Here’s a link to the paper Mike Glavin cited (PDF):
It’s worth reading in the context of this discussion, e.g.
In any case, user information should point out the value of thermal insulation of the standard mattress to make the user aware of the importance of considering the mattress type, and that user temperatures are only valid if a mattress with a thermal resistance similar to the standard mattress (0.85 m2K/W) is used. Furthermore, a note should be added informing that a mattress with low thermal resistance (<0.23 m2K/W, e.g., 10-mm foam rubber) may increase temperature limit values by 5–6 °C compared to temperature limits on the label.
FYI: 0.85 m2K/W is R 4.8, 0.23 is R 1.3, and 5-6 °C is 9-11 °F for non-metric people.