In theory, stacking two egg crate camping sleeping pads does allow their R-values to be combined.
In practice, however, egg crate pads have a unique characteristic when stacked: the peaks and valleys of the two surfaces tend to nest into each other. As a result, the combined thickness is not twice that of a single pad laid flat, and the combined R-value is likewise not double the single-pad value. The table below shows test data for thickness and R-value of a Mountain Spring double-wide egg crate sleeping pad in both flat and folded configurations:
Above: Mountain Spring double-wide egg crate sleeping pad — R-value and thickness test data in flat and folded configurations.
In-use thickness: The compressed thickness of the egg crate pad under the body weight of an adult lying on it.
Static thickness: The vertical height measured from the base to the highest egg crate peak, with the pad placed horizontally under a pressure of 100 Pa.
The test data shows that when folded, the in-use thickness, static thickness, and R-value of the egg crate sleeping pad are approximately 1.7 to 1.8 times those of the pad laid flat.
Conclusions:
First, when two identical egg crate sleeping pads are stacked, the combined R-value should be approximately 1.7 to 1.8 times that of a single pad. Second, when two identical ridgerest sleeping pads are stacked, the combined R-value should be close to double that of a single pad.