Why does Kuuma recommend against using screen or mesh over the top of my chimney?

Water vapor is a by-product of combustion, and the Kuuma is designed to let as little heat as possible escape up the chimney.  Due to this minimal heat, any screen or mesh will provide an opportunity for water vapor to condense, which can then freeze, allowing Carbon Monoxide to enter your home.  We would recommend a rain hat with at least a 4″ opening.  This is the one we use, and it works great!  Famco (Wind Directional Chimney Cap)