I would expand just a little on Ludwik's response in terms of what a
student might do.
I would build a box, perhaps one cubic foot, from the material I want to
determine the R-value of.
I would mount a standard light-bulb socket inside. And insert a
thermometer (or probe for a digital thermometer). I would have a
selection of light bulbs (15 watt, 25 watt, 40 watt, 60 watt). I would
put a light bulb inside, close it up, turn the bulb on, and wait for the
inside to reach constant temperature.
This way you don't need to make any electrical measurements. You trust
the marked power of the bulb, you measure inside and outside temperature
once equilibrium is established, you measure the surface area.
You might have to experiment with thermometer placement, and probably
want to put a can around the light bulb to act as a heat shield to avoid
radiative hot spots (you want to warm the air and not specific
absorptive spots on the internal walls). You'll also have to decide
what to do with the bottom surface. Will this box be suspended in
mid-air, or will is sit on something, thus are you using 6 surfaces or
5?
Start with low power (15 or 25 watts) so you don't get so hot you melt
or scorch anything. With night-light sockets you can also get 4 watt
and 7.5 watt bulbs.
Anyway, light bulbs make good heaters of known power.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu