The partition is sheathed solid with no air path through it, leaving one space of 900 square feet and another of 750 square feet. The governing ratio is 1 square foot of net free ventilating area for each 150 square feet, and all 1,584 square inches of installed vents open into the larger space. What is the finding?
- AIt complies, because 1,584 square inches covers the 1,650 square feet of attic area in the house
The total is right and the distribution is not. Air does not cross a solid sheathed partition, so vents on one side do nothing for the space on the other.
- BThe smaller space needs 864 square inches of its own, and none has been provided for it
That figure is what the 900-square-foot space requires. Applying the larger space's requirement to the smaller one overstates it and shows the two areas were not kept apart.
- The smaller space needs 720 square inches of its own, since each separated space is ventilated on its own
- DThe smaller space needs 360 square inches, because a space under 1,000 square feet may use the reduced ratio
No provision reduces the ratio because a space is small. The reduced ratio depends on where the vents are placed or on a vapor retarder, and neither has anything to do with the size of the attic.
Why C is correct
Each enclosed attic space is ventilated on its own area. The 750-square-foot space requires 750 divided by 150, or 5 square feet, which is 720 square inches of net free area, and nothing has been provided on that side of the partition.
What this question is testing
Whether the candidate recognizes that a separated attic space is figured and ventilated independently, and computes the requirement for the space that was left out.
On the job
Separated attic spaces show up at party walls in duplexes and townhouses, over attached garages, at fire-rated separations, and wherever a bonus room's knee walls close off the space behind them. Each pocket has to breathe. The classic failure is the small attic over a projecting wing or a porch roof that is sealed at the main roof and given no vents of its own, which shows up years later as rusted fasteners and stained sheathing.
Memory technique
Air does not go through a wall to get to a vent.
Exam tip
Count spaces before you count vents. Solid partitions make two attics.
Where to look it up
IRC Chapter 8, roof ventilation, the requirement for enclosed attics and rafter spaces.