The bedroom measures 11 feet by 12 feet. With the lower sash raised, the clear opening measures 30 inches wide by 26 inches high. The framed rough opening for the unit is 34 inches by 62 inches. Natural ventilation requires openable area of not less than 4 percent of the floor area, and no mechanical alternative appears on the approved plans. What is the finding?
- AIt fails, because 780 square inches is 5.4 square feet against the 10.6 square feet that this room requires
The 8 percent figure belongs to natural light, not to ventilation. Applying it here roughly doubles the requirement and turns a complying window into a failure.
- BIt fails, because the openable area has to equal the glazed area, and the sash exposes only half of the glass in the unit
Nothing requires the openable area to match the glazed area; a double-hung opens half its glass by design and the code accounts for that by setting a lower percentage for ventilation than for light.
- CIt complies, because the rough opening of 34 by 62 inches gives 14.6 square feet, which is well over what is required
The rough opening includes the frame, the sash stiles and the fixed upper sash, none of which pass air. Measuring the hole in the framing rather than the clear opening is the most common way this calculation goes wrong.
- It complies, because the 30 by 26 inch opening is 5.42 square feet against the 5.28 square feet required
Why D is correct
Required openable area is 0.04 times 132 square feet, or 5.28 square feet. The sash leaves 30 by 26 inches, which is 780 square inches, or 5.42 square feet. The opening exceeds the requirement, so the room passes on natural ventilation.
What this question is testing
Whether the candidate measures the clear sash opening rather than the rough opening or the glass, converts square inches to square feet, and applies the ventilation percentage rather than the light percentage.
On the job
Openable area is measured at the opening, which means an inspector raises the sash and puts a tape across it. Casements deliver nearly their whole glazed area, double-hungs deliver about half, and horizontal sliders deliver half as well. Awning units in basements are the ones that surprise people, since the sash swings out into a well and the clear opening can be much smaller than the glass.
Memory technique
Rough openings frame walls; clear openings move air.
Exam tip
Measure what opens. Then divide square inches by 144 before comparing.
Where to look it up
IRC Chapter 3, light, ventilation and heating, the openable area required for natural ventilation.