Thermal mass is essential in passive solar spaces designed to store heat. Given the diversity of climates with useful cool-season sun, climate-responsiveness in thermal mass design might be expected; however, rules developed in the dry, sunny American Southwest dominate teaching and practice throughout the country. Evidence from the UK, Alaska, and western Oregon now suggest that conventional thermal mass rules require substantial revision for rainy, cloudy climates. To address this issue, we here employ a series of field-validated Pacific Northwest sunspace models to quantify limitations of conventional thermal mass design in the region and to reveal more suitable parameters with respect to the sizing and ground configuration of floor-based thermal mass. Results favored thermal mass in far smaller quantities, and with much-reduced ground contact, than specified by conventional rules, with optimal parameters varying by design priority: daytime warmth, evening warmth, or early-morning warmth. A subsequent field test confirmed model predictions and elucidated underlying mechanisms, supporting specific revisions of contemporary passive solar design guidelines for the Pacific Northwest and related West Coast Marine climates. (c) 2015 Elsevier Ltd. All rights reserved.