The primary characteristics you want from a seat cushion are comfort and durability. However, different types of foam will suit a different range of cushions. For example, Dry Fast foam is perfect for outdoor use.
When shopping for a foam cushion, there is some terminology that you will need to know. The first is the density of the foam. The density of the foam is the amount air trapped within the foam. The key metric for foam density is pounds per cubic foot. The higher the value, the higher the density, and the more durable.
The other property is the IDF (Indentation force deflection) value of the foam. It is the amount of weight it takes to compress the foam by 25%. IDF values range from 30 (soft) to 70(firm). A foam with an IDF of 40 will take 40 pounds to compress by a quarter.
Indoor foam cushions
The best foam for indoor use is the affordable polyurethane foam. It does not handle moisture well, which is why it is better suited for indoor use. Regarding density, polyurethane foam ranges from 0.8 to 6.
Open and closed cell foam both work for outdoor use. However, for seat cushions, open cell foam is a better option. An open cell foam like Dry Fast foam is the best. According to The Foam Factory: Dryfast foam is specialized for outdoor applications because of its superb ability to allow water and other liquids to flow through it very easily.