R-Value: The Number That Matters More Than Thickness
If your sleeping bag keeps your top side warm but your underside is cold, the problem is not the bag -- it is your sleeping pad. Heat loss to the ground occurs through conduction, and compressed sleeping bag insulation (the part you are lying on) provides almost zero resistance to that heat loss. You are essentially lying on a thin fabric layer between your body and the cold ground.
R-value measures thermal resistance. The higher the number, the more resistance to heat flow. Under the ASTM F3340 standard (adopted in 2018 and now used by Therm-a-Rest, Sea to Summit, NEMO, Big Agnes, and Exped), all sleeping pads are tested using the same methodology, making R-values directly comparable across brands.
What the numbers mean in practice:
| R-Value | Best For | Example Pads |
|---|---|---|
| 1.0-2.0 | Summer only (50F+/10C+) | Therm-a-Rest Z Lite (R-2.0), NEMO Switchback (R-2.0) |
| 2.0-3.5 | Three-season (30-50F/-1 to 10C) | Sea to Summit Ultralight Insulated (R-3.1), Big Agnes Rapide SL (R-4.2) |
| 3.5-5.0 | Cold three-season / light winter (10-30F/-12 to -1C) | Therm-a-Rest NeoAir XTherm (R-7.3), NEMO Tensor Alpine (R-4.8) |
| 5.0-7.0+ | Winter / expedition (below 10F/-12C) | Therm-a-Rest NeoAir XTherm NXT (R-7.3), Exped DownMat (R-7.0-8.0) |
The ASTM F3340 standard tests pads on a heated plate in a controlled chamber. The pad is placed between the heated plate and a cold plate above it. Sensors measure the heat flow through the pad at a stable temperature differential, and the R-value is calculated directly. This replaces the older, inconsistent manufacturer-specific testing that made cross-brand comparisons unreliable.
Self-Inflating Pad Storage: The Critical Detail
Self-inflating pads (Therm-a-Rest ProLite, Sea to Summit Comfort Deluxe) contain open-cell foam that expands when the valve is opened, drawing air in. The foam is the insulation and the inflation mechanism.
The storage rule that most people get wrong: Store self-inflating pads with the valve open and the pad laid flat or loosely rolled, partially inflated. Never store them compressed in their stuff sack with the valve closed.
Why: If stored compressed for extended periods, the open-cell foam loses its ability to re-expand. The foam cells develop a "memory" of being compressed and will not fully re-inflate when the valve is opened. A pad that has been stored tightly rolled for a year will come out of the stuff sack half its normal thickness and may never recover.
The correct procedure: At home, unroll the pad, open the valve fully, and let it self-inflate. Once partially inflated, close the valve. Store the pad under a bed or leaning against a closet wall. Some people store them standing upright against a wall with the valve open at the top -- this is fine. The key is that the foam is not being compressed.
Temperature considerations for storage: Self-inflating pads should not be stored in uninsulated garages or attics where temperatures swing dramatically. The foam and the adhesives that bond the layers together do not handle freeze-thaw cycles well. Delamination -- where the foam separates from the shell fabric -- is accelerated by temperature cycling.
Air Pad (Non-Self-Inflating) Care
Pads like the Therm-a-Rest NeoAir XLite, Sea to Summit Ether Light XT, and NEMO Tensor are air-construction pads -- they inflate by mouth or pump sack and have no internal foam. These pads are more delicate than self-inflating pads but also simpler to maintain because there is no foam to degrade.
Inflation best practice: Use a pump sack whenever possible rather than your breath. Your breath contains moisture, and that moisture condenses inside the pad. Over months, the accumulated moisture can lead to mold growth inside the pad (visible as dark spots through translucent fabric) and can degrade the reflective films used inside insulated air pads like the NeoAir series.
The Therm-a-Rest NeoAir pump sack, Sea to Summit Airstream pump sack, and Exped Schnozzel Pumpbag all double as stuff sacks. They add roughly 2 ounces but eliminate moisture from breath inflation. For pads costing $150-250, the investment in a pump sack is reasonable insurance against internal degradation.
Valve Maintenance
Sleeping pad valves are mechanical failure points. The flat valves used by Therm-a-Rest (WingLock) and Sea to Summit (multifunction valve) rely on silicone or rubber flappers that can degrade over time.
Cleaning a sticky valve: If the valve does not seal properly or air leaks out slowly, dirt or debris may be lodged in the valve mechanism. Open the valve fully and inspect the sealing surface. Clean it with a damp cloth or a cotton swab moistened with isopropyl alcohol. Do not use oil-based lubricants -- they attract dirt and can swell the silicone flapper.