My windows & doors U=.25 = R-4
My walls 6" Polyiso foam is R-2 / inch @ 0*F and R-6 / inch @ 70*F
My wall average @ 0*F outside is about R-4 / inch = R-24
So my windows & doors loose heat about 6X faster per square foot, than my walls.
Conduction Heat Loss: Q = Area x U-value x Delta-T
Comes close for walls, windows, & doors, but ignores some BIG factors such as infiltration, wind washing, and Clear Cold Night Radiation.
http://www.healthyheating.com/Radian...m#.WNRBVvkrKUk
http://www.sensiblehouse.org/nrg_heatloss.htm
Radiant slab heat loss down & horizontal is very complex.
Depends on under slab & perimeter R-values, soil type, water content, soil temperature, outdoor temperature, slab temperature, room temperature, slab size, etc, etc.