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When buying a sleep system it is important to consider the R-rating, referring to the thermal resistance of the product(s) and how that compares to the conditions that one will use them in. Some people layer two or more sleeping mats/pads in order to use them at lower temperatures than they are rated for individually. Is the cumulative R-rating of a layered sleep system a simple addition of the R-ratings of the respective layers or is the correlation non-linear.

For example if I have a sleeping mat with an R-rating of 2 and a sleeping pad with an R-rating of 4, if I place the pad on top of the mat do I get an R-rating of 6 (as in 2+4) ?

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Yes, they should be additive in this situation and its pretty commonly done.

As long as the materials involved are dense solids in direct mutual contact,[9] R-values are additive; for example, the total R-value of an barrier composed of several layers of material is the sum of the R-values of the individual layers.

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Yes, the R-values of sleeping pads are additive, provided that there is no air gap between them.

Source

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