test protocol, and their compressed volume is calculated according to the ASTM standard.

So – when Sea to Summit wants to determine how well its sleeping mats will insulate you from the cold ground, it tests production versions of the mats at the EMPA laboratories in Switzerland.

The test method is fairly straightforward. In essence, heat is applied to a plate on one side of the mat, and measurements are taken via a plate on the other side of the mat to determine how much energy per given area is necessary to raise the temperature of the second plate by a given amount.

The result is a definitive measurement of the resistance to heat loss through the mats.

The laboratory testing is, as mentioned, straightforward (if expensive). Unlike the EN testing used to determine temperature ratings for sleeping bags, there is currently no internationally agreed-upon protocol for sleeping mat R-Value test conditions. Those conditions could include variations in ambient temperature or humidity, the inflated pressure of the mat being tested, and exactly where on the mat the plates are placed. In practice, however, the leading sleeping pad manufacturers all test to very similar protocols.

Some brands use the lack of a protocol as the rationalization for not testing their mats at all. They are simply estimating a temperature 'rating' for their products. It's a seductive idea: a 'temperature' is much easier for a consumer to understand than a number relating to heat retention.

However, there are several major flaws with sleeping pad temperature 'ratings'. It's unclear whether they are supposed to relate to air temperature or ground temperature. And there is absolutely no standard for testing them at all (which is ironic, because the justification for not using an R-Value is an apparent lack of a standard).

To make it easy for an end-user to determine which R-Value they will need, we’ve created the following table:

Season R-Value needed
Summer 0 – 1
Late Spring or Early Fall (possibility of a mild frost) 1 – 2
Early Spring or Late Fall (possibility of a hard frost) 3 – 4
Winter (frozen ground) 5 or above

You will note that we use frost or frozen ground rather than merely ‘temperature’ as reference points. This is because:

  • The temperature you are interested in is the ground temperature rather than the air temperature. The ground may well be several degrees colder than the air, and as we mentioned at the beginning, thermodynamics means that your warmth is being drawn down into the ground beneath you.
  • Using frost as a guideline includes the concept of humidity in the equation. Frost forms when moisture from the air freezes on the ground, and moist air conducts heat better than dry air. You don’t have to worry about the physics involved – if you have a Sea to Summit sleeping mat with an R-Value of 4 (and the right sleeping bag/liner, of course) you will sleep right through the hard frost forming and wake up to iciness outside your tent in the morning.

Of course, the R-Value isn’t the only thing you need to know when selecting the sleeping mat which is right for you. You can read about other factors and get some recommendations here.

As this article has hopefully shown, Sea to Summit’s efforts to provide clear, unbiased product evaluations frequently start with testing in an independent laboratory – but they don’t end there. If you have any questions on insulation in sleeping mats, you could always just…

Ask Baz.

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