Panda plastic and/or Mylar work great (since before grow tents were popular). I am thinking about this myself now - whether insulation on a closet's walls is beneficial for growing inside. Layers of insulation. It is already inside so I am thinking about the other side of the drywall. The in between spaces in other words, connected to the attic and maybe even under the home's crawl space presumably. If I line the thing with foam it may end up with more heat than I want since the grow room is always warmer than the house or than what humans enjoy. To be specific I mean it is quite easy to overheat a grow room, much easier than having too much air circulation or an air conditioner than works too well. With that in mind I may skip the foam. Same with the floor and/or ceiling - the floor can give its cold energy, or something about conserving heat, to the reservoir(s). On the other hand I am not sure if the res should get that cold at night and surrounding it top and bottom with insulation foam could help it stay cool and in the same temperature range throughout the day. Air is a good insulator but foam and reflectix have these silver coatings on them and I don't really know how much difference in a test if you sealed the spaces or used packing air bags anything really. It doesn't seem like a structural material although it is strong so strong and also insulates well. I am on the fence about the foam maybe just for the reservoirs top & bottom. For the rest of the room plastic sheeting for the floor like for a paint job on a house masking film don't need greenhouse plastic, and reflectix, Mylar. I was thinking about all three layers but the grow room most likely will need to stay cool not hot which means I can use the walls to release some of the heat or use the extra volume for growing created by losing the foam surround in a smallish closet box which is already enclosed/insulated based on where it is in a house. Hole in the ceiling in the closet and hole in the door for air circulation with an extension cord running under the door should be enough. Maybe a reservoir chiller if necessary so digital thermometers that record highs and lows are a good idea. So many parts and pieces to help stabilize reliability and add features these days. I don't think a lot of this stuff or Amazon existed when I was still in school.