Parking Garages? To model or not to model?

He all

I am wondering if folks have any particular thoughts on the ‘right’ way to handle parking-garages under buildings? In particular, do you think we should we be modeling the garage area and setting it to ‘Unconditioned’, or should we simply leave out that zone and model only the conditioned areas of any garage floors (stair cores, etc), then set any ‘exposed’ surfaces to a normal exterior-air BC?

I note that in ASHRAE 90.1 Section 3 it defines:

unconditioned space:
“An enclosed space within a building that is not a conditioned space or a semiheated space. Crawlspaces, attics, and parking garages with natural or mechanical ventilation are not considered enclosed spaces.”

which to me, seems to indicate that we should not model it as a ‘space’?

As an example, if I have a large subgrade parking area under a multi-unit residential bldg with some access stairs / cores such as:

I’m not sure what the extents of the rooms/zones should be in this case. Right now, I am modeling the entire parking area and simply tagging the parking rooms as ‘unconditioned’ when I create the Honeybee Rooms. So then I get Surface Type conditions like:

and then BC conditions of:

it seems to me that including these parking zones as ‘unconditoned’, then creating / applying a ‘parking’ program for lighting, ventilation (fans) etc. would be the appropriate method? But - I also wonder if doing it that way might undercount the heatloss through the floor above the garage (floor between the garage and the residential units above)?

Just wondering any thoughts folks have on the ‘right’ way to handle these types of zones / conditions? Any input is much appreciated.


Hi @edpmay,

I do not know the exact conditions of the parking garage. If it is well ventilated and you could use natural ventilation (you will need some operable windows) and set the room to unconditioned. If the floor between the apartments and the garage is well insulated, the difference between the two options wil not be that big. When a floor is regarded as a groundfloor the outside temperature will be set to 18 degrees. If you want to mimic outside temperures in the garage you can move the building a bit upward.

Thanks @Erikbeeren

Thats helpful. I was diving into some other guideline docs and I think I’ll move forward ‘not modeling’ the Parking as a ‘space’ and just setting all the floors as exposed to outdoor air. In the 90.1 Section 11 and Appendix G Review Manual V01b they seem to state pretty clearly that for figuring out U-Values / Assemblies at least:

so seems that it would make sense to keep those as ‘exterior’ surfaces for the model as well.

thanks for the input!

This is one of those “Art of Zoning” questions and, as the word “Art” implies, there’s a lot left up to interpretation.

Modeling the whole heat exchange through the garage is probably a bit truer to physics but no one is going to fault you for not including it and, for ASHRAE 90.1 purposes, I think it’s definitely acceptable to leave it out. After all, ASHRAE 90.1 is probably going to make you put a good amount of insulation in the exposed floor slab so Erik’s criteria would probably be met.