Hello sander,
That sounds like the foam board I was talking about that I wouldn't use. I don't know what exactly what it's called, but I think they use it mostly in the HVAC industry. It's aluminum foil on one side, and some other material on the other side, and the foam itself is a yellow/orange color. Looks a lot like this
Foam insulation. the foam is an open cell product, which means each tiny air bubble has an opening that will allow it will absorb water like a sponge. You want to use a closed cell foam. If Your not sure, I would take a piece and submerge it in a bucket of water for at least 24 hours and see if it absorbed any water.
As for the T5's you want them as close to the plants as you can get them withough't burning the plants. Lettuce plants are typically a low light requirement plant, and that's why they work well for lettuce, but the lumin drop off with florescent
lights is still significant. So you still want to keep the T5's as close as you can. Hopefully around 6 inches if possible, but I wouldn't want them much more than 12-14 inches away from the plants. Beyond 2 feet the plants will only get about 25% (75% drop off) of the lumins or less depending on distance. If your using multiple twin bulb light fixtures, I would space them no more than two feet apart.