I cannot dry my futon in the sun in my house.

View of a typical urban residence in Tokyo with futon drying (facing south)

If your home does not have a balcony or sunny yard, or if the weather makes it difficult to hang your futon outside, please use furniture inside your home. 
The purpose is to air the underside of the futon. A well-ventilated area is even better. You can also place it in a place where the air conditioner heating wind is blowing.
High humidity is not good for futons, so always try to keep them dry.

Single size futon mattress