R13 is regular insulation used in home construction. It is cheap and yes it is used to hold the heat it. Foam boards would work but the R value is a lot lower.
wow mate, this is an awesome ray tank! im over in australia and i have heard but never seen in person a tank made from wood. i guess we are behind the times haha! look forward to seeing some more pics.
I did use GE and I used the II instead of the I. I got the two mixed up going by memory. I used the black version. So what I am doing to correct it is I am using a thin layer of aquarium safe silicone around the edges of the acrylic on the inside of the tank to seal off the II silicone from the water.