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The R-value is a measure of specific thermal resistance, and R10 = 10 ft²*°F*hr/(BTU*inch). I much prefer the metric meters*Kelvin/Watt. The USCS (not metric) system discourages good practices by making it harder to check your units during a calculation.
The equation for heat flux is:
Q = ( surface area * deltaT ) / ( R-value )
If you have an 8' cube, its surface area is 384ft². Assume 9" thick of fiberglass batting at R4 per inch, giving R36, plus two surfaces of a wall with R 0.6 each.
A healthy human at 2000 Calories/day puts out 330BTU/hr, so we'll set heat flux equal to that.
330BTU/hr = ( 384ft² * deltaT ) / 37.2 ft²*°F*hr/(BTU)
deltaT = 32°F, and the body heat of a healthy person will hold a 32°F temperature differential across the R37.2 barrier. What will the temperature be on the other side of the wall?
This mathematical model of a room is too simple to yield anything but a first approximation. If any of the room's six walls are exterior walls or floors, or if sunlight is incident up on them, the math changes. There are also transient effects to consider: the room will take a long time to reach its steady-state temperature.
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