An inspector is reviewing a proposed new single-family dwelling. The plans include a habitable attic space. What is the minimum required aggregate area of operable skylights or windows for natural ventilation in this habitable attic, assuming the floor area is 400 square feet?

Correct answer: C. 20 square feet

IRC R303.1 requires every habitable space to have natural ventilation provided by openings to the outdoors equal to not less than 4 percent of the floor area served. For a 400 sq ft attic, 4% of 400 is 16 sq ft, but the openings must be operable. Operable area for ventilation must be at least 4% of the floor area. The question asks for the minimum aggregate area of operable skylights or windows, which is 4% of the floor area. 400 sq ft * 0.04 = 16 sq ft. However, the options are in increments of 5. The closest and minimum correct answer from the options is 20 sq ft, as 16 sq ft is not an option and the next higher increment would be required to meet the 'not less than' clause if options were exact. Re-reading IRC R303.1, for natural ventilation, the 'openable area' shall be not less than 4% of the floor area. Since 16 sq ft is not an option, and the question asks for the minimum, an answer of 20 sq ft would satisfy the requirement of being 'not less than' 16 sq ft. Let's re-evaluate the question and options. The question asks for 'minimum required aggregate area of operable skylights or windows'. The code states 'not less than 4 percent of the floor area served'. For 400 sq ft, 4% is 16 sq ft. Since 16 sq ft is not an option, the smallest option that is 'not less than' 16 sq ft is 20 sq ft.