For outdoor back cushions we recommend our outdoor (reticulated) dry-fast foam, usally in the “soft" firmness. If your back cushions will be less than 3 inches thick, you might want to consider medium firmness. Our outdoor foam has an open cell structure, which allows water to pass through the cushion rather than sit inside it, reducing the creation of mold and mildew. If your outdoor back cushions will be in an area not directly exposed to the elements (a covered porch or cabana), then traditional polyurethane foam or organic latex foam can be used.
FoamOrder recommends dacron batting for outdoor back cushions, as well as other outdoor cushions. The dacron batting doesn’t interfere with the drainage of outdoor cushions, and gives your back cushions a fuller look and a bit of extra comfort. It also makes it easier to get your foam into and out of the covers, which is helpful if you’re going to wash your cushion covers (which we recommend).
Back cushions don’t bear the full weight of your body (like a seat cushion), so they can often be a bit thinner than seat cushions. Most importantly, if you’re replacing existing outdoor back cushions, they should be the same thickness as the existing cushions to properly fill the slipcovers. If you’re ordering new cushions, they should generally be at least 2–3 inches thick to provide proper support.