'Roof space convective R for ceiling: '0.25000
'Roof space convective R for roof: '0.33300
'Roof space radiative R non-reflective: '0.16670
'Roof space radiative R reflective: '2.73330
This simplified method assumes a linear relationship between heat flow and temperature difference which is roughly OK with convective/conductive heat transfer but radiative transfer varies with the 4th power of the absolute temperatures of the surfaces so this averaged approach would be stretched on very sunny days when the roof temperature is very high.