Abstract: Apparatus for mounting an environment controlling screen, sheet or membrane, is provided comprising separate frame sections secured in mutually abutting relation to the inner periphery of an opening, mutually abutting strip of Velcro hook material are affixed to each frame section, and a flexible sheet, dimensioned to fit the frame section, is affixed to the frame section by means of a strip of Velcro pile material attached to the margins of the sheet. A mosquito-proof joint is provided by the abutting Velcro material even though the frame sections are not joined directly one to the other. Quick installation and removal of the sheet material is feasible. Storage of the sheets is convenient, simple, and takes up little space.