(Building Heat Loss in BTU's per hour) =
[(Building Total Surface Area in sq.ft.) / (Surface Area "R" value)] x (Temperature Difference)
Size of one wall = 8 X 8 = 64 ft sq.
Area of room = 64 X 6 (including ceiling and floor) = 384 ft sq.
Assume you start with a room temperature of 72 degrees.
Heat loss per hour = (384/7.7) X (72-20) = 2500 btu/hr.
A human on a 2000 cal/day diet gives off about 330 btu/hr as heat.
Thermal Efficiency of a Human Being
2500/330 = 7.56. So we would need to increase the R value to 7.56 times the current value to maintain 72 degrees in the room with only one person in it. Or decrease the surface area by 7.56 times. Or increase the number of persons to 7.56 times (add 6.56 people to the room).
Figuring it another way, we can rearrange the equation to determine the temperature that will be maintained with one person in the room: (Temperature Difference) = (Building Heat Loss in BTU's per hour)/[(Building Total Surface Area in sq.ft.) / (Surface Area "R" value)].
Substituting our known values we have: (Temperature Difference) = 330/(384/7.7) = 330/49.9 = 6.62 degrees. So the body heat of one person will maintain a 6.62 degree temperature difference in the room. 20 + 6.62 = 26.62 degrees in the room with a 20 degree outside temperature.