Just recently got `free` cavity wall & loft insulation and would like to
further improve the insulation and sound insulation in a couple of rooms.
In one bedroom there are 2 windows which although double glazed have
minimum spacing between the glass (approx cm) I also hear traffic noise
through them.
I was thinking of using some 10mm polycarbonate sheet in a frame to make
a third `pane` . I am using this method as it is something I could do
myself.
Am I likely to gain much by way of heat loss and sound insulation or
would it not be worth the effort?

thanks