Kitty Tube The Double Insulated option shell not included Generation 3 Double Insulation option with SCRATCH GUARD. - THIS IS NOT REPLACEMENT INSULATION. THIS IS USED TO DOUBLE THE EXISTING INSULATION. Must be used in conjunction with the main insulation. Fits easily into all previous model Kitty Tubes with a Kitty Tube logo on the side. The double insulated assembly also creates a secondary insulated air space between the ceiling and the outer lid. This assembly will drastically keep the Kitty Tube warmer in the winter and cooler in the summer. Up to 2 cats will comfortably fit inside the Kitty Tube when the double assembly is installed.