Building area method ASHRAE 90.1

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How does the building area method work?I just got a comment from the review team for a project pursuing LEED certification.
I used the building area method for the baseline case and in the proposed case I put the LPD from the lighting design. However they told me that the same method has to be used in both cases.
Any help would be appreciate.

Oscar B.'s picture
Joined: 2011-11-12
Reputation: 0

The building area method applies a uniform LPD to the entire building. If you do that in your baseline, you need to apply a uniform LPD to you proposed building too.
In general, I don't recommend using the building are method - the space by space method is a better approach.

Vikram Sami, LEED AP BD+C

Sami, Vikram's picture
Joined: 2011-09-30
Reputation: -1

as long as a lighting system has been designed:

according to ashrae/appendix g the uniform/whole building method applies
a uniform lpd to the BASELINE building only while the PROPOSED uses what
is designed - so long as the design is based on the whole building area
method. the lpd of the PROPOSED design should not be applied uniformly
to the PROPOSED building simulation.

see appendix g, table g3.1, section 6 lighting, subsection b, page 173
(2004), page 179 (2007). user's manual pages g-17/18 (2004 & 2007)

i would suggest to quote table g3.1 when replying to the reviewer's
comment. i have had reviewer's tell me i have to use the space-by-space
method in a simulation for both proposed and baseline buildings. this
is not correct. what is correct is that the simulation reflect the
methodology used by the lighting designer. if the lighting design is
based on the whole building method then the whole building method
maximum lpd is used in the BASELINE building. if the design is based on
the space-by-space method then the space-by-space maximum lpd for each
space type is used in the BASELINE building. in either case the
PROPOSED building should reflect what is designed. by 'what is
designed' i mean look at the lighting plans, lighting schedules, and
enter the lpd for each space/zone (thermal block) based on the number of
fixtures, watts per fixture, and square feet of space.

Patrick J. O'Leary, Jr.'s picture
Joined: 2011-09-30
Reputation: 200

I agree with Patrick that building-wide LPD should never be used for the
proposed design. In the simulation, the lighting power density is always
coupled with a schedule, thus improving lighting that is on 24/7 should
result in higher kWh savings than equivalent improvement to lighting that is
on for just a few hours a day. Using uniform building-wide LPD in the
proposed design makes even less sense for projects that have daylighting. We
never use building area method for the baseline model either, which is an
easy way to avoid review comments like the one that Oscar got. Building area
method makes sense for prescriptive compliance with 90.1, but it is
surprising that it made its way into Appendix G.


Maria Karpman's picture
Joined: 2011-09-30
Reputation: 0

Thanks for your comments.

I think the most sensible thing to do is change the Baseline mode to space by space method.


Oscar B.'s picture
Joined: 2011-11-12
Reputation: 0

Hmm, I think I'm on the fence here.

My practice is identical to Vikram's description for both energy modeling and when documenting compliance for my lighting designs: Choose whatever method you wish, but always use the same approach for baseline and proposed. This is pretty clear outside of Appendix G, when documenting compliance. For modeling, I don't use either approach predominantly - it depends on the project.

Since Patrick is pushing one side, I'll play devil's advocate =): I can affirm I've used "whole building" averaged LPD in proposed models for successful LEED submission without incident multiple times, documenting that clearly along the way, but I was using "whole building" for the baseline as well in each instance. I do not personally read 90.1 or LEED to explicitly require LPD be defined with space-by-space for a proposed model. Patrick, I just checked each of your citations and the only specific call for either method is when the lighting system has not been designed, in which case the whole building approach is prescribed. Keep in mind both methods should sum to the same total installed watts for the proposed design.

Back to the neutral perspective: I'll emphasis I do use both approaches.

To Oscar's case: My general experience has been the whole building method is less generous in net allowable watts when you run the numbers both ways. In other words, you may stand to earn more LEED points by making your baseline more detailed, using space by space. My suggestion for Oscar is to simply go with the reviewer's flow and possibly walk away with another point tucked under your arm... it'll probably be a similar amount of effort on your part relative to composing an opposing response, and you won't have to worry about the reviewer disagreeing =).

I agree space-by-space is 'better' for that reason alone - if different at all, it tends to yield a better performance rating. I will acknowledge space-by-space is also "more accurate," notably so if you're simultaneously defining distinct & accurate lighting schedules space-by-space, but whether the corresponding additional time investment and resulting "accuracy boost" are advantageous for a given LEED model is a toss-up. I personally feel the role accuracy plays in a LEED model is often overblown to a point of silliness, but that's a personal call we each need to make and a whole 'nother discussion.

Hot related tip: energy modelers and MEP designers alike need to be aware of COMcheck. I find it an invaluable time saver for speeding up takeoffs for whole bldg & space by space calcs, and it's only as costly as eQuest.


Nick-Caton's picture
Joined: 2011-09-30
Reputation: 805

actually nick, i agree with you. since i don't design lighting systems
i always ask the lighting designer which method they're using before i
start building a model. almost all of the lighting designer's i've
worked with use the whole building area method, though as you point out
the space-by-space method can actually do a bit better for energy savings.

my point is that there isn't a mandate to use either the space-by-space
or whole building area method and there is no justification as far as
90.1-200x is concerned for a usgbc reviewer to claim that the
space-by-space method be used. the requirement per 90.1 is that the
method be consistent in both the proposed and baseline models.
space-by-space in both or whole building in both. this is what i've had
to point out to reviewer's when i've received comments. just quote
chapter & verse to show that the method (space-by-space or whole
building) used meets the 90.1 app g requirement and is applied the same
in both models.

as far as comcheck, comcheck reports from lighting designers are only as
good as the individual filling them out. i've had lighting designers
(with all of their extra letters including pe after their names) fill
them out incorrectly. i.e. not having all the lighting fixtures the
same in drawing schedules as input into comcheck, not having same number
of fixtures in drawings and in comcheck, not having the same floor areas
in drawings and in comcheck, and worst of all, not using the same
methodology in comcheck that they've used to design the lighting system
in the first place. yes, i've seen comcheck reports that indicate
space-by-space when the lighting designer has told me whole building
method. and vice versa. so i always end up confirming my lighting
take-offs (from the lighting plans) and methodology with the lighting
designer/electrical engineer and their comcheck report.

Patrick J. O'Leary, Jr.'s picture
Joined: 2011-09-30
Reputation: 200

I'm laughing along with you over COMcheck woes - I've run into much of the same working with others' reports!

To clarify, and Patrick is absolutely correct on all counts regarding GIGO, the point I meant to raise is COMcheck is a tool that can assist, save time, and reduce human error risk for the competent energy modeler or designer interested in doing takeoffs correctly. It is not a substitute for code or design familiarity, but is useful in the context of supporting/supplementing takeoffs and referencing certain prescriptive req's.

Best regards,


Nick-Caton's picture
Joined: 2011-09-30
Reputation: 805

I have been following this thread and other than weighting in to say that
either method is acceptable, but the same methodology should be used for
both the Baseline and Proposed models (see Table G3.1#6 lighting for the
Baseline case).


With respect to your original question.If you have a mixed use building (say
a warehouse with an office area) and both building types are listed in Table
9.5.1, the LPD for each building type listed should be applied to the
associated area in the Baseline case. Assuming you told the reviewer you had
used the Building area Method in both models, you may want to see if this is
the focus of your comment.

Good luck,

Cam Fitzgerald

Cam Fitzgerald's picture
Joined: 2011-09-30
Reputation: 0

Building Area and Space-by-Space are not methods for designing lighting systems. They are prescriptive requirements for demonstrating lighting energy compliance in 90.1. The LPD allowances in Tables 9.5.1 and 9.6.1 do not need to be complied with if using energy modeling to demonstrate compliance for 90.1 and for LEED. (Only the Mandatory Provisions of 9.4 need to be met for the lighting design.) Energy modelers only need to know the lighting power and space use categorizations of the design as shown on the drawings (along with schedules and controls), not the process used to design it (which typically considers light levels in footcandles or lux).

I think that if a lighting system has been designed, a strong argument can be made that the space-by-space method needs to be used in both the proposed and baseline cases, and that lighting power needs to be entered individually for each space/zone.
"If construction documents are complete, the proposed building lighting system power is modeled as shown on the design documents." (ASHRAE 90.1 User's Manual, p. G-17)
"The LPD for the proposed design is taken from the design documents for the building. The LPD specified in the models must correspond to the spaces within each thermal block." (ASHRAE 90.1 User's Manual, p. 11-14 and also p. G-18)
The only obvious case for using the Building Area Method to determine modeled LPD is the case that Nick mentioned where lighting neither exists nor is specified.

As Maria Karpman, Nick and Patrick have mentioned, you are likely to show higher energy savings using the Space-by-Space method. Beyond that, using Space-by-Space allows you to give valuable feedback to the design team, which I would argue is a responsibility of energy modelers. It is routine for me to point out areas of potential improvement of the lighting design in every project I model, based on the allowances in Table 9.6.1. "Yes, Ms. Architect, that is a lovely looking light fixture, but 2.6 W/ft2 of lounge lighting is more than twice the baseline allowance." I don't know how you give helpful feedback if you are just comparing two building-averaged lighting power densities.


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Bill Bishop's picture
Joined: 2012-02-25
Reputation: 7

Classification: UNCLASSIFIED
Caveats: NONE

Do both and use the one that gets more points!

Jeurek's picture
Joined: 2010-10-07
Reputation: 0

Would think it's more about accuracy than points ?


Demba Ndiaye's picture
Joined: 2011-09-30
Reputation: 200
Pasha Korber-Gonzalez's picture
Joined: 2011-09-30
Reputation: 600

referencing this comment: "The only obvious case for using the Building
Area Method to determine modeled LPD is the case that Nick mentioned
where lighting neither exists nor is specified."

or when the lighting designer/electrical engineer has higher lighting
densities that exceed one (or more) of the space-by-space maximum
allowable lpds but compensates for it by having lower lpds in other
spaces such that the whole building lpd does not exceed the maximum
allowable by the whole building method.

Patrick J. O'Leary, Jr.'s picture
Joined: 2011-09-30
Reputation: 200

i am also reminded that there is a cir from 2007 that addresses how to
simulate the exterior lighting loads. tradeable surfaces are to be the
same as designed in the proposed model and the maximum allowable in the
baseline model. nontradeable surfaces though, per the usgbc cir, are
energy neutral & will have the same lpd in both the proposed and
baseline models. if i recall correctly without looking it up it should
be the nontradeable uses maximum 90.1 allowable in both
proposed/baseline models. need to check though.

Patrick J. O'Leary, Jr.'s picture
Joined: 2011-09-30
Reputation: 200


I agree with Bill that it is irrelevant for energy modeler whether
space-by-space or building area method was used to document compliance of
the lighting design with the energy code (see his discussion on mandatory
versus prescriptive requirements of 90.1). As a side note, when compliance
with energy code is documented using prescriptive path (for example via
ComCheck), exceeding LPD allowances for individual space types does not mean
that project fails to comply using space-by-space method, as you seem to
imply below. Space by space method does allow trade-offs between spaces,
because it compares the total specified lighting wattage for the entire
building to the sum of space-by-space allowances (see section 9.6.1 d). I
also agree with Bill that space-by-space method is the only way to provide
meaningful feedback to the design team. It also helps to catch issues with
LPD calculations, such as treating partial or temporary lighting in core and
shell spaces as complete lighting system, or failing to include unspecified
plug-in lighting in hotels into LPD calculations. Space-by-space method also
comes with a carrot of increased lighting allowances as described in 9.6.2.


Maria Karpman's picture
Joined: 2011-09-30
Reputation: 0

and maria is correct "Space by space method does allow trade-offs
between spaces, because it compares the total specified lighting wattage
for the entire building to the _sum_ of space-by-space allowances (see
section 9.6.1 d)." this is also noted in the user's manual in example 9-p.

Patrick J. O'Leary, Jr.'s picture
Joined: 2011-09-30
Reputation: 200

Patrick, the space-by-space LPD aren't mandatory requirements, so you'd be able to trade lighting power through space-by-space or BAM. Someone will have to perform the take-off either way to calculate the BAM weighted LPD for the Proposed case, so you aren't saving much time on the Proposed model. (Only saving the time to actually assign the Watts to the zone in the model.)

There will be a small time savings in Baseline model creation by not determining and entering space-by-space power usage into the model.

I agree with Bill that 90.1's wording about "...if a lighting design exists..." points me towards space-by-space if at all possible. But GBCI seems to accept both, regardless of possible BAM inaccuracies which as Nick pointed out may or may not be significant, so the main result of this gigantic thread is:

1. Use the same method in both cases.

2. Several people think 90.1 suggests space-by-space if the lighting system is designed.

a. Not always (ever?) enforced/requested by GBCI.

b. When the model is being used to inform the design or calculate incentives, this is the more accurate approach if there is variance in the spaces for control types, LPD values, and occupancy schedules.

3. Although BAM may not provide the most accurate predictions of energy usage, it may still be "legal" for EAC1 point calculation purposes.

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

David Eldridge's picture
Joined: 2012-05-08
Reputation: 1

David's summary looks pretty good to me!

I want to emphasis a great point that came up along the way: Space by space can result in more LEED points by 'padding the baseline,' sure... But "real" savings result from closely reviewing the proposed design, which in turn generates opportunities to identify specific means of design improvement.

I'll share a related strategy. Consider: Engaging the lighting designer and improving design doesn't require tallying the space-by-space totals. If I recognize significant improvements can be made with revised layouts, fixture reselection, and/or tweaked control schemes, I have found it VERY productive to simply share with the lighting designer (and design team leaders, if necessary) posed scenarios: "If you can reduce your installed watts by just 10%, the LEED models will earn 2 more LEED points." "If you define your astronomical timeswitch to shut off non-critical lighting after 2AM, the project earns a LEED point." Such 'carrot on a stick' proposals normally get the intended results with minimal friction, engage the designers in a positive way ("Hey, I just earned the easiest LEED point ever!") and performing the exploratory simulations to compose these proposals can be a lot less effort on the modeler relative to a standard-focused space-by-space analysis (I'm thinking of big buildings).

Hope that was illuminating!


Nick-Caton's picture
Joined: 2011-09-30
Reputation: 805

I hope everybody is having a great day! I came across this thread and a bunch of questions popped into my head. (Well,  I´m new to the energy modeling business so I always have questions I guess) Anyway, regarding Nick´s post, I would like to know: Why would someone bother modeling a building where the lighting systems has not been designed/specified? (Given that the building does need to have a lighting system in the first place).

[another question] In my case, I´m modeling  a building 7 stories tall with a 4 level basement for parking. This is an office building. Nevertheless, I wonder if Standard 90.1 (Table 9.5.1) contemplates the idea of having parking lots for an office building. That is, does it mean that the 1.0 value of LPD listed there considers a building made up of office spaces only?. Im inclined to use Building Area Method because this is a C&S project.(lighting layout is already defined all spaces, but we don´t know if tenants are going to stick to the type of luminaries that were recommended)

[another question] Following up on the previous one: ´´does it mean that the 1.0 value of LPD listed there considers a building made up of office spaces only?´´ If the answer is yes, then how should I go about finding the LPD value for my project? (baseline building)

[another question] As far I understand, Standard 90.1 is meant to be used to come up with the baseline building (among other goals). However, if we want to get more points for EA credit 1, one of the things we could do is reduce the energy consumption due to lighting systems. In that sense, we may have some type of luminaries that reduce the energy consumption, but at the same time yield a smaller value of LPD. Consequently, this new value of LPD is the one that must be accounted for in the proposed building simulation. That being said, how do I find the new LPD value for my proposed building case. Recall that I´d like to use the Building Area Method. (Sorry, no data of LPD can be found in the design documents. Long story)      

Thank you all for your time!



ibrid_alex's picture
Joined: 2012-08-21
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