Most indoor pillows are filled with a puffy, polyester fiber or with foam rubber. Neither of these fillers works well for outdoor pillows. Foam rubber acts like a sponge, retaining any moisture it comes into contact with. Similarly, the puffy polyester stuffing will retain moisture, and it is not firm enough to hold up to year-round use.
Look for material that is nonabsorbent, or that drains well. The material should be a polyester-blend fiber that is woven in a way that will be dense and firm, yet non-absorbent. This kind of filler comes in blocks or sheets instead of bags filled with fluff.
Mold can form quickly in filler that does not have the right draining properties. When purchasing filler, look for a product that will resist mold and mildew over a long period of time.
Since your outdoor setting might include fire-pits or candles, make sure that the material has passed standard fire safety tests and is flame-retardant.
Some of the foam pillow filler that is on the market today can contain CFCs (chlorofluorocarbons) or may release CFCs during the manufacturing process. These are bad for the earth’s ozone layer. If this concerns you, look for products that are free of CFCs. Some products are even recyclable.