I have an odd insulation question. I am taking down the fiberglass insulation from the top of my bus. So far it is doesn't look moldy or damaged, but it is pretty thin, probably only 1 in. I also have several other bags of R-19 fiberglass, but I didn't want to use fiberglass on the bus because it absorbs moisture, can get moldy, and doesn't insulate well if not install perfectly.
I am currently putting up styrofoam and spray foam (2-3 inches?) to insulate the bus roof. To save money, I have been collecting foam for a while now, and I recently built a hot wire foam cutter. I am cutting the foam into little blocks and gluing them to the inside roof. I will use Tytan expanding spray foam to fill in all the spaces between the foam, that way I can use as much free foam as possible, and as little expensive spray foam as necessary. I have 5 more garbage bags of foam blocks ready, but I don't think it will be quite enough. Which got me thinking about all the fiberglass I have.
I recently made a trade for a giant roll of poly tubing. It looks like the photo below except my roll is 4 ft high. Based on the weight, I think there is about 1200 ft of plastic left on the roll.
So my question is: can I take (3) 1 inch fliberglass mats, stack them together, put them inside the plastic tube, seal up the ends, and install it in the roof as insulation?
I don't know much about fiberglass. Will the plastic bag interfere with sound dampening? Will it trap condensation? Or will it help reduce airflow and improve the insulating of the fiberglass? I know that most people don't do this, because the cost of the plastic plus the cost of the fiberglass would be more than spray foam. But in my case, since the fiberglass and the plastic is free, would this be a good solution?