Scenario: A person lies flat on a camping sleeping pad with an R-value of 2.5. The skin temperature on their back is 33.5°C, and the ambient temperature (assumed equal to the ground temperature) is -5°C. Under these conditions, will the sleeper feel cold along their back? Assume that the front and sides of the body are already covered — for example, by a sleeping bag — and have reached thermal equilibrium.
Solution:
$$R\text{-value} = 2.5 \times \frac{°F \cdot ft^2 \cdot h}{Btu} = 0.4403 (K \cdot m^2/W)$$
$$\Delta t = 33.5°C - (-5°C) = 38.5 K$$
Under a simplified model, based on the definition of R-value:
$$R = \frac{\Delta t}{q}$$
We get:
$$q = \frac{\Delta t}{R} = \frac{38.5}{0.4403} = 87.4 W/m^2$$
According to reference data, the average basal metabolic rate (BMR) for Chinese adults is as follows (in W/m², source: Baidu, units converted):
The calculated heat flux q far exceeds the basal metabolic rate across all age groups, meaning the body loses heat faster than it can produce it — and the sleeper will feel cold.
Of course, in the calculation above we made an impossible assumption: that the person is lying "bare" on the sleeping pad. In reality, most campers wear clothing inside a sleeping bag. The total effective R-value of the system must account for the R-value of the sleeping pad itself, the equivalent R-value of the sleeping bag's bottom layer, the equivalent R-value of the clothing worn, and the insulating air layers trapped between them.
That said, a healthy human body has a remarkable ability to self-regulate and maintain thermal equilibrium within a certain temperature range. Beyond that range, however, the body becomes vulnerable to cold injury or heat injury.