I wrote to Wiggys and Jerry Wigutow promptly replied that my idea wouldn't work:

it would actually be 'detrimental to the function of my bags'

'If you use a waterproof layer on the bottom of the bag you will get condensation and then water. If cold enough the water will freeze.

'a waterproof material that has the capacity to be vapor permeable [does not] exist.

oh well it seemed a good idea at the time.
QJS