Window Ventilation in Multifamily Residential unit

Classic case of a LEED reviewer inventing new requirements. ASHRAE 62.1-2007 requires the windows be operable, it does not require that they be open. Your energy model does not need to include ventilation through operable windows. Push back on the reviewer, ask for a clarification. We have successfully fought this interpretation recently.

Christopher,
Was this the case when you did not have any other ventilation in a space, just natural? I assume if you only have natural ventialtion in a space, that should be modelled somehow.

If you have an exhaust, set the makeup flow through the windows equal to the exhaust. This volume flow rate is then your minimum ODA requirement and is set equal in both baseline and designcase models. Due to your exhaust, you actually have no natural ventilation...it's mechanical.

Let's not confuse "natural ventilation" with passive cooling. You meet the requirements of 62.1-2007 with operable windows. Appendix G only says that the ventilation rates should be identical between baseline and proposed. Model what you think will actually happen - if you think the windows will be closed all winter there is no requirement that you size your systems or model the building as though the windows will be open.

I aggree. The requirements of 62.1 are met and we're not talking cooling. So we make sure the fresh air rates are equal for baseline and design. However, I will stick one point: you can't make the fresh air rates null. This would reduce the total energy consumption. Then if you save only a little in another area such as lighting, it makes a bigger percentge savings. That's why we don't ignore process loads. So to avoid gaming, I would make sure to include justified ventilation rates. 
If you have no mechanical fresh air delivery system in the building and a bathroom or kitchen exhaust fan, the makeup air comes from outside, whether indirectly from adjacent spaces or through cracks or through windows. Energy wise this ODA is heated to temperature and consumes energy. Maybe not all in that zone, but you can account for it in the zone.
If I were a reviewer, this would be the minimum ODA I would expect to be accounted for... There are even better tools for the job: there are plenty of simulation tools dating back to BLAST or normed infiltration calculations, which are based on regressions from building data sets based on the additional energy consumption from envelope infiltration and window and door usage. ASHRAE doesn't say how you must take account of these "infiltration" effects, but it does say that you must include for them.

98% of the time what we see is a corridor ventilation system that balances the toilet exhausts. The designers have no actual intention of using the windows for ventilation - they are just there to meet code. I'm not saying this is a good design, but it meets all the existing code requirements, and to arbitrarily add additional ventilation would move the model further from reality. We'd model the corridor system as the only source of outdoor air. Of course we'd also include a reasonable number for infiltration. What we would NOT do is to set the infiltration equal to the 62.1 ventilation rate, which is what the reviewer is asking for.

Two quick notes - (1)ASHRAE 62.1-2010 fixes this problems - openings must be open during occupancy. So move up to v4.
(2) we have a client who has run blower door tests on some larger commercial (non-residential) buildings. The tests are consistently showing that even well constructed buildings have more infiltration than what us modelers generally use. We've found the same to be true when we've calibrated models on existing buildings. So us modelers are undercounting infiltration everywhere, not just on residential buildings with operable windows.

100% Agree. If the floor is mechanically fresh air supplied, this should per design be where the fresh air comes from for makeup. Infiltration is then to be assessed on its own then. Perhaps this was not clear to the reviewer and that is why they are asking for a description.
Goodluck.

Christopher,
You said you have successfully fought the interpretation about including the natural ventilation rates in the model.
We are getting the same comment from the USGBC. The corridors In hour building are unconditioned because are exposed to outdoor.
Can you share how did you answer this comment?
If I have to include natural ventilation to the model. Should I do it as infiltration? Should I follow the minimum rates shown in Table 6-1 (ASHRAE 62.1) even if mechanical ventilation is not applicable?

Section 6 of Standard 62.1 applies only to mechanical ventilation systems. A natural ventilated system should NEVER follow or meet the values in those tables. If it does, it is badly designed. In fact, I would compare a good naturally ventilated building to a demand controlled ventilation. It is good practice to include (natural) ventilation rates to reflect the natural ventilation that you have. It is not prescribed how to do this. INFILTRATION is not ventilation. It is regulated in ASHRAE 90.1 2010, so you could look there for guidance.

You should also note that the ventilation rates of both models must be equal (outdoor air minimums). This means that as soon as they are not, you are taking credit and that means an exceptional calculation which for LEED means proving sufficient comfort levels. Ventilation is not to be confused with cooling.

As a side note, I'm working on a paper that takes the adaptive comfort driven window opening algorithm (Humphreys window opening behavioural algorithm in ESP-r, P. Tuohy, H.B. Rijal, et al) to the next level. If it ever gets published I'll post a link on the forum.

Erika,
Did you submitted a CIR? or you just addressed the GBCI comment with your explanation?