Vic Dura wrote:

Rv = 0.67(Tw-Ta)/(70-Tw).

For instance, if you measure a Tw = 67.5 F wall temp with your $50 Raytek
MiniTemp when it's Ta = 30 F outdoors, Rv = 0.67(67.5-30)/(70-67.5) = R10.

Would this be valid for cooling?

Sure, with no sun shining on the wall, eg at night.

Ta=90F, Inside temp = 70F, Tw=72.5F

That makes Rv = 4.7 (not much.)

Nick