Short answer
You do not want to use insecticides and plug-in repellents for long in a closed room. Stopping mosquitoes at the window in the first place is the cleanest solution.
Why might an ordinary insect screen not be enough?
Old or torn mesh, gaps at the edges, and smaller insects such as sand flies can get through an ordinary insect screen. Mosquitoes and micro insects – we describe these tiny insects on that page.
Made to measure: hook-and-loop or roll

