Firstly, they fit tightly into your window with no gap at either the top or bottom and only a 3-4mm gap at each side which prevents the circulating current described above from being able to start as the air cannot be moved close to the window. Curtains with gaps at the top and bottom or roller blinds with gaps on all sides of up to 20mm do not seal your windows in the same way.

The second feature is the honeycomb design. Still air is an excellent insulator and with a Thermacell blind you trap a layer of still air between the glass and the blind and then another still layer of air within the honeycomb fabric. If you choose a double cell, then you have two still layers of air in the fabric and if you select a room darkening fabric you get the additional silver lining in the fabric that also reflects the heat back into the room.

As well as keeping your home warmer in winter, you can also use Thermacell blinds to prevent heat gain in summer and reduce your energy bills when running fans and air conditioners.