Popular design approaches to develop ambient or artistic digital artifacts such as speculative design have revealed various serious problems hidden in our modern convenient life. Such digital artifacts developed based on speculative design appeal to audiences in appropriately curated museums but are difficult to deploy in our everyday lives. This is because the users of the artifacts may not understand the expected meaning of how to use the artifacts, and the design approach does not offer a guideline to explicitly design the roles of the artifact explicitly in our daily lives. This paper proposes a design concept for developing ambient digital artifacts based on the lens of postphenomenology and sociomateriality. The affordance concept in the proposed approach is defined on two levels: agential affordance and interactive affordance. Each affordance is examined through the key concepts of postphenomenology and sociomateriality: material agency, multistability, and transparency. We discuss the benefits of the proposed concept based on case studies to analyze three ambient digital artifacts. The analysis reveals some insights for developing the semantics of better future ambient digital artifacts.