DIY Removable Insulated Window Panel

Measure the width and height of the window where you want to put an insulated panel. Take the measurements from opposing inside faces of the window frame. Transfer these measurements to the face of a piece of rigid foam insulation. Foam panels are 24 inches wide, so if your window is larger than this you will have to either duct-tape sheets together to make a wider panel or make multiple panels for the window.

Cut the rigid foam to the size of the window opening. Angle your cut slightly so that one face of the panel is larger than the other. This way, when you put the panel into the window opening, the sides of the foam will butt up tightly against the inside faces of the window opening to create a tight fit.

Improve the appearance of your insulated window panels by sewing fabric covers for them. Measure a piece of fabric that is the same height as the panel and twice the width plus a few inches. Fold the fabric over and sew the side together using a sewing machine to create a bag with one open end. Slide the rigid foam inside the bag, then sew the end shut.

Put your insulated panels into the windows any time it's very cold outside. Leave them out of the south windows during sunny weather to take advantage of solar heating.