Are you sure you want to use insulation? I don't understand why you'd need to keep a grow rack insulated in California. I thought you wanted light, not necessarily heat. A fully insulated, six foot tall grow rack lit with fluorescents at every shelf would roast. It would be little different than a giant terrarium, only the insulation probably wouldn't handle the humidity as well as glass. I kept my old apartment warm in the winter by leaving my grow rack running late at night. If you're just looking for light, you should go to the sporting goods store and pick up some mylar emergency blankets, and stretch them around the outside of the rack. Attach with packing tape, and they last pretty long (mine survived being ripped off and put back up twice, and I could probably put it up again.) Shiny side in for brightness and heat; shiny side out will give you roughly the same light with less heat and glare. Covering all but one broad side of a 4 x 1.5 x 6 foot rack takes four blankets tops; when I did it I spent less than $20, including the tape.