The situation is like this:

4-story office building (+20C indoor) with two underfloor levels of parking where +5C temperature is maintained during winter by means of ventilation. 

To comply with ASHRAE 90.1 should we advise architects to consider making U-Value of the first floor (over the heated parking garage) as for Mass Floor in Table 5.5-1 (Building Envelope Requirements)?

 It is mentioned in 90.1 description of "unconditioned space" that "...parking garages with natural or mechanical ventilation are not considered enclosed spaces" and therefor can't be considered being inside the envelope.

Architects argue that since the parking garage is heated then the floor over the parking can't be considered an envelope.

My assumption is that U-value between two heated spaces won't make a difference energy-wise and therefore shouldn't be considered an envelope. No additional heat losses are transferred to the environment through the floor over heated underground parking garage whether they are insulated form one another or not.