ASHRAE Journal - October 2013 - 80

technical feature

the construction teams can take them
table 4 Sample building envelope design criteria.
all into consideration. The sample
Item
DesIgn crIterIa
HVAC criteria here are based on stan• Fenestration percentage. The percentage of glass on the building’s exterior shall be 40%.
dards adopted by Minneapolis.
• Window specifications. All exterior windows shall be specified to help prevent unwanted solar
heat gain in summer, while still harnessing direct solar radiation in winter. Visible light transmitWhile the OPR might generally
Windows
tance (VLT) shall be at least 0.75.
state that “the building will meet
• Insulating value. The U-value of all exterior windows shall be at least 0.55.
all applicable ASHRAE standards,”
• The solar heat gain coefficient (SHGC) of all exterior windows shall be 0.40.
the BOD must be more specific—so
• Interior wall panels shall be 5/8-in.-thick gypsum board over metal framing (16 in. on
the designers know exactly which
center).
Walls
• Exterior curtain wall assemblies shall be double-pane insulated and fully climate-appropriate.
code to follow in their designs. As a
• Insulating value. The maximum U-value of all exterior walls shall be 0.032.
result, the BOD might state that the
building “shall meet or exceed ANSI/
• Framing. The roof shall be metal-framed.
• Overlayment material shall be ¼-in.-thick protection board.
ASHRAE Standard 62.1-2010.”
• Moisture barrier shall be either white thermoplastic polyolefin (TPO) membrane or GalRoof
By specifying the exact codes and
valum® deck.
standards required, the BOD identi• Insulation shall be entirely above deck with continuous layers of open-cell foam as needed to
achieve a minimum rating of R-30.
fies how the design team will meet the
owner’s requirements. In addition, a
careful delineation of codes and standards not only helps
only minimally addressed in the previous article, that
prevent “cutting corners,” but also helps answer questions
doesn’t mean it shouldn’t be a key element of the BOD.
about why something was designed in a certain manner.
As with every aspect of the BOD, defining the envelope
HVAC design. The BOD must explain how the buildcriteria must be a team effort. For example, a key enveing’s HVAC systems will be designed in accordance
lope design element is establishing the sealing levels
with the project RFP, the OPR, and all applicable codes,
that will achieve the owner’s desired infiltration rate—
standards, and regulations—as well as any other essenwhich is data that should come from the architect. Although a
tial design criteria (which should also be described in
mechanical engineer or energy modeler can evaluate
the BOD). For example, the sample OPR in the previous
the performance of various building materials, they may
article called for “total energy consumption of less than
not understand the physical or budgetary impacts of (for
70 kBtu/ft2 per year.” The BOD must explain how the
example) changing a roof’s insulation specification from
components will meet this requirement (Table 1).
R-20 to R-40 (see Table 4 for sample criteria).
Indoor and outdoor design. The OPR should include
Utility baselines. The BOD should include projected
detailed charts that describe the owner’s expectations
monthly utility baselines for electrical, gas, and water
for the building’s interior and exterior environments
usage. These baselines—which you can extract from the
(temperature, humidity, CO2 levels, etc.). The BOD must OPR—will provide a starting point for whatever monitoring
restate those considerations and refine them as needed. tool the owner wants to use to track resource consumption.
For this series we’re assuming our sample building
Additional energy-usage factors. The BOD must also
is being constructed in Minneapolis; so its indoor and
address any other systems or equipment that will affect
outdoor design criteria are based on ASHRAE data for
the building’s energy use, such as loads for occupants,
Minneapolis (see Table 2, Page 78, for sample criteria).
miscellaneous equipment, and any specialty areas such
Lighting and electrical systems. The previous article
as kitchens, fitness rooms, and server rooms.
made little mention of owner’s lighting expectations; but
Sustainability goals. A key element of the BOD is a
those specifications must be thoroughly documented in
description of the owner’s sustainability goals. Typically,
the OPR and BOD. Certainly, the building owner wants
this includes the owner-mandated LEED rating, as well
to maximize light output for occupant comfort. But the
as the version of LEED to be used in the project. The
owner also wants to minimize energy use and maintedescription should include specific details, such as:
nance requirements. So the BOD must give the design
“The facility is required to achieve LEED-NC Gold rating
team the needed data to translate the owner’s desires into
under LEED Version 3.0.”
actionable design (see Table 3, Page 78, for sample criteria).
LEED certification is not the sole responsibility of the
Building envelope. While the building envelope was
design team—every party and person involved in the project
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ASHRAE Journal - October 2013

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