Grass seeds are hygroscopic, meaning that they absorb moisture from the air, so a dry environment is essential for storage. They require a combined temperature and humidity equal to or less than 100; for example: 70 degrees F with 30 percent humidity or 40 degrees F with 55 percent humidity. Any value greater than 100 will greatly diminish the seed's viability, according to Penn State Cooperative Extension.
Some grass seed should not be stored to begin with. Any grass seed that has been exposed to moisture should be used or disposed of. Nor should you store any seeds that are cracked or otherwise bruised.
You need to know exactly how old your bag of grass seed is before you consider storing it. The Federal Seed Act of 1936 requires all seed to be labeled with the date packed, germination rate and lot number, among other identifying information. If you cannot verify how old your seed is, do not expect great results.