Sustainability communication has become a top priority for B2B corporate communication strategies. The building sector, in particular, faces a challenge in effectively conveying their environmental responsibility amidst the saturation of sustainability messaging and the public's raised expectations. To establish a positive reputation and cut through the noise, companies in this industry must convey credible environmental messages. The primary scope of this paper is to investigate environmental communication in the building sector, with a focus on the communication process between building material suppliers and architects. The study discusses the central concept through the lens of signaling theory to understand the formation of environmental communication messages (signals) and the impact of communication context (signaling environment and information asymmetry) on stakeholders' perceptions of a firm's environmental impact. The thesis contributes to Corporate Communication literature and expands the literature on Corporate Social Responsibility by seeking responses to the B2B sustainability communication literature gap. The research problem and questions were the guiding factors in selecting this study's research interpretive philosophy and qualitative methodology. The study collects primary data from 13 expert interviews in two phases: Phase 1: Four expert interviews of communication decision-makers in a Finnish building material supplier (case organization). Phase 2: Nine semi-structured interviews with Nordic architects (targeted stakeholder group). The finding of this study demonstrated that the observability and qualities of costly environmental signals are critical drivers behind persuasive communication messages. Secondly, the study reveals the role of signaling environments and information structure on the environmental messages' credibility. Finally, the study presents the signaling process between suppliers and architects to offer practitioners insights into transmitting impactful sustainability messages to architects.