How much active cooling is needed in Estonian multifamily apartment buildings to comply with Category II thermal comfort?

This paper examines summertime overheating and indoor temperature requirements for residential buildings in Estonia, comparing technical solutions needed to comply with current and forthcoming editions of regulations. Over the past decade, advancements in building energy efficiency have allowed thermal comfort to be maintained during hot summer periods by adhering to best practices and guidelines, relying on passive cooling methods. However, with the updated climate test reference year, Estonian local regulatory +26°C target temperature and < 150 K·h excess preventing overheating may become increasingly challenging. This change affects both new constructions and existing residential buildings undergoing major renovation. The study compares overheating risks under both regulatory editions using various room orientations and window-to-wall ratios. Additionally, it provides and discusses optimal strategies for selecting cooling source, cooling system and local room cooling units. For the first time, active cooling is becoming integral part in residential spaces in the Estonian construction market. Worst-case simulations of a single room without solar shading indicate that passive measures alone are insufficient, as the new test reference year meets summer temperature criteria only partially on the north façade, highlighting the need for combined passive and active strategies.