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Section III- Envelope
a.) Summary:
It is very important to ensure the envelope is designed correctly before it is built.
Changes to the insulation or windows are very difficult after the envelope has
been established.
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Exceed code
o Wall roof insulation levels
 Continuous insulation
Windows
o Percent of wall area
o Low e-coating
Reduced infiltration
o Air sealing
o Air barrier
o Vestibules
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b.) Technical Information:
Building envelope includes the exterior walls, roof, and exterior windows, all of
which need to be optimized to improve the energy performance and thermal
comfort of a building. Code advances have been ratcheting up the requirement for
wall and roof insulation levels, including continuous insulation. While going beyond
code is always encouraged, the costs need to be balanced with the benefits.
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The building envelope is the boundary between the inside and outside of the
structure through which heat transfer could take place. To improve the building
energy performance, the design of building envelope is the first line of defense. In
terms of the climate design, envelope could be a “closed shell” or barrier to provide
maximum separation between inside and outside particularly in harsh climates
(opaque envelope that can be heavily insulated), or it could be an “open frame”,
which is more appropriate for temperate climates where a higher degree of contact
between interior and exterior is desirable (transparent envelope).
Heat flow phenomenon could occur within a building envelope (opaque or
transparent) or via air exchange. For an opaque building envelope, the heat could
conductively transfer through the mass. Given the non-homogeneous wall layers,
heat transfers along the path of least resistance—this phenomenon is generally
referred to as thermal bridging. However, the wall construction could potentially
possess a beneficial thermal property called thermal mass. Thermal mass is a
combination of wall material density and specific heat. This material property could
result in time lag with respect to heat transfer, which in turn causes the wall’s
response to the thermal force to be transient. In hot climates, thermal mass could
delay the heat transfer from outside to inside and in cold climates, it could perform
otherwise and keep the heat inside, both of which could considerably improve the
thermal comfort in buildings and save cooling and heating energy.
For a transparent building envelope, heat transfer occurs through
windows/skylights that generally have lower R-values than other envelope
components. Despite this inherent limitation, the fenestrations could admit
sunlight to the building for heating and daylighting, which could improve building
energy and thermal comfort performance. Heat transfer also could take place
through air exchanges to the outside, mainly in a form of infiltration or ventilation.
Infiltration is an unintended flow of outside air into the building through leaks in
envelope, normal opening and closing of doors, etc, and ventilation is a
purposeful introduction of outside air for fresh air or conditioning purposes.
Generally, the infiltration is to be minimized through envelope air-tightness codes
and standards; however, natural ventilation could help save cooling costs in
buildings where opening windows would be appropriate.
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Stack Effect:
Stack ventilation is a vertical movement of air through buildings—a natural phenomenon
caused by air vertical density differences. In other words, as the air gets hotter, it becomes
lighter and rises, which in turns causes vertical air movement within a building. Stack
ventilation keeps inside air temperature slightly above outside air temperature and could
be enhanced by coupling with thermal chimney effect to increase air flow.
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c.) Case Study: Science and Technology Magnet School in Champaign, IL
Our case study school is very well insulated in the walls and roof with 3-6” of
continuous polyiso insulation.
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R-18 to R-24 walls
o 3” continuous insulation (polyiso)
R-37 roof insulation
o 6” continuous insulation (polyiso)
o 3” minimum at drains
Windows U-0.27, SHGC-0.31
Vestibules reduce air infiltration from doors
Designed with heat exchanger to make up for reduced air infiltration
d.) Potential Issues: Case Study: Hospital (Built in 2005)
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The hospital was designed before continuous insulation was required by code. The
image below is from a thermal camera in one of the walls, which illustrates the
difference in temperature of the insulated cavity and the steel studs. The temperature
goes from 64 degrees to 75 degrees. Patients expressed that they were uncomfortable
in their rooms. This was partly due to the cold areas acting like a heat sink absorbing IR
heat from the room.
When examining a window in one of the rooms, it was discovered that parts of the
frame were 42 degrees on a day when the temperature outside was in the upper 20’s.
This lack of insulation could also lead to water condensation and mold, which could be
especially dangerous inside a hospital.
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