Insulation material in a roofspace has a tendency to compress -especially if you want to use a double layer.
This greatly reduces the effectiveness of heat retention.
Today’s invention is a cheap plastic fork with needle-like tangs (red).
When the insulation is laid, and has had a chance to expand or ‘loft’, many of these forks would be pressed into the sides of each batte in order to maintain its thickness (and air retention) over time.
A refinement of this I just thought up would be to make these items with a bimetallic element, so that, as as the temperature outside falls, so the vertical spacing between the tangs (and thus the air content within the insulation) increases.