- Who is ASHRAE?
- What is a "carbon footprint?"
- How are anodized finishes applied?
- Other than LEED® credits related to natural ventilation, what are the benefits of operable windows?
- What are the key elements of air barrier interface at window frames?
- How are window U-Factors determined?
- How is finite element computer thermal modeling used?
Q: Who is ASHRAE?
The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) is a non-profit society with headquarters in New York City. It is made up of 50,000 members, both companies and individuals, principally from the mechanical engineering profession.
ASHRAE’s reach extends beyond the mechanical trades; in the past 30 years, it has become the authoritative source for energy performance criteria in the built environment. Because of the importance of energy efficiency and comfort in sustainable design, ASHRAE also was instrumental in moving our industry toward green building practices.
The ASHRAE "Handbook of Fundamentals" is recognized worldwide as the finest overall technical manual of its type. Wausau’s design engineers refer to the Handbook frequently for thermal performance characteristics and information.
The International Energy Conservation Code (IECC) references ASHRAE 90.1 "Energy Standard for Buildings Except Low-Rise Residential Buildings" as an alternate compliance methodology. The 2010 version of this standard will require non-residential buildings to perform 30% more efficiently than current standards.
Q: What is a "carbon footprint?"
A "carbon footprint" is a quantified assessment of the greenhouse gas emissions of an individual person, product, organization, group, material, or supply chain. The global warming potential of other greenhouse gases such as methane, nitrous oxide and hydroflurocarbons are converted to normalized "carbon dioxide equivalents," hence the term "carbon footprint." Technically, water vapor is also a greenhouse gas, but because of weather-related variables, is not traditionally included in a carbon footprint.
International standards for the uniform determination of carbon footprint are in development to include Scope 1 direct or primary emissions from manufacturing facilities, distribution and materials, as well as Scope 2 and 3 indirect or secondary emissions such as electrical power consumption, heating fuels, waste disposal, employee commuting, suppliers, and business travel.
Organizations use results to identify and track improvement opportunities, qualify materials and focus on product "design for the environment."
Q: How are anodized finishes applied?
In the anodizing process, the material to be finished is attached to an aluminum rack, thoroughly cleaned and etched to create a "matte" finish, and then immersed in a sulfuric acid and water solution called "the electrolyte." The aluminum is made the anode and cathodes are located about the tank perimeter (or the side lining of the tank itself).
When a direct electric current (DC) is applied, the electrolyte decomposes at the surface of the anode (the aluminum work piece), which releases oxygen. This oxygen, in turn, combines chemically with the surface aluminum to form aluminum oxide.
At the same time, the oxide coating, thus formed, becomes quite porous due to the dissolving action of the electrolyte.
Coloring of an anodic film enhances the appearance of the material and broadens the application of anodized aluminum. In electrolytic two-step coloring, anodizing is followed by the electro-deposition of metal. Alternating current (AC) is used to slowly deposit metallic tin at the bottom of the pore. Wausau’s finishing partner, Linetec, has computer-controlled rectifiers to ensure that deposition rate is uniform and repeatable. The intensity of the color is dependent on the amount of tin deposited and its packing density. This process step is not used if a clear anodic coating is desired.
After the coating is completed, the aluminum is immersed in a hot water tank, which effectively seals the porous aluminum oxide coating.
The aluminum oxide anodic finish is an integral part of the aluminum and is very hard, relatively transparent, and offers excellent abrasion and corrosion protection to the underlying metal.
Q: Other than LEED® credits related to natural ventilation, what are the benefits of operable windows?
Many operable window units are designed so that maintenance personnel can clean the outside glass surface from the interior. These include:
- side-hinged, in-swing windows;
- vertically pivoted windows;
- top-hinged, in-swing windows;
- and tilt-sash, double-hung windows.
Ease of maintenance not only improves building appearance and staff safety, but also represents a life cycle cost savings.
In hospitals, where evacuation may be slow and cumbersome, operable windows are primarily intended for emergency ventilation use in the case of fire. They also avoid the need to "break out" exterior glazing in the case of fire, for firefighter access or rescue.
If of sufficient size, operable windows can be used to shorten egress pathways in schools and residential occupancies.
In addition, nasty spills or smells may need to be quickly "aired out," a power failure or equipment failure may temporarily disable fans, or a breath of fresh air on a pleasant day may simply be "just what the doctor ordered." Psychologically, most room occupants prefer to have some control of their own environments, including connection to the outdoors.
In many parts of the U.S., natural ventilation offers a seasonal opportunity for air-conditioning energy savings, if included as part of the HVAC design, balancing and operating plan.
Q: What are the key elements of air barrier interface at window frames?
Know the appropriate air-vapor barrier detailing for your climate zone.
Take the time to coordinate air and/or vapor barrier membrane interfaces with the responsible subcontractor and suppliers.
Consider that frame cavity insulation improves overall thermal performance, especially with retrofit panning, but know this must be placed to exterior of the vapor barrier membrane.
If wall cavity air is expected to be cold, isolate the window frame inboard of the thermal barrier. Non-permeable air barrier membranes should "tie into" window frame perimeters inboard of the thermal barrier
Don’t install windows if air barriers are damaged, torn, discontinuous, or otherwise ineffective. Have the air vapor barrier installer mark the location of studs when critical for window anchorage.
Don’t forget that window frames and adjacent wall substrates usually move relative to one another. This must be accommodated in interface detailing. Keep window anchors clear of the membrane system.
Ensure that sequence of installation is workable, and ideally, does not depend on the window installer to complete the waterproofing contractor’s work.
Watch out for specifications disallowing sealant in weather-resistive elements. This stricture will require special interface detailing.
All parties’ respective scope of work and warranty obligations needs to be clearly defined in the bid documents, and the general contractor or construction manager must "buy" the project accordingly.
Q: How are window U-Factors determined?
Before the advent of finite element computer thermal modeling tools, the only reliable way to determine thermal performance characteristics of complex frame-glass assemblies was in a "guarded hot box" test facility, constructed per ASTM C236. A "guarded hot box" is essentially a large, highly insulated room divided into a "cold side" and a "warm side." An insulated wall separates the two, and it is into this wall that the test specimen is mounted.
On the cold side, a large fan simulates a 15 mph wind directed at the exterior face of the test specimen. Cold side temperature is held constant at about 0°F, per ASHRAE standards.
On the warm side, temperature is held constant at 70°F by a small electric heater. By measuring the energy necessary to heat the warm side, an indirect measurement of heat loss through the specimen (in BTUs per hour) is inferred. Because both the insulating wall surrounding the specimen and the warm side’s chamber walls are highly insulated against extraneous heat loss, only small adjustments need to be made to these heat loss measurements. Therefore, the warm side is "guarded" against heat loss, hence the name of the apparatus.
U-Factor is a measure of air-to-air heat flow per unit time, area and temperature drop, and is the reciprocal of the familiar "R-Value" cited for insulation products. With U-Factor, "…the smaller, the better."
The energy input measured in the guarded hot box test, which approximately equals test specimen heat loss, is divided by specimen area and the 70°F temperature drop air-to-air to yield a useful "normalized" result called U-Factor, with units of BTU/hr-sqft-°F. U-Factor, in turn, can be used with project-specific parameters, such as wall area and weather conditions, to calculate building performance.
Q: How is finite element computer thermal modeling used?
In the early ’80s finite element computer modeling was developed to predict thermal performance of untested frame-glass combinations on custom systems. Thanks to the efforts of the National Fenestration Ratings Council, Lawrence Berkeley National Laboratories (LBNL) and the Department of Energy, this software is now publicly available and is widely used for system design. Many manufacturers, test labs and glass fabricators have modeling capabilities in-house.
The key is "system" design: Glass manufacturers calculate and publish "center-of-glass" U-Factors inferred from the basic physics of solar-optical properties. For insulating glass, these represent the expected properties of two, infinite size lites of glass separated by an air space of given width.
Of course, in the real physical world, thermal conductance of insulating glass edge spacers and framing can detract from the center-of-glass value. Modeling software like LBNL’s THERM program takes these complex edge effects into account and allows the calculation of a "weighted average" or "overall" system U-Factor. Always use this overall system U-Factor when citing thermal performance to avoid misleading claims.
Finite element thermal modeling accurately predicts U-Factors, as well as surface temperatures of glass and non-conductive framing materials such as PVC and wood. Modeling is not as accurate in predicting surface temperatures of aluminum framing, and caution is advised in applying results to field condensation performance prediction.