(b) More than 50 percent of the combined surface area of its sides is covered by closed physical barriers. In calculating the percentage of side surface area covered by closed physical barriers, all solid surfaces that block air flow, except railings, must be considered as closed physical barriers. This section applies to all such enclosed indoor workplaces and enclosed parts thereof without regard to whether work is occurring at any given time. The term does not include any facility owned or leased by and used exclusively for noncommercial activities performed by the members and guests of a membership association, including social gatherings, meetings, dining, and dances, if no person or persons are engaged in work as defined in this section.