Hi all, I have a project that seems only slightly different than many of the others I've seen posted here. However, it seems like a common enough issue that I can't imagine it's unique, and I'm hoping someone has come across it before.

The project is an apartment/condo building. Each residential unit has a small air cooled heat pump, and provides the maximum amount of ventilation air that is possible, per manufacturer recommendations.
Unfortunately, the mechanical ventilation only covers about 1/2 of the air that would be required for mechanical ventilation alone (ASHRAE 62.1-2007).
The residential units also have operable windows, but the openable area is not quite enough to assume the space is fully served by natural ventilation, either.
So we must assume the ventilation is met through Concurrent Mixed Mode ventilation, per CIBSE AM10 and AM13.
This is not something I've really seen before, but I think I'm ok with the design concept up to this point... And I think the description meets the requirements of ASHRAE 62.1-2007, IMC 2009, and LEED IEQp1.

My real question comes from the insistence of others that this qualifies for IEQc2, because, I'm told, it has been done in the past.
First; as I read through AM10, it appears that our single sided, single vented residential units can only claim natural ventilation up to 2x the ceiling height from the exterior wall: the condos have 10 ft ceilings, so natural ventilation serves the area up to 20 ft inside of the exterior walls. It would need to be assumed that the remainder of the space (open plan kitchen and foyer over 20 ft from the exterior) is ventilated by the mechanical system... In practice, I understand the idea that most homes are provided with much less ventilation, and the heat pump will circulate the OA. However, I do not intuitively see how this could meet the "letter of the law" requirements for Increased Ventilation.

LEED does not require the Natural Ventilation (Case 2) EXCEED any minimum levels, only that the design meet the recommendations of CIBSE AM10 or AM13.
Best I can tell, CIBSE AM10 only appears to be more stringent than ASHRAE 62.1 in the distance from the exterior wall: ASHRAE states a flat 25 ft, CIBSE states 2x the ceiling height (20 ft).
Reviewing the required area of window opening, it appears ASHRAE simply states that the openable area of the window/door must be at least 4% of the floor area served.
The buoyancy calculations in CIBSE AM10 appear to only require about 1/3 of what was required by ASHRAE for the prerequisite...
CIBSE AM10 was a little vague about the required air flow rates that should be used in the buoyancy calcs, so I assumed the airflow required by ASHRAE for mechanical ventilation. These air flow numbers turned out to be very close to the amount of ventilation required to maintain CO2 concentration levels per CIBSE section 4.5: Reservoir Effect.

Bottom line, ASHRAE appears to require 12 sf of openable window area for the prerequisite, and CIBSE appears to require 3.5 sf of openable window area for "Increased Ventilation"...?
I'm at a loss. Am I missing something?