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. In particular, warm air production during daytime hours, for delivery to adjacent spaces, requires minimal mass: although greater mass facilitated slightly greater warm-air production in sunnier spaces in late-spring months, thermal mass generally counteracts the purpose of heating adjacent spaces. For evening warmth, 5.1 cm thick floor mass is optimal, with performance increasing markedly from the lowest mass to the optimum and diminishing gradually as thickness increased. Early-morning warmth, in turn, requires greater mass, with each increment of additional mass over 7.6 cm, yielding progressively smaller benefit. In addition, it has been found out that thermal mass at the levels recommended by conventional rules, including both massive floors and massive common walls, is oversized by a factor of two or more for the Pacific Northwest. Finally, conventional perimeter insulation is not an effective strategy to limit a massive floor's heat loss to moist soils. Instead, mass in solar-collecting floors must be fully isolated from moist soils through insulation, drainage, or internal position: otherwise, the combination of warm solar-heated mass in contact with high thermal diffusivity soil creates a significant heat loss pathway.