The Internet-of-Things, which designates the interconnection of numerous physical devices, is a growing research direction faced with many challenges. One of these challenges is to provide constant quality-of-service despite IoT devices being used in a constantly changing physical environment. In order to answer this problem, we introduce a quality-of-service driven self-adaptation framework, which can simultaneously handle changing adaptation strategies, monitoring infrastructure and physical environment while guaranteeing constant quality-of-service. Because of its formal guarantees, our system is particularly suited for the control of critical IoT-based systems, and we thus demonstrated its practicality by applying it to an e-health case-study where the safety of the monitored patients must be assured.