Sleeping Pad R-Value: What ASTM F3340 Measures and What It Does Not

R-value tells you how well a sleeping pad resists heat leaving your body and going into the ground, and since 2020 it has been measured the same way by every brand that bothers. The method is ASTM F3340, the Standard Test Method for Thermal Resistance of Camping Mattresses Using a Guarded Hot Plate Apparatus. Higher is warmer. That is the whole useful summary, and everything else on this page is about the edges of it, because the edges are where people get cold.

Most campers get their sleeping bag right and their pad wrong. You lose heat downward by conduction into ground that is often colder than the air, and a sleeping bag cannot help you there because your weight crushes the insulation underneath you flat. The pad is what stands between you and the dirt. If you have been cold at night in a bag rated well below the temperature, the pad is the first thing to check.

What the test actually does

Mountain Equipment’s explanation of the standard describes the apparatus clearly: the mat is held under pressure between an upper hot plate representing a person lying on top and a cold plate underneath representing cold ground, and the rig measures the energy required to hold the hot plate at a steady temperature. From that, the R-value is calculated.

Sea to Summit, which states it has used ASTM F3340-18 since January 1, 2020, notes that the protocol pinned down things that used to vary between brands, including the ambient temperature and the internal mat pressure during the test. That is the point of the whole exercise. Before 2020, every brand tested its own way and the numbers were not comparable across companies. NEMO has published an account of helping develop the standard for that reason.

We did not verify the specific plate temperatures, applied pressure, or test duration from a source we could fetch, so we are not publishing those numbers. The ASTM standard itself sits behind a paywall and their site refused our request. If you need the exact parameters, buy the standard from ASTM.

What the number does not include

This is the part that matters in the field. ASTM F3340 measures steady-state conductive resistance, on a new mat, in a lab, in a controlled setup. Mountain Equipment lists the exclusions directly: the test does not measure how comfortable the mat is, how durable it is, whether it is easy to inflate, or whether it is pleasant to sleep on. It gives a measure of warmth with the mat in an ideal position and does not account for compression of the mat or sleeping posture. And it tests new mats, which may perform differently after a few weeks of use.

Add the things no lab test can carry:

• Wind. Convection under and around a pad in an open bivy or a drafty tent is not in the number.
• Wet. A pad sitting in standing water in a tent, or a condensation-soaked underside, loses heat faster than the test predicts.
• Ground. Frozen gravel, a rock slab, and dry duff are not the same cold plate, and the test uses one temperature.
• You. Two people on identical pads in identical bags sleep at different temperatures. NEMO puts this plainly: R-value is an insulation factor that never changes and remains independent of environmental factors and personal sleep temperature.

So the R-value is a real, comparable, useful spec, and it is a spec about the pad, not a promise about the night.

There is no R-value-to-temperature chart worth trusting

You will find plenty of charts online mapping R-values to degrees. None of them come from the standard. ASTM F3340 produces a thermal resistance figure and stops there, and the brands that helped write it do not publish a temperature map either. NEMO’s own guidance is deliberately relative: the lower the temperature, choose a higher R-value.

We are not going to invent the table. Reason from your own experience instead, which is more reliable anyway:

1. Note what pad you were on the last night you were cold from below, and roughly what the overnight low was.
2. If you were cold, go up. Doubling the R-value is a large step, not a marginal one.
3. Change one variable at a time. If you swap the pad and the bag at once, you learn nothing.
4. Assume the ground gets colder than the forecast air temperature, especially in a valley bottom or on sand in the desert.

R-values add, and that is the cheapest upgrade you can make

Therm-a-Rest’s own guide states it directly: R-value is additive, so if you stack two pads you can add their R-values together for a higher overall rating. That single fact solves most cold-sleeping problems without buying a new expensive pad.

Their published figures make the arithmetic easy. The Z Lite SOL closed-cell foam pad is R-2.0, weighs 14 oz in the regular size, packs to 20 x 5 x 5.5 in, and is 0.75 in thick. The NeoAir XLite NXT is R-4.5. Put the Z Lite under the XLite and you have R-6.5, which is more than the R-value of any single pad in most people’s closets.

Therm-a-Rest also says do not stack two inflatable pads, because you will likely slide off overnight. That matches experience. A firm foam pad under an air pad is stable, adds warmth, doubles as a sit pad and a camp-chair cushion, and is the thing you sleep on when the air pad punctures at 1am. For a winter setup, see winter camping.

Picking one, with real numbers

Three published examples across the range, all from manufacturer spec pages:

Pad	R-value	Thickness	Weight
Therm-a-Rest Z Lite Sol (regular)	2.0	0.75 in	14 oz
NEMO Tensor All-Season	5.4	3.5 in	not published on the product page
Exped MegaMat (MW)	8.1 (labeled “R-Value (ASTM)“)	10 cm	2070 g

The pattern is the tradeoff. The MegaMat is the warmest of the three by a wide margin and weighs about 4.6 lb, packing to 67 x 20 cm. That is a superb pad for sleeping in a vehicle or a basecamp tent and an absurd pad to carry on your back. The Z Lite is a quarter of an R-value class and weighs 14 oz. NEMO does not publish weight or packed size on the Tensor All-Season product page we checked, so we are not going to fill that cell from a review site.

Do not buy the thickest, warmest pad by default. A 10 cm pad raises you well off the floor of a small tent and eats space in a vehicle build. And do not buy a high R-value expecting it to compensate for a summer-weight bag on a cold night: the pad handles the ground, the bag handles the air, and neither covers for the other.

If you sleep in the vehicle rather than a tent, the platform underneath matters as much as the pad, because a hard flat deck changes how much cushioning you need and where condensation collects. See building a platform and sleeping in your car at rest areas.

The one buying check that saves you

Before you compare two R-values, confirm both brands say which test produced them. Exped labels the spec “R-Value (ASTM)” on its own product page. Sea to Summit and NEMO both publish their use of the standard. If a brand prints a bare number with no method named anywhere on the site, you do not have a comparable spec, you have a marketing claim, and the honest move is to treat it as unverified.