Thermal Performance

Identifying the true thermal performance of fenestration systems and products has been the focus of a not-for-profit trade association known as The National Fenestration Rating Council (NFRC). Since 1989 they have championed the process of fairly and comprehensively rating windows, doors, and skylights including multi-slide glass doors. Prior to the formation of NFRC, window manufacturers used different tools to measure and report the energy efficiency of their products. In 1993, NFRC developed the first consensus method for evaluating the thermal transmission of windows. NFRC 100 “Procedures for Determining Fenestration Product U-factors” is now the accepted standard for rating windows, doors, and skylights for U-factor. This standard establishes standardized environmental conditions, product sizes, and testing requirements, so that architects and others can make informed choices by comparing the performance of different products fairly and accurately.

One of the most important improvements NFRC 100 offered the industry was that the determination of heat loss of the entire window unit, not just the glazing. A multi-slide glass door that is tested and subsequently rated in accordance with NFRC 100 gets credit for all of the energy-efficient features including low-e glass, thermally improved frames, and even the spacer used between layers of insulated glass. However, if a manufacturer is deficient in any of these areas, the testing will reveal that as well. Therefore, when comparing performance between different manufactured systems it is advisable to always look for products that have U-factors determined in accordance with NFRC 100.

Beyond direct thermal transfer through materials, codes and standards, along with the NFRC, have increasingly recognized air leakage (AL) as a very significant factor in fenestration performance. Therefore, it is becoming required for fenestration products to meet minimum standards for this leakage or air infiltration and be tested, certified and labeled as such, which the NFRC does. The total system must be able to withstand wind pressures associated with their location and air leakage must be controlled not only for energy performance but for occupant comfort and long-term durability of the fenestration system.

In addition to the AAMA testing discussed, air infiltration testing under ASTM E-283 will indicate cubic feet per minute per linear feet of crack. Multi-slide glass doors address this need to control air leakage, and in the process weather leakage, by using an appropriate mix of perimeter seals on the door panels, usually custom fabricated to match the doors. Top seals can be brush type to allow for the smooth operation of the door but still restrict the transfer of air and weather. The bottom or sill seal may be of low-friction coated rubber applied to the bottom of the door, creating full seal but still allowing for the smooth operation of the door panels.

As the panels come together and interlock, continuous seals along the edges can include compression rubber gaskets, fin brush seals, or both. In all cases, the intent is to provide a tight seal when the doors are closed and maintain the pressure and weather resistant performance as tested.

NFRC testing also looks at other overall performance characteristics including the solar heat gain coefficient (SHGC) which measures how much solar energy passes through a particular glazing (versus being reflected away) creating heat gains inside a building. This performance factor is usually adjusted through coatings applied to the glass to allow more light (and resulting solar heat) where it may be desired in cold climates, or to allow less where it is not wanted in hot climates. NFRC testing also responds to green building standards by assigning a value to the visible light transmittance (VT) that comes through a tested product. This VT determines the effective light available for daylighting so finding the right balance between how much light and resulting solar heat is transmitted versus how much to exclude needs to be assessed on a project by project basis.

At the end of all of this testing, a comprehensive view of thermal performance of tested products is determined providing a more accurate, credible, and uniform energy rating for fenestration products. The test results are documented through the NFRC Product Certification Program (NFRC 700, PCP) which sets forth the specific requirements for rating, certification, and labeling of a manufactured fenestration product. Manufacturers who participate in the NFRC Certification Program have their products listed in the NFRC Certified Products Directory that contains thousands of certified products and is available on-line at www.nfrc.org. In addition, each tested unit is then shipped with a standard label that identifies the key elements of performance, much the way mileage rating stickers are applied to cars or nutrition labels are applied to food.

The ultimate balancing activity in any design is how much fenestration to use as a percentage of the overall wall area of a building. The energy codes tend to use 40 percent as the benchmark. In some cases, all of this glazing can be concentrated on one or two facades as appropriate to a location. Building designs that use multi-slide glass doors with more than 40 percent window to wall ratios are possible but offsetting measures elsewhere in the building envelope are required. The rationale for this benchmark seems to rely on the fact that a well-insulated wall will inherently perform better thermally than any type of fenestration. Nonetheless they also recognize that only fenestration allows the natural daylight and potentially beneficial solar heat gain into buildings. Simple energy modeling of different scenarios during the earliest stages of design will help to determine the most appropriate balance of opaque wall to fenestration. This is particularly important when different façade orientations (i.e. north, south, east or west) are taken into account since the best performing buildings usually do not treat all facades equally. The talent, experience and skill of the architect used in finding this appropriate balance will determine not only design quality and code compliance, but in a very real way help determine the overall energy performance and utility costs experienced by the owner and users of the building.

Green Building Contributions of Multi-Slide Glass Doors

The green building movement in this country has given rise to quantifiable rating systems that seek to establish specific levels of achievement in the creation and performance of green buildings. The best known of these green building rating systems has been developed by the US Green Building Council and known as the LEED® rating system. The LEED 2009 system is in place until the year 2015 which overlaps with LEED version 4 introduced late in 2013. Since buildings are currently being designed under both systems, it should be noted that credits may be earned somewhat differently for each when seeking certification. With that in mind, the contributions that multi-slide doors can make are generally described within the common green building categories as follows.

Optimize Energy Performance

As part of an overall building envelope design, multi-slide doors can have a big impact on the energy performance due to their contribution to desirable passive solar heat gain when the building is designed to take advantage of it. By incorporating passive solar strategies such as overhangs and building orientation, solar gain can be used to an advantage in the heating season, and reduced in the cooling season. The result is reduced use of purchased fossil fuel energy. In commercial buildings where daylighting is desired, then multi-slide doors can be used to provide an abundance of natural light into the space. With proper controls and lighting arrangements, this can reduce not only the amount of artificial lighting needed but also reduce the associated cooling load imposed from heat given off by electric lighting. The net result is a reduction in electrical energy use. The actual optimized energy performance can best be determined by using energy modeling and adjusting the size and configuration of multi-slide doors accordingly.

Materials and Resources

In some cases multi-slide doors are available with recycled content positively impacting a life cycle assessment of the product. During the construction process, the pre-fabricated, custom nature of the product limits or reduces scrap and construction site waste. During their use in the building, their ability to close and seal tightly will reduce air infiltration and heat transfer. At the end of their service life, the door panels can be salvaged and re-used in other buildings. Manufactured in the USA means regional material contribution is possible and the impact of transportation is lessened.

Indoor Environmental Quality

Large format multi-slide doors provide the notable and desirable addition of daylight and views which contribute directly to indoor environmental quality. In the open position, they also provide ample natural ventilation and exchange of fresh air into the building. Finally, since little or no paints, coatings, adhesives, or harmful sealants are needed in the product, they do not contribute to VOC content in the indoor air.

Innovation in Design

With green design, size matters. Building size is a significant contributing factor to resource efficiency and in the end will affect energy use also. The move to smaller and more efficiently planned spaces makes flexibility, functionality, and expansion capabilities all the more important. Multi-slide glass doors provide a variable connection to the outdoors allowing for living spaces to be smaller while still feeling large.

Specifying Multi-Slide Glass Doors

When specifying multi-slide glass doors, there are choices and specification details to pay attention to so that the best products are selected for the building where they will be installed. The Master Format section number commonly used is 08 32 13 “Sliding Aluminum-Framed Glass Doors.” Some of the relevant items to address in a standard 3-part specification format are highlighted as follows.

Part 1 General

When it comes to specifying the most appropriate multi-slide glass door for a particular building, especially a green building, the first place to look at is testing performance criteria. Make sure to compare similar testing methods between manufacturers and specify accordingly. The appropriate AAMA Product Performance Class should be indicated along with the minimum DP rating. Water penetration should be tested by ASTM E-547 and air infiltration testing under ASTM E-283. Structural Load Deflection testing is carried out under ASTM E-330 and performance criteria should be related to the size of the panels. Finally, and in particular look for NFRC certified and labeled systems to address thermal performance and require those accordingly.

Submittals will be similar to most window and door products for product information and shop drawings if needed. They should indicate full coordination details with the construction drawings and the sizes should be double checked since they are custom fabricated to suit the designed opening. Working with a selected manufacturer during the design process will allow for better coordination in the spirit of integrated project delivery. Other general specification items should include warranty requirements for the glazing, the other materials, and the system overall consistent with available 10 year warranties in the marketplace.

Part 2 Products and Options

Identifying the specific multi-slide door products, materials and options that are desired requires covering several items. If multiple product types are selected for a building, then obviously they each need to be specified and their locations identified on the drawings.

▶ Frame materials: The choice needs to be made between the three commonly available frame materials. Aluminum only frames generally carry the lowest cost, thermally broken aluminum frames address energy efficiency, and aluminum wood frames (with or without a thermal break) address exterior maintenance and interior appearance. Wood only frames may also be available from some manufacturers.

▶ Frame colors / finishes: Since the frames are extruded aluminum, they can be finished in common aluminum color choices such as bronze or clear anodized or white paint. They are also available from some manufacturers in colors to match popular window brands allowing for a consistent exterior or interior appearance with the rest of the building. This would include some of the earth tones, reds, greens, and browns found on popular window brands. If none of these are appropriate, then it is also possible to use a Kynar paint finish to customize the frames to virtually any color. One manufacturer's innovative frame design offers split finish options for the interior and exterior to provide maximum design flexibility. If aluminum wood is selected for the frames, then the wood color or finish should be specified from such options as vertical grain Douglas fir, mahogany, maple, oak, walnut, or even dark cherry.

▶ Glazing: With the frame selected, the glazing infill options need to be finalized. Typically, most applications use double pane insulating glass units with low-E coatings and inert gas fill for energy efficiency. Single and triple glazing are also options as the location may warrant. Specialty glazing may also be available with obscure, acoustic control, impact resistant, art glass, or even solid material possible to be specified.