Gross vs. Conditioned Square Footage

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Per 90.1, G3.1.1 the baseline system shall be based on... number of floors, conditioned floor area... etc.

I have a 5 story building with a total gross square footage of ~153,172 square feet. I received a LEED review comment (LEED 2009) that states the baseline system should be system 8 rather than system 6 (as I have modeled). The SVA report indicates the floor area as 153172 square feet, but the LSC report indicates the conditioned area as 144775 square feet for calculations. Supply/return shafts, elevator shafts, etc are excluded from conditioned floor area as well as wall thicknesses. Is the use of system type 6 appropriate since the conditioned floor area is less than 150,000 square feet or, as the comment suggests, should the baseline system be changed to system 8 since the gross area exceeds 150,000 square feet? Changing the system type will substantially impact the savings as the proposed system is a packaged rooftop unit rather than a chilled water plant.


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Bernie Hont's picture
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Hi Bernie,

In my opinion, the reviewer is correct. While table G3.1.1A does not explicitly use the term ?gross square footage,? the left hand column says Building Type. There is no reference to heated/cooled square footage, only to building square footage. Perhaps this could be clarified in future 90.1 releases, but I believe that gross square footage is implied.

Best to you,


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Dave Weigel's picture
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Hi Berine

I think you are right you could argue that according to 90.1, G3.1.1 the
baseline system shall be based on? number of floors, conditioned floor
area? etc
Also in 90.1-2007 User Manual pag G-22 said:

For nonresidential building the baseline building system type depended
not only on the heating source for the proposed building, but also on
the number of floors and the total CONDITIONED floor area of the building.

I would show in a drawing the areas that are excluded (not conditioned)
to the reviewer.

Good luck.

MSc Oscar Buchely
Ingeniero de Proyectos
Soluciones Energ?ticas Sostenibles
Bogota - Colombia
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Oscar Buchely L.'s picture
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Following paragraph is from page G-22 of the ASHRAE 90.1-2007 User?s Manual.

?For nonresidential buildings, the baseline system type depends not only on
the heating source for the proposed building, but also on the number of
floors and the *total conditioned floor area* of the building.?

Base on the ASHRAE language I think your system selection for the baseline
is correct.


Morteza Kasmai's picture
Joined: 2011-09-30
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Just to further clarify, you?ll need to explain to the reviewer why SV-A has a larger area than LS-C. You may want to include a corresponding conditioned area total as noted in the architectural plan as supporting documentation ? after all, it isn?t really what the area is in model space that is the determinant, it is the actual conditioned area of the proposed design.

Although the LEED reviewers tend to take some flak from time to time, I don?t think they were necessarily off-base to ask you about the area when they saw the larger value in SV-A. If the conditioned area as designed was included in the other credit uploads, you can probably point the reviewer to that document. (Hopefully it is equal/close to 144,475 sf.)

There might be some debate about whether elevator shafts are conditioned or not, as these receive cooling indirectly from the rest of the building, and the area of the elevator cab takes the air directly from the rooms it opens up towards, if not having a small packaged A/C unit mounted on the cab.


David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
Grumman/Butkus Associates

David Eldridge's picture
Joined: 2012-05-08
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David makes a good point about elevator shafts being conditioned spaces. Per the space definition in 90.1, an indirectly conditioned space is considered conditioned. It doesn?t have to be served by an HVAC system, or heated or cooled. It just needs to be ?enclosed? and have greater heat transfer to the interior of the building than to the exterior, based on wall area and U-factor. I generally interpret this as meaning that everything within the building envelope is conditioned space, except crawlspaces, attics and parking garages (per the definition for unconditioned space), and non-heated/cooled spaces along the exterior of the building that have insulated interior walls (like unheated storage and garages). Indirectly conditioned spaces are easy to model in eQUEST. Just define the space/zone as ?conditioned? but don?t assign thermostat schedules, and zero out the air flow.
I don?t see an allowance for interior wall thickness in 90.1, so I think that should be included in ?total conditioned floor area? as well. There is no definition for floor area, net in 90.1, only floor area, gross.


William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
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Bill Bishop's picture
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The definition of conditioned square footage is interesting and I wonder how it would apply to a medical facility that I performed some HVAC commissioning for.

The building housed some sort of MRI or CAT scan type machine that needed some intense shielding. The exterior walls and roof were a minimum of 3 feet reinforced concrete (some of the walls were 5 feet thick!). Additionally, there were interior walls with concrete 2 to 3 feet thick. If you took the net habitable area, it was somewhere in the neighborhood of 60% net.


Jim Paulino's picture
Joined: 2012-03-15
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If the facility had to be benchmarked in that way, there might be justification to use the midpoint of the walls.

However this facility may not be a great fit for using area at all, if the whole occupied area is the imaging suite.

The client will have a better idea if the facility is area dominated or process donated after they have operated for a few months.

If they figure out that area is appropriate, then there still may not be a useful peer group, so the main issue will be tracking against their own facility over time.

Net or gross in this case is only important if a comparison is made to a similar facility.

Let us know how it turns out. This building could be a useful case study for energy use by a standalone imaging facility.

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David Eldridge's picture
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