Let the Sunshine In
November 19, 2013
COMMERCIAL : CODES & STANDARDS
The recent AAMA Fall Conference featured various speakers on the topic of energy efficiency, including Karma Sawyer of the U.S. Department of Energy’s Building Technologies Office and Stephen Selkowitz, leader of the Windows and Envelope Materials Group at Lawrence Berkeley National Laboratory.
Both Sawyer and Selkowitz challenged the belief by some that glazed openings in the building thermal envelope always impact the energy efficiency of the building negatively and should be limited to the greatest extent possible. As Sawyer explained during her presentation, it is true that heat can be lost or gained through fenestration, which typically does not resist heat flow as well as the opaque thermal envelope. When this happens, fenestration does have a negative effect on the energy efficiency of the building. However, depending upon the season and the building’s orientation and location, desired solar heating can be gained through fenestration, in addition to daylight that reduces the electrical lighting load. In such cases, fenestration has a net positive effect on the energy efficiency of the building.
Due to the combination of these factors, the role fenestration plays in the energy efficiency of a building is rather complex. There is no straight-line, simple solution.
Presumably keeping this in mind, the U.S. Department of Energy brought forth proposals for the 2015 International Energy Conservation Code for commercial fenestration that promote the use of well performing fenestration to contribute to the energy efficiency of commercial buildings by providing daylighting in all climate zones. Another proposal permitting use of dynamic glazing was also brought forward. With the support of AAMA and other fenestration industry representatives, many of these proposals were approved during the recent ICC Group B Public Comment Hearings. The result will be some significant changes in the 2015 IECC that better reflect the role glazing can have in improving a commercial building’s energy efficiency.
Increased provisions for daylighting in commercial buildings will be among the most significant changes for fenestration in the 2015 IECC. Prior to the 2012 IECC, the role of daylighting in commercial buildings was not considered at all. As a direct result of energy modeling funded by AAMA and the lighting control industry, provisions requiring mandatory toplighting in large, open public spaces were added to the 2012 IECC. Provisions that permit increases in the Window to Wall Ratio of vertical glazing, and the skylight roof area when used to provide daylighting, were also introduced into the IECC for the first time in the 2012 edition.
The mandatory toplighting provisions of the 2012 IECC require that skylights be used to meet part of the lighting load in spaces for specific occupancies that are more than 10,000 square feet in area, with ceilings greater than 15 feet in height, in Climate Zones 1-5. The occupancies this applies to are offices; lobbies; atriums; convention centers; transportation concourses; retail stores; gymnasium and fitness centers; automotive service areas; and workshop, manufacturing, storage and distribution areas. The energy modeling co-funded by AAMA found that when skylights were provided in such spaces in combination with automatic lighting controls, energy savings as great as 40 percent could be achieved in some climate zones. Although energy savings occurred in all climate zones, they were not as significant in Climate Zones 6-8.
In spaces that meet this description, 50 percent of the floor area is to be toplit by skylights with a minimum VT of 0.40, in at least 3 percent of the roof area.
No provisions for the use of sidelighting through vertical glazing, however, are included in the 2012 IECC. Also, if the space is less than 10,000 square feet, there are no requirements at all for daylighting of the space. Therefore, the provisions only apply to fairly large, open spaces, and the potential scope of the provisions is limited.
Mandatory Toplighting Threshold in the 2015 IECC
The DOE supported significantly expanding the provisions for mandatory daylighting in the 2015 IECC. This expansion includes the following:
- Reducing the size of space required to be daylit from 10,000 square feet to 2,500 square feet. This dramatically increases the number of buildings in which mandatory daylighting will be required.
- Adding an exception to the requirement for buildings in which the total area, minus the area daylit by sidelighting, is less than 2,500 square feet. The change will be beneficial to both skylight and vertical glazing manufacturers. For example, if a designer chooses to provide enough sidelighting to a space that is greater than 2,500 square feet, but less than 10,000 square feet, to reduce the area without daylighting to less than 2,500 square feet, the change is beneficial to vertical glazing. If, on the other hand, the designer chooses to provide skylights such that at least 50 percent of the floor area is toplit, then the change is beneficial to skylights.
In both cases, daylighting of the space would not have been required under the 2012 IECC. So in this manner, the new provisions of the 2015 IECC will encourage the use of both vertical glazing and skylights.
Increase in Window to Wall Ratio
The maximum WWR is limited to 30 percent of the exterior wall under the prescriptive provisions of the 2012 IECC. An exception is provided if 50 percent of the building area is daylit with automatic lighting controls. Achieving 50 percent of building area daylit through a combination of toplighting and sidelighting is relatively easy in low-rise buildings of one or two stories. This is due to the ability to achieve close to 100 percent toplit area in the top floor through the use of skylights or tubular daylighting devices. Achieving 50 percent daylit area becomes more difficult as the building becomes taller because the lower floors have to depend upon sidelighting, or lighting brought into the building core through the use of a large vertical opening such as an atrium. In the 2015 IECC, the WWR can be up to 40 percent of the exterior envelope if 25 percent of the building area is daylit, in buildings greater than two stories. The criteria that 50 percent of building area must be daylit to increase WWR to 40 percent will remain for one- and two-story buildings.
An additional proposal was approved that will vary the maximum solar heat gain coefficient permitted for glazing in commercial buildings based upon both orientation and projection factor.
The maximum SHGC permitted for commercial fenestration in the 2015 IECC will be as shown in Table 1.
Dynamic glazing will meet IECC 2015 requirements if the ratio of highest SHGC rating to lowest SHGC rating is greater than 2.4, or if both the highest SHGC rating and lowest SHGC rating are less than that permitted by the prescriptive provisions of the IECC.
Alternative Building Thermal Envelope Compliance Path
Lastly, a new alternative building thermal envelope compliance path will also be added to the 2015 IECC, permitting the trading off of the area weighted average of the building envelope U-Factor (UA), thermal conductance (CA), perimeter heat loss (FL), window area and skylight area.