The PCM will cool if the temperature is over 70F,
and will give off heat if the temperature is under 70F.

STT-521-500 Saf-T-Temp Controlled Room Temperature (CRT) PCM Paks - 500g; 16 / carton
http://www.saf-t-sup...hopperRegionID=
$77.75

PCM = Phase change material
Phase Change = solid to liquid, or liquid to gas
or gas to liquid or liquid to solid
= = melting or freezing

If you rebag the 1000g PCM into two 500g packs, the rebagged 500g packs are $5.40 each.

My calculations
90F outside of the styrofoam box
1.5 inch thick x 8 x 10 x 10 styrofoam box
gains heat at a rate of 1W
PCM will absorb 210,000 J of heat energy
= over 29 hours of continual cooling.

But even better, the PCM only cools when it needs to
and can even recharge if the box gets cold overnight.