Bivouac sleeping pad and R-value: physics of ground heat transfer and sleeping system sizing
The bivouac mattress is systematically presented as a comfort accessory. This is a framing error that leads to frequent sizing mistakes. Thermally, the mattress is as important as the sleeping bag itself. The reason is physical: the ground absorbs body heat through conduction at a rate 25 times faster than air. The underside of a sleeping bag, compressed under the body's weight, loses almost all of its insulation. Only the mattress provides insulation from the ground.
A practitioner who chooses their sleeping bag with care and then places their bag on a mattress with an insufficient R-value may find themselves feeling cold on a night that their certified bag should cover without issue. Understanding the R-value, knowing how to calculate it, and matching it correctly to the certified performance of one's bag is just as important as knowing how to read an EN 13537 comfort temperature. The interactive selection tool integrates both parameters into its recommendations. The range of bags available on the sleeping bag shop is presented with their certified temperatures to be compared against the recommendations in this guide.
The physics of heat transfer to the ground
Conduction vs. convection: why the ground is the true thermal enemy
Thermodynamics distinguishes between three modes of heat transfer: conduction (transfer via direct contact between solids), convection (transfer via movement of fluids), and radiation (transfer via electromagnetic waves). A sleeping bag primarily insulates against convection (cold air) and radiation (loss to a cold environment) via layers of air trapped in the down or synthetic fibers. The ground, however, transfers heat through direct conduction: a mechanism 20 to 25 times more efficient than convection for an elongated body.
The thermal conductivity of dry air is 0.024 W/m·K. That of compacted soil is 1.5 W/m·K. That of rock is 2 to 7 W/m·K depending on the nature of the rock. This means that a cm² of ground in direct contact with the body extracts 62 times more heat per second than a cm² of air. Even with a sleeping bag fabric interposed, the compression of the filling under the body's weight reduces its thickness from 6–8 cm to 0.5–1 cm, nullifying 85% to 95% of its insulation. The ground therefore acts as a negative radiator—a surface with high thermal extraction capacity—against which compressed down offers almost no protection.
Ground temperature in a bivouac
The surface temperature of the ground varies according to the season, altitude, type of substrate, and exposure. In summer at 2,000 m altitude in the Alps, the ground surface temperature at the end of the night is between 4 and 10°C, depending on exposure and substrate. Bare rock (granite, limestone) can drop to 1–3°C on windless nights, even in July. Loose soil remains more stable (8–12°C) because its moisture provides greater thermal inertia. In winter or during shoulder seasons, these values frequently drop below 0°C, and the ground can be frozen at the surface.
A sleeper lying without a mattress on granite at 5°C, with an ambient air temperature of 12°C, undergoes thermal extraction through the ground that is far greater than that from the ambient air, despite the similar temperature difference (5°C at the ground, 12°C in the air). It is the combined effect of the high conductivity of the rock and the compression of the filling that makes the ground so critical.
⚠️ A practitioner using a 0°C certified bag with an R-value 1.5 mattress may feel cold on a night with an actual temperature of 8°C. It is not the bag that is at fault: it is the sleeping bag-mattress system that is undersized. The 0°C certified bag was tested with an R-value 5.5 mattress in the EN 13537 protocol. Using an R1.5 mattress is equivalent to removing 4 units of R-value from the test system, which equates to several degrees less thermal capacity.
The R-value: definition, calculation, and standardization
Physical definition
The R-value (thermal resistance) expresses a material's resistance to the heat flow passing through it. It is defined as R = ΔT / q, where ΔT is the temperature difference between the two sides of the material (in Kelvin or °C) and q is the heat flow passing through (in W/m²). The SI unit is m²·K/W. The higher the R-value, the more the material resists thermal transfer.
For a camping mattress, the R-value measures the mattress's ability to block heat transfer between the sleeper's body (approximately 37°C at the contact surface) and the ground (variable). An R3 mattress blocks this transfer 3 times more effectively than an R1 mattress. In practice, each additional point of R-value allows the sleeper to maintain thermal comfort on nights approximately 3 to 5°C colder, all other things being equal.
The ASTM F3340 standard: unification of measurement
Until 2020, each manufacturer measured the R-value of its mattresses according to its own protocol, making comparisons impossible. A Therm-a-Rest R4 was not comparable to a Sea to Summit R4 because the measurement protocols differed. The ASTM F3340-19 standard (Test Method for Thermal Resistance of Camping Mattresses) standardized the measurement in 2020: warm temperature 35°C, cold temperature 5°C, precise stabilization duration, and standardized calculation of thermal resistance.
Reputable manufacturers (Therm-a-Rest, Sea to Summit, Nemo, Exped) have adopted this standard and now indicate an ASTM F3340 R-value on their product sheets. Manufacturers that do not indicate the reference standard used or that provide an R-value without a protocol are suspect: their values are likely measured under advantageous conditions (less extreme temperatures, reduced stabilization durations) that artificially inflate the result.
💡 The R-value of a mattress is not additive in a perfectly linear way. Superimposing an R2 mattress and an R3 mattress does not result in exactly R5: thermal bridges at the edges, air gaps between the mattresses, and material differences introduce corrections. In practice, stacking provides 80% to 90% of the theoretical sum, i.e., R4 to R4.5 for an R2+R3 stack.
The R-value in the EN 13537 protocol
The EN 13537 (and the supplemental ISO 23537) thermal testing protocol for sleeping bags uses a reference mattress with a specified thermal resistance of 0.688 m²·K/W, which is approximately R3.9 in imperial units or R5.5 if using the ISO convention. It is against this reference mattress that a bag's comfort, limit, and extreme temperatures are measured. Using a mattress whose R-value is below this reference threshold is equivalent to testing the bag in conditions more unfavorable than those of the test: actual performance will be lower than the certified values.
Concretely, if a bag is certified for 0°C comfort with an R5.5 reference mattress, using it with an R2 mattress lowers its effective comfort performance by 5 to 10°C depending on the conditions. The same bag may behave like a bag with 5–10°C actual comfort with this undersized mattress.
Types of mattresses and their typical R-values
Closed-cell EVA foam
Closed-cell EVA foam is the simplest, most robust, and least expensive type of mattress. Its closed-cell structure traps air in isolated bubbles that cannot compress under body pressure (unlike inflatable mattresses, where insulation depends on the thickness of air maintained). A 1 cm thick EVA mattress offers an R-value of 1 to 1.5. A 2 cm thick model reaches R2 to R3. The market reference model for closed-cell foam (Therm-a-Rest Z-Lite Sol, 51 × 183 cm) weighs 410 grams for a certified R-value of 2.0.
The advantages of closed-cell foam are absolute robustness (no possible punctures), durability (15 to 20 years), price (20 to 60 euros), and the possibility of use as seating during breaks. The disadvantages are the incompressible volume (the foam folds accordion-style but does not roll into a small volume) and the limited R-value compared to inflatable mattresses of the same weight.
The self-inflating mattress
The self-inflating mattress combines an open-cell foam core (which expands spontaneously upon opening the valve) and a waterproof nylon shell. Open-cell foam offers superior insulation to closed-cell foam at the same thickness because the open cells form a three-dimensional network that traps more air. A 3.8 cm thick self-inflating mattress (the standard hiking format, e.g., Therm-a-Rest Trail Scout) offers an R-value of 3.1 to 3.8 depending on the foam density.
Self-inflating mattresses occupy a compressed volume of 3 to 5 liters depending on the size, weigh 600 grams to 1.2 kg, and cost 60 to 180 euros. Their main advantage over air-inflatable mattresses is their behavior in the event of a partial puncture: the internal foam maintains residual insulation even with the valve open, whereas an air-inflatable mattress becomes inert.
The air-inflatable mattress (with or without insulation)
Pure air-inflatable mattresses use only the air column for insulation. The problem is that air convects inside the chambers: warm air rises toward the body, cools in contact with the ground, descends, and rises again, creating a thermal circulation that reduces effective insulation. Large-chamber pure air-inflatable mattresses (standard camping type) have R-values of 1 to 2.5 despite their apparent thickness, because internal convection cancels out part of their potential insulation.
To counter this phenomenon, technical inflatable mattresses use horizontal baffles that reduce the size of the air chambers and limit convection, or they incorporate insulating filling (down, Primaloft) that physically blocks convection. An inflatable mattress with 70 g/m² Primaloft filling in an 8 cm thick chamber reaches an R-value of 4 to 6 for a weight of 400 to 600 grams. This type of mattress (Therm-a-Rest NeoAir XTherm, Sea to Summit Ether Light XT Insulated, Exped SynMat) represents the optimal performance in weight-to-insulation ratio: less than 500 grams for an R-value of 4 to 7 depending on the model.
🚫 An air-inflatable mattress without insulating filling and without anti-convection baffles does not effectively protect against the ground below 10°C ambient. Its thickness creates an illusion of comfort that disappears as soon as the ground cools. For bivouacs below 10°C, a mattress with insulating filling or closed-cell foam is essential.
Recommended R-value according to conditions
Sizing table
Mattress sizing should be done in parallel with the sleeping bag, not independently. A -5°C certified bag with an R2 mattress can result in a cold night at an actual 0°C. The same bag with an R5 mattress correctly covers the conditions for which it was certified. The recommendations below apply to a bivouac in a tent (protection against wind), with standard sleepwear (long underwear):
| Conditions | Min. Ground Temp | Min. R-value | Recommended R-value |
|---|---|---|---|
| Summer camping low altitude (<1,000 m) | 12-18°C | R1 | R1.5 to R2 |
| 3-season trek (May-Oct, 1,000-2,500 m) | 3-12°C | R2 | R3 to R4 |
| Shoulder season (Mar-May, Oct-Nov, 1,000-2,000 m) | -2 to 6°C | R3.5 | R4 to R5 |
| Accessible winter (ski touring, snowshoeing) | -8 to 0°C | R4.5 | R5 to R6 |
| Winter mountaineering and extreme cold | < -8°C | R6 | R7 to R10 |
Interaction with the sleeping bag
The relationship between the mattress R-value and the effective performance of the bag is not linear, but it is predictable. For each R-value point missing relative to the EN 13537 test reference mattress (R5.5), the effective performance of the bag degrades by approximately 1 to 2°C of actual comfort depending on the ground conductivity and ambient temperature. A 0°C certified bag used with an R3 mattress (2.5 points less than the reference) sees its effective performance reduced to approximately 5-7°C of comfort under normal conditions.
This interaction explains why the Aegismax G2 trekking (certified -2°C comfort, 800 cuin) used with an R4 mattress in the Alps in September comfortably covers bivouacs at 0°C: the mattress R-value is within the test's reference range, and the certified temperatures are representative of real conditions. The same bag on an R1.5 mattress may be disappointing on a 5°C night, not because the bag is at fault, but because the system is undersized on the mattress side.
The logic also applies in the other direction: an R6 mattress can allow the use of a slightly lighter (and thus less insulating) sleeping bag while maintaining the same overall thermal comfort. Investing in an R5 mattress rather than an R3 is sometimes more effective in terms of weight and cost than investing in a higher-performance sleeping bag.
Weight, compressibility, and trade-offs by profile
Choosing a mattress involves the same trade-offs as a sleeping bag: weight, insulation, and price are in tension. For car camping, weight and volume are not constraints: a 1 kg R4 self-inflating mattress is optimal (superior comfort, moderate price). For trekking with a sub-10 kg backpack, an inflatable mattress with insulating filling (400-550 g, R4-R6) is the only format that combines sufficient insulation with acceptable weight. For extreme ultralight, superimposing a light EVA foam (R1.5, 200 g) and a light inflatable (R2, 300 g) yields R3 to 3.5 for 500 grams: often more rational than a single expensive high-performance mattress.
The compressed volume of the mattress is as real a constraint as that of the sleeping bag in a trekking backpack. A 3.8 cm thick self-inflating mattress occupies 3 to 5 liters compressed. An 8 cm inflatable with filling fits in 1 to 2 liters. This difference in volume is as decisive as the difference in weight for practitioners using 30 to 40-liter backpacks.
Frequently Asked Questions
Does a mattress's R-value decrease over time?
Yes, but slowly. For EVA foam mattresses, the progressive decompression of the foam (crushing of cells after years of use) reduces the R-value by 10% to 20% over 10 to 15 years. For inflatable mattresses, the insulating filling (down or Primaloft) retains its properties if the mattress is correctly dried after each use. Moisture entry into the baffles (via micro-leaks in the valve or internal condensation) is the primary cause of premature degradation of the R-value of inflatable mattresses with filling.
Can one go without a mattress if the sleeping bag is sufficiently warm?
No, not below 15°C ambient. Even a -20°C comfort certified bag will have its underside compressed to almost zero insulation under the body's weight. Without a mattress, the only insulation against the ground is the bag fabric (a few tenths of a millimeter), which is insufficient relative to the conductivity of the ground. The mattress is not a comfort option: it is an essential component of the nightly thermal system as soon as the ambient temperature drops below 15°C.
Does a layer of clothing underneath replace the mattress?
Partially. A coat or down jacket placed under the lower back and buttocks (the areas of contact with the ground that generate the most heat) improves local insulation by an estimated 1 to 2 R-value units. This makeshift solution covers nights where the temperature is not extreme. For a night at 5°C or less, it does not replace a correctly sized mattress: the body's contact surface area with the ground is too large to be effectively covered by clothing not optimized for this use.
Is a higher R-value needed on snow?
Yes, significantly. Compacted snow has a thermal conductivity of 0.1 to 0.6 W/m·K depending on its density (fresh snow vs. firn), compared to 0.5 to 1.5 W/m·K for compacted soil. Snow is therefore less conductive than compacted soil, which might seem paradoxical. But snow melts upon contact with body heat and creates a layer of liquid water which, in turn, is very conductive (0.6 W/m·K). On snow, a minimum R-value of 6 is recommended for winter nights, and a waterproof mattress underside is essential to avoid dampening the mattress's own filling.
Can the same mattress be used in summer and winter?
An R4 to R5 mattress can be used for 3 seasons with good comfort and in winter with acceptable conditions (above -10°C at the ground). Below -10°C at the ground, an R6 or higher is necessary. The common trade-off for versatile practitioners is an R4 to R4.5 mattress supplemented by an R1.5 EVA foam sheet during winter trips: the same stacking principle mentioned previously, with low additional cost and weight.
Conclusion
The bivouac mattress is not a comfort accessory: it is the component that determines whether the certified performance of the sleeping bag will actually be achieved in real conditions. The ground absorbs heat 25 times faster than air; the EN 13537 protocol tests bags with an R5.5 mattress, and each missing point of R-value degrades the effective performance of the bag by 1 to 2°C. A 0°C certified bag with an R2 mattress can make you feel cold at an actual 6°C. This interaction between bag and mattress is not optional when sizing a bivouac sleeping system.
The recommended R-value logically follows these conditions: R3 to R4 for 3-season trekking between 1,000 and 2,500 m, R5 to R6 for the shoulder season and ski touring, R6 and above for winter mountaineering. An inflatable sleeping pad with insulation is the optimal format for carrying (400-550 g, R4-R6, 1 to 2 liters compressed). EVA foam remains the gold standard for robustness and durability, with the drawback of its non-compressible volume. To calibrate your entire pack-and-pad system according to the target temperatures, our sleeping bag shop and its trekking collection allow you to find the thermal certifications for each model.
⚡ Verdict: for a 3-season trek in France (May-October, bivouac up to 2,500 m), aim for an R3 to R4 pad. For the shoulder season and autumn bivouacs, R4 to R5. Never choose a pad without comparing it to the reference R-value used for sleeping bag testing (R5.5 according to EN 13537): every missing point of R-value results in a degradation of the sleeping bag's effective performance.
