Anyone ever use this stuff to protect figs? I have a cheap Harbor Freight greenhouse that completely collapsed last winter. I rebuilt it so I know it will not fall this year but I'm thinking of making a box inside of it of rigid insulation. Any thoughts? I know there are different "R" ratings. I will have heat controled with a thermostat set to 40 degrees but I also worry about the possible upward temp spikes. It's a 10'x12' greenhouse. I will line the walls 4' up and cover the top with 10' lengths. At least that's what I'm thinking.

Something I read about, to even out temps in a Green house, is put concrete blocks in so they absorb heat during the day and release at night. I also read a large barrel of water will do the same thing and is supposed to be better at it.

Aaron, the good thing about a box made of that stuff is that it will protect the plants from temperature spikes too, it'll warm up and cool down at a slower rate than the surrounding area.
I used this stuff to make my rooting 'hot boxes' and the ballasts from the lights keep them at a balmy 70-75 degrees, even though my unheated basement gets down to the 40's. In the morning, when the lights have been off for 6 to 8 hours, the potting mix is still in the 60's.
It does get stuffy in there, with the moisture from the potting mix rising out of the pots.
Good for rooting, and I doubt you'd have to air it out much, since your stored plants would be kept at a lower temperature, and with a lot less moisture.

How big will you build it (them)?
They will bulge if they're packed with plants, are you considering building a frame to stiffen the walls?

I store my figs in a structure with basically 2.5 inch foam insulation from November till april with no sunlight and heat with light bulbs to maintain apx 36 degrees f .  
Ive posted about it here

http://figs4funforum.websitetoolbox.com/post/figloo-update-5040799?pid=1280174529#post1280174529

My 10' x 12' greenhouse now has a 2x4" internal skeleton. The panels will attach to the walls and will then be topped by more panels. All of my figs are in pots. I am not worried about bulging out the sides. What "R" rating should I be shooting for? This winter it's about 100 pots from 3-15 gal.

The thicker the foam, and the higher the R factor more it will protect from temp swings.