An unconditioned space is an enclosed space that is neither directly nor indirectly conditioned. These are either (1) spaces that have neither a heating nor a cooling system or (2) spaces that are not cooled and have a heating system output capacity less than 3.4 Btu/h-ft2 (10 W/m2) and a cooling system with sensible output capacity less than 3.4 Btu/h-ft2 (10 W/m2). Examples include stairways, storage spaces, unoccupied adjacent tenant spaces, and attached sunspaces. Ventilated parking garages, attics, and crawlspaces are defined by Standard 90.1-2019 as unenclosed spaces and are not considered as unconditioned spaces. Modeling requirements for ventilated parking garages, attics, and crawlspaces are documented in the section below.
Unconditioned spaces shall be modeled if they are part of the permitted space. Permitted space includes all spaces in the building that are being analyzed for compliance with the use-case, which could be a beyond code program, a code compliance program or any other application which requires compliance with Standard 90.1-2019 PRM. All applicable envelope information shall be specified in a similar manner to conditioned space. If the unconditioned space is not a part of the permitted space, the space may be explicitly modeled or its impact on the permitted space may be approximated by modeling the space as outdoor space and turning off solar gains to the demising wall that separates the permitted space from the adjacent unconditioned space. The baseline envelope of conditioned, semi-heated, or plenum space adjacent to any other “unconditioned” enclosed space would be semi-exterior. Fenestration on these surfaces would be included in the fenestration area calculations for semi-exterior surfaces. For unconditioned spaces that are explicitly modeled, all internal gains and operational loads (occupants, water heating, receptacle, lighting and process loads) shall be modeled as designed if known or as specified in COMNET Appendix B (COMNET 2017) if unknown.
Return air plenums are considered indirectly conditioned spaces and shall be modeled with equipment, lighting power, and occupant loads at zero. Where recessed lights are used, heat from lights can be modeled to be transferred to the plenum.
Indirectly conditioned spaces can be either occupiable or not occupiable. For spaces that are not occupiable (such as plenums), lighting, receptacle, and occupant loads shall be zero. For indirectly conditioned spaces that are occupied, such as retail spaces, portions of restaurants etc., these loads shall be as designed. Indirectly conditioned zones will not have thermostat setpoint schedules. The allocation of zones into conditioned, indirectly-conditioned, and unconditioned zones shall be the same in baseline and proposed building models.
Unconditioned spaces may not be located in the same thermal zone as conditioned spaces. Conditioned spaces and indirectly conditioned spaces may be located in the same zone; when this occurs, the indirectly conditioned spaces will assume the space thermostat schedule of the conditioned space.