For years I have thought about building my own insulation panels for my house windows, as shown here:
But that chap uses heat-shrinkable plastic to cover the frames.
I always intended to cover the frame with a thin sheet of hardboard both sides, then fill the space within the 'box' with some insulating material, such as loft insulation. These panels would only be in place during the night in winter, say from 4:00 pm till 8:00am. They would be light in weight, so easily placed or removed. During the summer they could be stacked in the garage.
Question 1: Is it better to use the heat-shrink method with air as the insulation medium, or to fill the void with insulation material?
Question 2: What would be an alternative suitable material to fill in the void?
MM