Example Proof of U Value Calculation
Property: 
Roof = profile sheet steel sandwich with 50 mm of fibre glass insulation.
Target U value = 0.025 W/(K•m²) 
Accepted best/worse case k values in W/m/K
Fibreglass: 0.04 <= kf< = 0.045 = range of thermal resistivity of fibreglass 
Polyurethane Foam: 0.020<= kp<=0.026 = range of thermal resistivity of polyurethane foam  
Given R = d/k where d is depth in metres and k is thermal conductivity in W/m/K

Then worse case: 
Rf = d/k = 0.050/0.045 = 1.11
Rp = d/k = 0.075/0.026 = 2.885
Rt = total thermal resistivity
Total Rt = Rf + Rp = 1.11 + 2.885 = 3.99 K•m²/W
R = 1/U or U = 1/R or U given by proposal is 1/3.99 = 0.25 W/(K•m²) = target U value.

Then best case: 
Rf = 0.050/0.04 = 1.25
Rp = 0.075/0.020 = 3.75
Total Rt = 5.00
Therefore U value = 0.20 W/(K•m²)   U value range, best case/worse case: 0.20 <= U<= 0.25   Therefore U value = 1/R = 0.25 on worst case assumptions and therefore 75 mm of polyurethane coating with existing insulation will meet U target of 0.25 W/(K•m²) without allowing for positive contributory thermal resistivity of steel, roof covering, suspended ceiling, air void all of which will help reduce the U value further. Therefore 75 mm of polyurethane foam (Class 1 material) is considered adequate in meeting overall U value of roof to <=0.25 and will in fact better this by some margin.