Use thermal curtains. Thick thermal curtains drawn across the windows during the night noticeably elevate the window's Total R-price. ✔ Challenge: Modest gaps or missing insulation develop "thermal bridges," where by heat moves by uninsulated sections. J–K: johns manville thermal and sound control insulation, kingspan thermal insulation, knauf space blanket, https://fakeeyelashes47901.tinyblogging.com/5-easy-facts-about-retrofit-windows-described-78382058