New couch cushions update the sofa, while sagging ones are a signal that the foam needs to be replaced. Choosing foam for new cushions is not difficult, once you decide how firm or soft you want the cushions to be, and you understand how foam is graded. The thickness of the cushions, the degree of firmness or softness, and the desired appearance of the finished cushion determines what type of foam is best for you.
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One measurement of foam is density. This is the weight of the foam per cubic foot of material, written, for example, as #30, which refers to foam that is 3 pounds per cubic foot. The density of the foam relates to the quality. The higher the density the longer the foam retains its ability to reshape after compression. Quality foam has a density of 3.1 to 3.3, or #30 and higher.
The technical term for the foam compression rating is Indentation Load Deflection, or ILD. This measure is the amount of weight that must be applied to foam to compress it by 25 percent. For soft foam, this is 18 to 30 pounds; for medium foam 30 to 50 pounds; and firm foam 50 to 85 pounds. Sofa seat cushions are usually in the 30 to 50 range. For softer cushions, choose a compression rating of 30, but not more than 40, for 4" to 6” foam. For firmer feeling cushions, choose a foam with a higher compression rating.
Foam cushion inserts are wrapped with a polyester quilt-bat-like material that softens the edges of the foam, and adds loft to the cushion. The wrap also protects the foam from disintegrating with the friction of the upholstery fabric. A cushion that has not been wrapped will have a hard-edged appearance and be less comfortable than one that has been wrapped.
A centerpiece of foam surrounded by foam chips and enclosed in a fabric case is an alternative to using one large piece of foam per cushion. Another alternative is a foam core with loose fiberfill in a fabric case. Both of these alternatives will produce a much softer, more down-like appearing cushion.