The best overall choice to satisfy most indoor furniture reupholstering needs is to go with a medium open-cell foam, for heavier chairs such as those made of wood, chairs used in a dining room or for more "cushiony" chairs such as those used in a living room. The foam should have a density of 2.1 or higher and accommodate a compression weight of at least 34 lbs. If the chair is lighter, like a kitchen chair, a light, open cell foam with a density of 1.0 and a compression capacity of 30 lbs. is the best choice.
Upholstering a chair for outdoor use by pools, patios and garden spaces requires additional considerations. This will be especially true for chairs placed outdoors in humid climates and areas that experience heavy rains. Mildew will be the most common threat. As a result, the best choice is an open cell foam that has a higher density of 2.0 or more and a compression of 55 lbs. This will enable the seat cushioning to accommodate and expel water, and be wrung out to dry quicker, with minimal loss of resiliency.
Open cell foam is also the best choice for upholstering outdoor and marine chairs. The caution is to be sure that the foam has the correct density and compression capacity and, most of all, that the foam is mildew resistant. The best cell foam choices should have a density of 1.5 or higher to reupholster common outdoor garden furniture seat cushions, and 2.0 and higher for cushions used on boats. Compression is also a key consideration for upholstering seating for boats; the best choices should have a weight capacity of 44 lbs. If the chair will have a dual purpose to serve as a flotation device, the best foam should have a high density of 4.0 or more and a weight compression of 25 lbs. (lower than the compression weight for "land" or "on-board" use) to enable the upholstered seat cushion to float in the case of an emergency.
High resiliency foam is best for upholstering chairs that will be used frequently, as in public places such as restaurants. High resiliency foam should have a density of 2.85 or higher and a compression of 55 lbs. or more. Look for high resiliency foam that is also fire retardant for upholstering chairs that will be used frequently by the public. Additional considerations are mildew resistance and antimicrobial capabilities for upholstering chairs used frequently and by public patrons and business customers.