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 <= k_f< = 0.045 = range of thermal resistivity of fibreglass 

Polyurethane Foam: 0.020<= k_p<=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: 

R_f = d/k = 0.050/0.045 = 1.11

R_p = d/k = 0.075/0.026 = 2.885

R_t = total thermal resistivity

Total R_t = R_f + R_p = 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: 

R_{f =} 0.050/0.04 = 1.25

R_p = 0.075/0.020 = 3.75

Total R_t = 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.