The Majestic UNIV-AS2 Attic Insulation Shield measures 5 inches in diameter. Its designed to protect your attic ceiling and roof from the intense heat that comes with burning wood or gas in your fireplace. Without the insulation, your homes roof may burst into flames! Since the exhaust from your fireplace is so hot, this insulation shield adds extra protection. Please note that no firestop is included. You can purchase it separately from Majestics product catalog. For over a decade, Majestic has been providing homeowners and contractors alike with quality product for fireplaces and indoor hearths. Enjoy the heat of your roaring fire along with the confidence that your fireplace and chimney are built from Majestics quality products!