There was research done on the Performance of Filling Material. This was conducted at the Institute for Environmental Research, Kansas State University. A report on this research can be found on the polarguard web site. Report on The Performance of Filling Material. Down can be stored partially compressed, but it should never be stored in anything watertight.
Condensation can occur on the inside of a waterproof bag and result in mildew.
Store your bag in a dry place that has fairly constant temperature. Do not expose the bag to sunlight except to dry. UV radiation degrades the nylon and heat evaporates the oils from down. You can store a sleeping bag either loosely bundled in a cotton storage sack, laid out flat under a bed, or hung vertically in a closet. We have cotton storage sacks available if you chose the former method.