How to insulate a bay window
According to the U.S. Department of Energy, windows can account for 10 to 25 per cent of your home's heating costs. Bay windows are especially prone to heat loss due to their large glass surface area and cantilevered construction.
Reduce the heat lost through your bay window using caulk, insulation and window treatments.
Place a tube of clear silicone caulk into a caulking gun so the nozzle rests at the opposite end of the gun from the plunger. Use a utility knife to cut the tip of the nozzle off ½ inch down at a 45 degree angle. Push a nail down through the tip of the nozzle to break the seal in the tube. Position the nozzle at a corner of window pane and the tip slowly along the edge of the glass, applying steady pressure to the trigger as you go to leave a line of caulk about ¼ inch wide along the glass where it meets the frame. Repeat along all four sides of the glass for all window panes.
Cut R-19 batt insulation to fit into the cavity below the window using a utility knife. Prior to replacing the siding material on the exterior of the home after the window is installed, tuck the batts into the cavity with the vapour barrier facing up. If the window has already been installed it is best to hire a professional to spray rigid foam insulation into the cavity.
Purchase and install insulating window treatments custom fit to your bay window. Honeycomb shades, blackout curtains, thermal panels and roman shades all offer varying degrees of thermal protection for your windows.