Sleeping pad R-values: the number that decides whether you sleep

R-value is thermal resistance: how much the mat slows heat leaving your body into the ground. The 63 pads here run from R 0.5 to R 8.5 with a median of 3.4. Higher is warmer, the scale is linear, and that last part is what makes it usable — R 6 lets half as much heat through as R 3, and the step from 2 to 3 is the same amount of insulation as the step from 7 to 8.

The figure comes from ASTM F3340, which puts the mat between a warm plate at roughly skin temperature and a cold plate, then measures how much power it takes to hold that difference. The less heat leaks through, the higher the number. It is a bench test on a machine, so it says nothing about comfort, durability or how the mat behaves when you roll over.

What F3340 changed is comparability. Before it, each brand measured R-value its own way and published numbers that could not be set against each other; a 4 from one company and a 4 from another were not the same claim. The standard gave everyone one method, the major brands adopted it, and pad ratings became something you can actually shop on. That is the entire reason this site records the figure at all.

Still air is a poor conductor of heat, which is the whole trick behind a sleeping bag: the fill does not insulate, it holds air still so the air can. Soil, rock and water conduct heat tens of times faster than still air, and the ground is in direct contact with you all night. It is also effectively infinite. It will keep taking heat until morning without warming up in any way you notice.

At the same time, the fill under your body is compressed to nothing by your own weight, so the bag has stopped working exactly where the losses are worst. This is why a warm bag on a thin mat is a cold night, and why sleeping bag temperature ratings are measured with a mat under the manikin. The rating assumes you brought one, whatever the bag is filled with: the down or synthetic argument does not change the physics underneath you.

• Wet ground is far worse than dry. Water conducts heat well and carries it away. A pitch that looks merely damp will take real warmth out of an under-insulated mat.
• Bare rock and hard-packed campsite pitches give you nothing. Grass, dry leaf litter and pine needles are worth a surprising amount, because they trap air.
• Snow is a cold reservoir you are lying on, but it can be shaped: level it, let it set, and it beats frozen rock.
• Wooden platforms and shelter floors feel civilised and insulate badly. Treat them as rock.
• The temperature drops after you pitch, and the ground lags behind the air. The coldest part of the night is usually the hour before dawn, on ground that has been losing heat since sunset.

Stack two mats and their R-values add, near enough to plan with: a closed-cell foam mat at R 2 under an air mat at R 3 gives you about R 5. That single fact is the cheapest way to buy winter warmth, and it is why the foam mats in this pool are worth more than their specification suggests.

It also buys redundancy. An air mat that punctures at midnight in February is a serious problem; an air mat that punctures over a foam mat is an uncomfortable night. Put the foam underneath, where it protects the air mat from thorns and grit, and where the one that cannot fail is the one touching the ground.

Those bands are convention, not measurement. ASTM F3340 defines how to measure an R-value; it says nothing about which one your night needs, and reasonable people put the boundaries in slightly different places. Where they agree is the direction to err: too much R-value costs you grams and euros, too little costs you the night. The finder asks for the lowest night temperature and the ground you expect to pitch on, and ranks all 63 pads on both.

The pool splits 46 air, 9 self-inflating and 8 foam. All three can be made warm; what separates them is weight, bulk, comfort and how they fail. Weights run from 60 g to 1,100 g with a median of 550 g, packed volumes from 1.8 l to 9 l with a median of 3.5 l, and prices from €30 to €280 with a median of €130.

One more difference worth planning for: air mats are noisier, both to you and to anyone sharing the tent, and they need five minutes of inflating at the end of a long day. Self-inflating mats are the quiet middle option and the reason a lot of people who camp from a car and walk from a trailhead own one of each.

Thickness here runs from 0.3 cm to 10 cm with a median of 7 cm. The thinnest entries are closed-cell foam, and the inflating pads start at 2.5 cm: read a foam mat as a mat rather than a mattress. Thickness is mostly a comfort figure and not a warmth figure — two mats with the same R-value are equally warm on the plate test, but only one of them keeps your hip off the ground.

That is where side sleeping changes the answer. On your back, your weight spreads across shoulders, buttocks and heels. On your side it concentrates on a hip and a shoulder, over a much smaller area and at much higher pressure, and you can press through a thin mat until you are touching the ground. When that happens you lose the comfort and the insulation at the same time, because a compressed mat at that point is no longer the mat you bought.

• Side sleepers: as a rule of thumb, look for 7 cm or more, which is the median of this pool and upwards. Under-inflate an air mat slightly so your hip sinks in rather than bouncing off.
• Back sleepers: 3 cm to 5 cm is usually enough, and the weight you save is real.
• Restless sleepers who move between the two want the thicker mat, because the side position sets the requirement.
• Regular lengths run 130 cm to 200 cm. The short ones are torso mats: you save weight and put your pack or spare clothes under your legs, which works better than it sounds and worse than a full-length mat in winter.
• The finder asks whether you sleep on your side or your back for exactly this reason — answer it and the thin mats drop down the list.