2006 IRC illustration M1703.2(2) shows the two vents and outlines the space between the top of the top vent and the ceiling, and the space between the bottom of the bottom vent and the floor, and for both of them it says less than or equal to 12 inches. So the top of the top vent can be a maximum of 12 inches from the ceiling and same for the bottom vent and the floor, so, in fact, the entire vent(s) can be just outside of the 12 inch space. At least, that's how it looks to me.

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