k (styrofoam): 0.010 J/s⋅m⋅ºC
Suppose you get a cup of coffee at a coffee shop. You get a phone call and know that the call is going to take a few minutes. You have a decision to make, do you mix the cream into the coffee right away or do you wait several minutes and put the cream in after the call, right before you drink it? The goal, we assume, is to have the coffee as hot as possible. So, the main question is how the coffee is going to cool as you wait until after your call.
I took a boat through 96 million black plastic balls on the Los Angeles reservoir to find out why they're there. The first time I heard about shade balls the claim was they reduce evaporation. But it turns out this isn't the reason they were introduced.