mm wrote:

Would it work to just buy infrared film for a standard 35 mm camera?

That kind of film won't show warm vs hot spots, and if it did, the heat
in the camera would expose it, unless the camera were cooled.

With 70 F house air and Tw and Ta wall and outdoor temps and a slow-moving
R0.67 indoor wall airfilm resistance and an Rv wall R-value, we have this,
viewed in a fixed font:

Tw
R0.67 | Rv
70 F ---www-------www--- Ta
------------
I = (70-Ta)/(Rv+0.67)

Tw = Ta+IRv, makes 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.

It's hard to measure an R30 wall this way, because the indoor wall temp
is very close to the indoor air temp (69.1 F). Aiming the IR thermometer
into a 1" hole in 1 ft^2 of 2" R10 Styrofoam pressed against the wall for
a few minutes could increase accuracy, making something like this:

Tw
R10.67 | Rv
70 F ---www-------www--- Ta
------------
I = (70-Ta)/(Rv+10.67)

with Rv = 10.67(Tw-Ta)/(70-Tw).

Rv = R30 and Ta = 30 makes Tw = 59.5.

Nick