The article and discussion found in the article referenced by @ThreePhaseEel indicates no code reference specifically to party walls. However, all major codes reference energy compliance for "Dwelling Unit's Walls". I think the article is misleading when it implies there is no energy code requirements for "party walls."
Yes, the code is very specific for fire separation for "party walls" in multi-family structures. (By the way, as of 2014 Code, fire sprinklers are also required for 3 units and more.)
However, all party walls have "sound wall" requirements. I'm not sure what your party wall's sound transmissions class (STC) is, but I'm sure the brick wall does not meet the required "R value" for walls in a "dwelling unit"...as required by all codes.
We try to "kill two (or three) birds with one stone" and use thermal insulation in a party wall design to meet sound wall requirements and fire wall separation requirements.
So, I'd suggest use the same thermal value in your party wall as your exterior walls to meet the energy requirements for dwelling units.