Fi-Foil's AA2 Vapor Shield is a multi-layer reflective insulation intended for use on furred-out masonry walls. The inside layer is aluminum foil. The outer layer is 35 lb. natural kraft paper coated with polyethylene, internal expanders separate the paper from the foil creating a 3/8" reflective air space. When installed on furring strips spaced 16" or 24" on center, a second reflective air space is formed. This air space is dependent upon the thickness of the furring strip selected. AA2 Vapor Shield is available in Standard & High-perm versions or perforated for applications not requiring a vapor retarder.