3M Indoor Insulator Kit, 2-Window – Link

Amazon has this currently marked down to its lowest price at only $4.98.

Note that this is an Add-on Item which ships with a $25 Amazon order.

With the 3M indoor window insulator kit, you can lower your heating bills during the winter and make your home more energy-efficient.

• Included: (1) 5.16′ x 7′, (2) rolls of tape each 1/2″ x 500″
• Insulates two 3′ x 5′ windows
• Lowers heating costs and saves energy
• Applies easily, shrinking tight, wrinkle-free and clear on glass
• Increasing the R-value of single pane window by 90%