90.1 Appendix G: Daylight Utilization

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Building-Sim users,

Perhaps somebody could shed some light on this issue for me.

In 90.1 Appendix G, Table G3.1, Section 6.f. states that credit can be taken in the proposed building for automatic controls due to daylight utilization. However:

In order to know how much of an effect daylight should have on the power used, a target luminance/lux level is required. If this target is met by daylight alone, the lighting consumption would drop to 0 (except for emergency lighting). Where daylight falls between 0 and the target lux level, lighting power can be scaled linearly.

The documentation gives no target lux levels and they cannot be derived from the baseline lighting power densities (from 90.1 Sections 9.5.1/9.6.1) without knowing the efficiencies associated with these values.

The only related luminance levels available are from the IESNA Lighting Handbook but these levels do not seem to sensibly correlate with the power densities of 90.1 Sections 9.5.1/9.6.1. Take, for example, a drafting office. The IESNA Lighting Handbook Section 10, Quality of the Visual Environment, suggests a lighting level of around 100fc (1076 lux). Achieving this 100 fc with an LPD of 1.1W/ft? (11.83W/m?), i.e. that quoted for an Open Plan Office, requires a lamp with an efficiency of approximately 91 lumens/watt. Using the same Lighting Power Density, and illuminance of 30fc for simple office tasks, we require lights that produce 27 lumens/watt. This gives us two spaces that are classified identically, but are subject to massively different limitations when it comes to selecting lamps.

Could somebody please provide some clarification on whether it is wise to assume these lux levels while performing a Building Performance Method simulation, bearing in mind that it could affect results in an unfavourable way if lux level is overestimated (as LPD will be constant in the baseline buildings).


Jonathan Catterall

Jonathan Catterall's picture
Joined: 2011-10-02
Reputation: 0

To further complicate things, the target footcandle level of the daylight harvesting system will vary depending on whether the space uses tasks lights or not. With task lights, the overhead lighting system can be configured to provide an even lower target footcandle level. I don't know the correct answer, but I've always used 30 footcandles for the target lighting level in typical office projects (most of them have task lights). If it's a GSA project, where they are very specific about the required workspace lighting levels, you might need to use predefined owner requirements for the target levels...

James Hansen, PE, LEED AP

James Hansen's picture
Joined: 2011-09-30
Reputation: 200

Jonathan, welcome to our world. To shed some light on the issue - this is a
"gray" area that is often taken advantage of when conducting energy
analysis. Looking at the IES handbook some lighting levels for particular
rooms range from 50 to 100 fc. This seems ridiculous but lighting is
perceived. It is not a hard science, factors like interior colors, occupant
age and health (glasses ? no classes), etc all contribute to what is an
appropriate lighting level. When conducting an energy analysis engineers
need to be reasonable and set parameters for their buildings that are
within the IES range and correct for their space's use. The engineer should
also have an idea of the interior colors, reflectances, demographic of the
occupants to be using the space, know if shading will be a factor in the
facility (internal or external) etc..

If I know a space will be brightly light (because I asked the electrical
engineer on the project) I'll use the higher level. If I know they will
have task lights, I'll use a lower level. If the space has some task lights
and I am not certain of parameters, I'll use a median value. I'm sure there
are engineers that are conservative and use the upper extreme, and others
that are very loose and want to earn more energy credit and thus will use
the lower extreme. A good rule of thumb when designing parameters for the
programs we use would be give us options. Maybe call them "loose" "median"
and "conservative" and give us a list of all the room types in the IES
handbook. I use Trane Trace and Revit, not Bentley but with Trace I wish
they had built in templates from IES, from ASHRAE, and from the AIA, for
lighting, for airflow, templates, outdoor air, etc.. it would make
conducting energy analysis a lot faster. Because this was not done by trace
or Revit, we've had to build them our selves as we work on projects. If a
software package designed for the US market had these built in (and were
accurate) it would be a huge selling point. The amount of time it takes to
build in a standards library is substantial. Trane has jumped on board
recently and released ASHRAE 90.1 equipment that meets the baseline
standard, however it does not have room templates or lighting level
templates there yet.

A question - does your energy analysis software interface with GBXML? We're
in the market for new - more accurate - software. Specifically Trace does
not do a building level air balance, and when you are using total energy
wheels your going to either over shoot or under shoot your energy savings.
This is a substantial deficiency when your building has 12+ air handlers
and is located in a very hot humid climate.

Good luck.

Shariq_Ali at URSCorp.com's picture
Joined: 2011-10-01
Reputation: 200