A praying mantis laid an egg sack a week ago. It was 60 degrees. The temp here will soon be falling to below 30. I'm in New York. I don't want to store egg sack in house and would like to keep it until spring so it can hatch properly.
There are several different species of Mantis, many do not survive the winter cold. Laid eggs that need to survive the winter should be kept from freezing. It may take several months for the eggs to hatch and they will only hatch if it is warm enough. If kept in a warm house they may hatch prior to food being available, they babies will then starve to death.
There are several suggestions found for overwintering the eggs. Most references I looked at agreed on these two points
Don't let the eggs freeze
Don't keep them in the house (if they hatch in winter the babies may starve)
The recommendation is to put the eggs in a container (some references suggested air holes) and store them in your refrigerator (NOT Freezer) or in an unheated garage or shed. When you are ready to let them begin hatching; the ootheca (egg case) should be suspended a bit more than 5cm or 2 1/2 inches above the floor of the cage.