Down and synthetic insulation have fundamentally different storage requirements because they are fundamentally different materials. Down is a natural product (the plumules of ducks and geese) composed of branching filaments that create loft by trapping air. Synthetic insulation is manufactured from polyester fibers (PrimaLoft, Coreloft, Thermolite, Thinsulate) engineered into various structures that mimic down's loft. Their aging mechanisms are different, their failure modes are different, and the storage that preserves one may damage the other.
How Down Ages in Storage
Down has a theoretically infinite lifespan if stored correctly. The clusters are essentially stable protein structures (keratin) that do not chemically degrade under normal conditions. Archaeological evidence of down-filled garments hundreds of years old shows intact down clusters. What ruins down in storage is not age but three things: compression, moisture, and biological attack.
Compression damage. Down clusters work by interlocking their filaments to create millions of tiny air pockets. When down is compressed for long periods, the filaments do not spring back fully. The clusters lose their structural integrity through a process called "compression set." This is not the down chemically degrading -- it is physical deformation of the filament structure.
The rule for down storage is absolute: never store down in a compression sack. A sleeping bag that is compressed to the size of a football for a backpacking trip for a few days is fine. Compressed to that size and left in a closet for six months, it will lose measurable loft that never fully recovers. The comparison is instructive: a Western Mountaineering AlpinLite bag stored properly (hung loosely or in a large storage sack) will maintain its original loft for decades. The same bag stored compressed for a year will lose 10 to 20 percent of its loft permanently.
Storage sacks are the solution. Most down sleeping bag manufacturers include a large mesh or cotton storage sack with the bag. Western Mountaineering, Feathered Friends, and Enlightened Equipment all ship bags in oversized storage sacks. Use them. Alternatively, hang down bags over a wide hanger or lay them flat under a bed where they will not be compressed.
Moisture damage. Down absorbs moisture from humid air (it is hygroscopic). At humidity levels above 65 percent, down clusters absorb enough moisture to lose loft and become susceptible to mildew. A down bag stored in a damp basement for a year will develop a musty odor and the down will clump. Mildew damage to down is permanent -- the fungal growth physically breaks down the plumule filaments.
The practical solution: store down gear in a climate-controlled living space (bedroom closet, not basement or attic). If you must store down in a humid environment, use silica gel desiccant packs and check them monthly. A humidity monitor (10 USD at any hardware store) placed in the storage closet gives you data instead of guesswork.
Biological damage. Moths do not eat down (it is keratin, the same protein as wool), but they will lay eggs in it, and their larvae will eat the down clusters as a protein source. Carpet beetles do the same. The damage is often invisible until you notice the fine dust at the bottom of a sleeping bag that used to be supporting loft. Prevent this by storing down bags in breathable cotton or mesh storage sacks in a clean, dry environment. Cedar blocks and lavender sachets have limited effectiveness -- the best prevention is a sealed storage container, but this creates the moisture problem (sealed means no ventilation, which means trapped humidity). A breathable storage sack in a dry closet is the best compromise.
How Synthetic Insulation Ages in Storage
Synthetic insulation ages through different mechanisms than down. It does not suffer from compression set in the same way (polyester fibers are more resilient to compression than down plumules are), but it has its own failure mode: mechanical fatigue.
Compression and loft recovery. Synthetic insulation can be compressed and recover its loft better than down can. You can store a synthetic bag in a compression sack for months and it will mostly recover when unpacked. "Mostly" is the operative word. Repeated compression cycles (stuff, compress, unstuff, repeat) cause cumulative damage to the fiber structure. Each cycle breaks some of the fiber bonds, and over hundreds of cycles, the insulation loses loft.
This is why synthetic insulation has a shorter useful life than down in applications where the item is repeatedly compressed (sleeping bags, insulated jackets). A PrimaLoft Gold insulated jacket that is stuffed into its pocket daily on a thru-hike will lose 20 to 30 percent of its warmth over a 5-month trail. A down jacket subjected to the same treatment will lose less loft but may develop leaks where the down escapes.