While it might be tempting to use your comfortable indoor cushions on outdoor furniture frames, this is generally a poor decision that can lead to ruined cushions and potential health hazards. Indoor cushions are not designed to withstand the outdoor elements. They lack the critical weather-resistant properties of purpose-built outdoor cushions.
The primary issue is moisture. Regular indoor cushions will absorb rainwater and humidity like a sponge. This leads to several problems: the growth of mold and mildew inside the cushion, which is difficult to remove and poses health risks; the development of a persistent, musty odor; and the potential for the cushion filling to rot and clump. Furthermore, the fabric of indoor cushions fades quickly in direct sunlight and is more susceptible to tearing.
Instead of risking your indoor decor, invest in proper outdoor cushions. These are crafted from solution-dyed acrylic or polyester fabrics that are water-resistant, UV-protected, and mold-resistant. Their fillings are typically made from quick-drying foam or fiber. For a temporary fix, you can use indoor cushions outside but only for a very short period in perfectly dry conditions, and you must bring them inside immediately afterward. For long-term enjoyment and peace of mind, always choose cushions specifically rated for outdoor use.