DON'T USE IT!
Regardless of whether or not you have a vapor barrier, moisture seeps into the fibergalss insulation. DO NOT USE FIBERGLASS BATT INSULATION! This moisture creates a moldy nightmare and also decreases your insulation value. The decreased insulation value results in higher electricity bills; your A/C now has to work constantly to cool the cooler, since the cold air is leaking out through the now uninsulated walls. Please do not use this!