Energy-Efficient Envelopes for Metal Building Construction
Mark Robins, Senior Editor,
Posted
10/27/2011
The basic function of any building's energy-efficient envelope
is to separate the outside environment from the building's
interior. The design of metal buildings makes them very compatible
with energy-efficient envelopes allowing for a fast and economical
blend of energy-efficient applications that starts with the
erection process.
An energy-efficient envelope is the foundation upon which all
other systems are designed to fit. "In new construction, energy
concerns or improvements can be made by adding insulation in both
roof and wall assemblies," says Jim Bush, vice president of sales
and marketing, ATAS International
Inc., Allentown, Pa. "In a retrofit situation, additional
insulation can easily be added during the renovation process."
Two primary energy-efficient envelope functions are air
infiltration and thermal performance. "Infiltration directly
relates to how effective the building envelope will be in providing
air and water tightness in the roof, walls and other envelope
components such as windows, doors and flashings," says Brad Rowe,
national marketing manager, Thermal Design Inc.,
Madison, Neb. "Before one can have good control of energy
efficiency, there must be functioning barriers in the envelope for
air, water and heat."
Energy-efficient envelopes produce a controlled environment
enabling occupant comfort and energy cost reduction. "To be a true
energyefficient envelope, it must minimize air infiltration which
can have an adverse affect on indoor air quality and rob the
building of valuable conditioned air which in turn increases energy
costs," says Mark Engebretson, director of marketing and business
development, Therm-All Inc.,
North Olmsted, Ohio. "Since the insulation in the roof and walls in
metal building solutions is often exposed, the acoustical
performance and aesthetics of these assemblies plays an important
role as well. Roofs and walls are not the only components
considered part of the envelope, though they typically represent
the largest surface areas. Doors, windows, skylights and foundation
are the other components that must be considered carefully to
complete an efficient design. Another component that has been
identified more recently as playing an important role, especially
in the southern climates, is cool roofing."
The effectiveness of all installed envelope components and the
seal minimizing air infiltration at each joint, seam and
penetration is critical. "Air infiltration is probably the biggest
problem of building envelopes regardless of how much insulation is
installed," Rowe says. "If the wall leaks between or through the
components due to pressures from wind, mechanical systems or vapor
differences, then energy efficiency will be difficult to achieve,
maintain and manage. The overall effectiveness is highly dependent
on using quality materials installed with quality workmanship.
Caulking that shrinks, tape that loses adhesion, staples that rust,
and weather seals that shrink, deform and disintegrate over time
are just a few examples of envelope component failures that reverse
the initial effectiveness."
Controlling interior space conditions at the least expensive
energy cost is the main goal of an energy-efficient envelope. It's
important to select the optimum levels of thermal performance based
upon building use, conditioning requirements and the systems used
in the enclosed space. Rowe believes optimal performance levels can
be effectively achieved and maintained by maximizing insulation
thickness, using correct vapor retarder applications and installing
effective air barriers. "With proper building envelope
optimization, all other building systems can then function properly
without massive over sizing due to functional uncertainties," he
says. "Improving the building envelope design and optimizing the
installed insulation performance will return more value to the
building owner than any other building material going into the
project."
An air barrier
Air barriers prevent airflow between the inside and outside of
buildings. Air passage due to differential pressure is the primary
cause of heat loss in insulated buildings. Air barriers enhance
energy efficiency and prevent moisture problems, which help improve
indoor air quality and protect longterm structural integrity. A
study from the National Institute of Standards and Technology shows
that air barrier systems in non-residential buildings can reduce
air leakage by up to 83 percent and energy consumption by up to 40
percent.
There are two broad types of air barriers: vapor impermeable air
barriers and vapor permeable air barriers. "Both types can be
either sheet membranes- mechanically-attached or self-adhered-or
fluid-applied," says Jane Wu, product marketing manager, Grace Construction
Products, Cambridge, Mass. "For any air barrier to be effective
in preventing air leakage, it must be continuous. Using
high-quality flashing systems at all openings, and following
manufacturer's application instructions and details are crucial to
ensure a continuous air barrier throughout the building
envelope.
"Perm-A-Barrier VPS is a self-adhered vapor permeable air
barrier membrane consisting of a breathable carrier film with a
specially designed adhesive. It protects against the damaging
effects of air and water ingress on building structures by creating
a solid barrier against air infiltration and exfiltration, which
minimizes associated energy loss and condensation problems.
Perm-A-Barrier Aluminum Wall Membrane is a self-adhered impermeable
air barrier sheet. It is specifically designed for up to 12 months
of UV exposure, allowing for extended job-site scheduling
flexibility and reduced concern for membrane damage and
replacement, resulting from exposure."
Foam insulation's impact
Foam insulation used in metal building solutions is typically
rigid foam insulation incorporated into the envelope via individual
rigid board panels, field-applied sprays, or factory-integrated
with the cladding, such as insulated metal panels. This insulation
retards heat transfer, either into or out of the building. "Foam
insulation can be highly effective in this regard because it has a
very high heat flow resistance per inch of material," says Robert
A. Zabcik P.E., LEED AP, director of research and development/Green
Building Initiative, NCI Group
Inc., Houston. "If utilized with facings or metal skins, it can
also be a very effective air barrier preventing outside air from
entering the building. Although all buildings need some air
infiltration for indoor air quality, bringing this air at a
controlled rate through the building's HVAC system instead of
unintentionally through the building envelope is a primary focus of
newer energy codes."
Prescriptive model energy codes are now recognizing and
requiring continuous insulation in metal walls in most climate
zones. "Rigid foam insulation such as THERMAX with its R-6.5 at
1-inch and wide range of facer configurations, delivers not only
the required R-value, but with proper detailing, can work as part
of an air barrier system," says Doug Todd, market manager at Dow Building Solutions,
Midland, Mich. "Available in thicknesses up to 4 1/4 inches and up
to 30 feet long, THERMAX provides the long-term, stable R-value
that is needed over the life of the building, and meets fire,
building and energy codes.
"Rigid insulation also offers a number of performance advantages
over other types of insulation. Because it doesn't sag or compress,
and is usually placed over the metal wall girts/roof purlins to
form a continuous layer of insulation, it maintains its R-value
over the long-term. By properly detailing the joints of THERMAX
insulation and at interfaces such as ceilings and floors, designers
can specify an easy-to-construct, field-tested air barrier that
many newer energy codes require."
Fiberglass cavity insulation
Fiberglass insulation is one of the most popular and
cost-effective, energy-efficient envelope materials for metal
building solutions. Fiberglass insulation has very desirable
attributes such as acoustic absorption, sound transmission
reduction, inorganic fibers, lightweight compaction for shipping
economy, ease of handling and overall low installed costs.
For many years taking a single layer of fiberglass with a vapor
retarder laminated to it, and compressing it between the roof/wall
sheets and the secondary framing was a very common application.
However, "the recognition that there was a tremendous loss of
thermal performance through compression has brought about changes
in how fiberglass is being installed," says Engebretson. "Because
metal building systems provide a natural cavity with the roof
purlins and wall girts, these cavities are being utilized to add
additional insulation and improve the overall thermal performance.
In conjunction with standing seam roofs utilizing a foam thermal
spacer block at the purlins, as well as the availability of taller
standing seam roof clips, the improvements have been vast. There
are now fiberglass alternatives to the most stringent energy codes
that can provide a bright, clean finished appearance."
Fiberglass insulation has been designed specifically for metal
building systems. Fiberglass manufacturers in the North American
Insulation Manufacturers Association working in conjunction with
National Insulation Association have developed important standards
for the manufacture and lamination process of fiberglass. NAIMA
202-96 and NIA "Certified Faced Insulation" help ensure the thermal
performance to both the contractor and building owner.
In spite of its many advantages, fiberglass insulation has
received some negative publicity in the last few years for "the way
it is commonly compressed in metal building assemblies, leading to
a loss in in-place R-value," Zabcik says. "That's unfortunate
because it is a very inexpensive way to insulate a building. Yes,
there is a loss in R-value but it does not go to zero like some
would have you believe. Additionally, most fiberglass insulation
manufacturers have been actively developing and testing new
proprietary systems that are highly effective and proven
performers. Many of them integrate continuous air barriers as well.
Our advice would be to spend some time familiarizing yourself with
ASHRAE requirements and researching the performance metrics for
each fiberglass system rather than eliminating fiberglass from
contention."
Pushing the envelope
Energy-efficient envelope product manufacturers are quickly
improving and releasing their products to accommodate a growing and
competitive market. "We are likely going to see the greatest impact
from new processes and ancillary products that augment our metal
roof and wall panels along with our complete building envelopes,"
predicts Zabcik. "In the next few years, improved performance in
foam insulations and incorporated blowing agents, as well as new
details and installation processes developed with energy efficiency
in mind, will become more prevalent. Innovative approaches
incorporating building integrated photovoltaics, phase change
materials and above sheathing ventilation could also be factors in
the coming years.
"Our main advice would be to not focus on one or two individual
performance metrics, but to consider every potential solution in
the context of how it affects the entire building. Again, whole
building modeling can be a big help in this respect, but even the
most optimized solution will flounder if it is not built correctly.
Good installation techniques and quality control, both in the
factory and the field, is critical to achieving the desired
performance level. That will never change."
The metal construction industry must be aware of new regulations
regarding energy-efficient envelopes to fully optimize their
performance. "Designers should keep abreast of model energy codes
changes and recognize that these only specify the minimum," says
Todd. "Moving forward, the codes will increasingly require more
energy-efficient approaches. For example, the new ASHRAE 90.1-2010
standard will cut energy use by 30 percent from 2004 requirements.
It recommends continuous insulation as a requirement in most
climate zones and has increased R-value requirements for metal
buildings nationwide. The model codes are also moving to mandatory
air barrier requirements. Fortunately for the design community,
there are prescriptive methods to detail the insulation, such as
THERMAX insulation, as an air barrier system and to meet the air
leakage and CI requirements in a single solution."
There are new computer software programs assisting in the
overall design of not only the envelope, but the entire building
performance. ASHRAE, IECC and the DOE are working together to
better educate designers and code officials, and are offering more
tools to assist in this process. "[Metal Building Manufacturers
Association] is working together with suppliers to help create more
energy-efficient envelope designs as well as assembly testing,"
says Engebretson. "There has been improved and increased usage of
infrared equipment and other technologies to locate weak areas of
the envelope for both heat loss and air infiltration. This is
leading to better construction techniques."
Rowe believes to maximize the full potential of energy-efficient
envelopes now and in the future, "all parties from insulation
manufacturers to the installers, including designers, must accept
their responsibilities to achieve the installed performance of the
products. So specify it, demand it, order it, install it, inspect
it and reject it if it does not meet the specifications and
expectations, otherwise it will not perform as expected. The
building owners deserve nothing less than the performance they are
led to believe they will get."
Interested in learning even more about energy-efficient
envelopes for metal building solutions? The National Institute of
Building Sciences, under guidance from the Federal Envelope
Advisory Committee, has developed a comprehensive federal guide for
exterior envelope design and construction for institutional/office
buildings titled the "Building Envelope Design Guide." This guide
provides information and resources regarding energy efficiency,
green building construction guidelines, daylighting, blast and wind
safety, flood resistance and indoor air quality.
Envelope Influences
Building function and climate location have a big influence on
energy-efficiency envelopes.
"Envelope energy efficiency is more important for a heated
office building with high human occupancy and air conditioned, than
for a warehouse with minimal human occupancy that is heated only
and maintained at a lower inside temperature," says Mark
Engebretson, director of marketing and business development, Therm-All Inc., North Olmsted,
Ohio. "How many hours of the day and days of the week the building
is operational is another design consideration. The shape and
building orientation can be important for capturing daylighting
from windows and skylights. The larger the building, typically the
more surface area is exposed to the outside environment which in
turn creates the need for a more efficient building envelope to
reduce energy costs."
Roof color has an influence on energy-efficient envelopes.
"Factory-coated metal cool roofs are excellent candidates to
consider, but sometimes, the best answer may very well be a darker
color that can help create warmth in a primarily heated
environment," says Robert A. Zabcik P.E., LEED AP, director of
research and development/Green Building Initiative, NCI Group Inc., Houston.
"As far as building sizing goes, minimizing the exterior
surface-area-to-interior-volume ratio is a good idea, but that may
not be an option aesthetically or from a site layout perspective.
Nor does that strategy do anything to address internal heat and
humidity loads. So how does somebody know what the optimal mixture
of solutions is? Whole building modeling is probably the best
option available. It allows a designer to pick the most efficient
design with confidence because it considers the performance metrics
of multiple solutions. That's why whole building modeling
requirements are being introduced into high-performance building
programs like LEED and standards like ASHRAE 189.1 and IgCC."