Holistic Approach Needed to Achieve Energy Efficiency Goals
If you have an ache in your belly, who do you go see—a family practitioner, a general practitioner or perhaps someone who specializes in gastroenterology or internal medicine. Medicine today has become so specialized that it is often difficult to know just who to call for help.
And it is not uncommon for the recommendations of one specialist to conflict with the recommendations of another. A surgeon may disagree with the prescription for blood thinner that a patient’s cardiologist has prescribed, for example. Although in most cases physicians working within the same facility will attempt to coordinate a patient’s care with each other, in the end it’s our responsibility as patients to make sure all aspects and potential ramifications of proposed care have been considered and that the decisions being made are what is best for our body as a whole. After all, we are the ones that have to live with the results.
In the same manner, a somewhat holistic approach is needed for good building design and construction. A friend of mine commented recently that each contractor thinks their work is the most important part of a project—a mechanical contractor will place a thermostat right in the middle of a blank wall because he or she feels it’s the most important element in that space. Over time, the elements of good design prevail—the mechanical contractor comes to accept that in most cases the building owner or designer will not want the thermostat in the middle of the wall. Structural engineers resign themselves to designing the challenging geometric forms architects keep pushing for, while longing for the “square boxes” of the good old days.
Incorporating energy-efficient design into buildings is a relatively new phenomenon in America. Perhaps we should not be surprised then that the advocates of energy-efficient design do not always understand that truly energy-efficient design can only be achieved when the performance of a building as a whole is considered, rather than the performance of the individual components of a building. Applying a holistic approach to the energy-efficient design of a building involves asking the question, “How do I achieve the greatest net energy savings for this building as a whole?” If one focuses upon only individual components without paying attention to how they interact with each other, that question will never be truly answered.
Up until recently, the prescriptive approach of the energy conservation codes in this country has been primarily component based. For residential construction, the two components of concern have been the building envelope and the building’s mechanical system. In commercial construction, the lighting system was added to this mix. It is true that by tightening up the performance of each of these components, significant energy savings over conventional construction has been achieved. But we are starting to reach the point where pushing too hard on one component can have a negative effect on another.
One example of this is the lowering of the solar heat gain coefficient of windows and skylights. Certainly solar heat gain is a concern, particularly in cooling dominated climates. Lowering the SHGC of the glazing, however, inherently lowers the visible light transmittance of the fenestration product as well. For skylights, this effect is almost a direct, one-to-one relationship. The 2006 International Energy Conservation Code establishes a maximum SGHC requirement of 0.40 for glass skylights in climate zones 1 to 3. Skylights that met this criterion have a VLT of approximately 0.40. For plastic skylights, the maximum SHGC permitted by the 2006 IECC in climate zones 1 to 3 is 0.35. Skylights that met this criterion have a VLT of approximately 0.30. In both cases, the amount of light admitted to the space below the skylight will only be about a third of the available daylighting.
SAVING ON LIGHTING LOADS
Restricting the SHGC of the skylights in this fashion, and thereby lowering the daylighting provided through the skylight, unduly restricts the ability of the skylight to reduce the lighting load for the building. This in turn has a negative impact on the total energy package of the building.
Restricting skylights to 3 percent of the roof area in the IECC prescriptive provisions for commercial buildings can also restrict the ability of skylights to reduce lighting load. The percent roof area that is permitted to be skylights is limited to 3 percent to lower the average U-value of the roof as a whole, which is appropriate to reduce heating loss during the heating season and heating gain during the cooling season. But in some cases, the potential heat loss or gain if the skylight area were increased is not as great as the potential savings in lighting load. This is particularly true if automatic lighting controls in daylight areas are used in conjunction with the skylights.
AAMA has been exploring this phenomenon for quite some time. Research funded by AAMA and conducted by Heschong Mahone Group and Carli Inc. revealed that increasing the skylight area from 3 percent to 6 percent of the roof in combination with the installation of automatic lighting controls saved energy in all but one of the 1,521 cases studied. What’s more, these savings occurred even if the skylight installed in 6 percent of the roof area had a less stringent U-value and SHGC factor than the skylights installed in 3 percent of the roof without the automatic photo controls. For example, in climate zone 4, a retail building with skylights that have a U-factor of 0.80 and an SHGC of 0.71 in 6 percent of the roof area and automatic lighting controls saved 15 percent in energy costs over the same building with skylights that have a U-factor of 0.60 and an SHGC of 0.40 in 3 percent of the roof area, but no automatic lighting controls. When the same comparison was made for a warehouse building, the savings in energy costs were 20 percent. Even in cooling dominated climates such as climate zone 2, energy savings were achieved when higher SHGC skylights comprising 6 percent of the roof area in combination with automatic lighting controls were used in comparison to lower SHGC skylights comprising 3 percent of the roof area and no automatic lighting controls.
The fact that energy savings could be achieved even though skylights with less stringent U-factors and SHGC values were used has been a difficult concept to get across to energy advocates. During the 2006/2007 ICC Code Change cycle, AAMA submitted a proposal that would have allowed the skylight roof area to be increased from 3 percent to 6 percent of the roof area and allowed the use of skylights with less stringent U-factor and SHGC than those required by the IECC, when automatic lighting controls were used in daylight areas. The proposal was backed by the information from the Heschong Mahone and Carli studies mentioned above. The IECC committee, however, was uncomfortable with increasing the permitted U-factors and SHGCs while also increasing allowable area of the skylights, and the proposal was disapproved.
During the same cycle, the IECC committee approved a code change proposal that increased the U-factor and SHGC stringency for glass skylights in climate zones 1 to 3, the U-factor stringency for plastic skylights in climate zones 1 to 3 and both the U-factor and SHGC stringency for plastic skylights in climate zones 4 to 8. Upon initial review, it was unclear how the energy performance of buildings equipped with skylights and automatic lighting controls that met the criteria in the AAMA proposal would compare with that of buildings equipped with skylights that met the new criteria.
Upon further review, however, it became clear that the comparison had already been made in climate zones 4 to 8. In those climate zones, the newly approved values for all skylights were merely the previous values for glass skylights. These values had been included as the basis for comparison in the Heschong Mahone and Carli studies. In the example given above for retail and warehouse buildings in climate zone 4, the values for the skylights in the less energy efficient building (U-factor = 0.60, SHGC = 0.40) were the same values that had been approved for all skylights by the IECC committee. As noted above, when skylights with a less stringent U-factor and SHGC were installed in 6 percent of the roof area, with automatic lighting controls, energy savings were realized.
Armed with this information, AAMA decided to come back with a code change proposal for the 2007/2008 ICC code change cycle that would permit an increase in roof area from 3 percent to 6 percent when automatic lighting controls are provided, without reducing the stringency of the U-factors for skylights from the prescriptive requirements of the IECC. The proposal replaces the maximum SHGC requirement of the IECC with a requirement for a minimum VLT of 0.40. It also requires the glazing in the skylights to have a haze value of 90 percent or better (higher haze values ensure the glazing effectively diffuses incoming light, providing broader, more efficient illumination of the space below the skylight). A detailed comparison of energy cost showed anticipated energy savings in every climate zone. In some cases, the energy savings was as great as 40 percent in comparison with the 2006 IECC, and 30 percent in comparison to the 2007 Supplement to the 2006 IECC.
With all the variables and interdependencies, this can be a difficult concept to understand. It’s becoming increasingly important, however, as AAMA’s Rich Walker notes in this month’s Industry Watch column (page 38), looking at the ambitious new energy efficiency goals being considered by our nation’s leaders. With more time to digest the benefits of increasing allowable skylight area, perhaps the IECC committee will recognize and accept this proposal during the current code change cycle. A similar proposal has been accepted by the ASHRAE 90.1 committee and is on its way to inclusion in the next edition of ASHRAE 90.1.
Code Arena is brought to you by the American Architectural Manufacturers Association. Julie Ruth may be reached through AAMA at 847/303-5664 or via e-mail at firstname.lastname@example.org.