Fresh Air and Daylight: Designing Natural Environments

Manufacturers are providing opportunities for fresh air in buildings while integrating daylighting techniques for increased energy efficiency.
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Sponsored by EFCO Corporation, La Cantina Doors, Milgard Windows & Doors, Nana Wall Systems, Inc., OpenAire Inc., Pella® Windows and Doors, Solatube International
Celeste Novak, AIA, LEED AP, En\compass Architecture

Labeling Daylight, Fresh Air and Energy Efficiency

Image courtesy NFRC

Many architects are familiar with ENERGY STAR, which is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy. Residential windows, doors and skylights are labeled with the ENERGY STAR logo, which provides the National Fenestration Ratings Council (NFRC) ratings for window performance.

The NFRC website states, "We're Changing the Way America Shops for Windows, Doors and Skylights."5 This nonprofit organization administers the independent rating and labeling systems architects rely upon to assure that they are specifying an energy-efficient window, door, or skylight. The NFRC was founded in the 1970s in response to that decade's energy crisis. In the 1992 Energy Policy Act, the NFRC was approved as the national performance and energy labeling organization. Manufacturers apply to the NFRC for certification. Products that achieve the NFRC label are tested in accordance with NFRC test methods and standards. The components on the NFRC label rate the design performance values that will maximize the energy efficiency of smart windows, frame components, and window walls.

An ENERGY STAR window with an NFRC label will have the following characteristics. The label will identify the manufacturer and the type of window, and this description will include the frame components, type of glazing, type of air or gas between the glazing and the type of operation. Energy performance ratings include values for the following factors:
  • The  U-Valueis a measurement of the resistance to heat loss. This factor includes the insulating performance of the entire window system. The lower the U-Value, the better the window will provide a barrier to thermal changes. In comparison, the insulation in a wall system is designed to resist heat gain and is measured as an  R-Value, the inverse of the U-Value. A high R-Value predicts better performance of the wall insulating system In a northern climate a recommended R-Value, will resist almost one-half of all heat loss. For example, designers might specify R-48 insulation values in Michigan or New England. In contrast, a window U-Value rates the heat gain through the entire window system, frame, glass, airspace and additional coatings or gas fills. An energy efficient window will have a U-Value below 0.35 and newer products are now approaching U-Values of .20

  • The  Solar Heat Gain Coefficient (SHGC) measures the fraction of incident solar radiation enters a building. This number ranges from zero to one. Different climate zones require different SHGC ratings. If a designer is using a passive solar strategy, then a higher SHGC rating is desired. Designers sometimes are challenged to trade visible clarity in windows with a low SHGC rating, which might be preferred in cooler climates

  • Visible Transmittance (VT) is the clarity of light through a window. This number is measured from 100 percent transmittance to zero. Single pane clear glass windows have about 90 percent visible transmittance, however, some coated windows have less than 50 percent transmittance.

  • Air Leakage (AL) determines how comfortable a person will feel next to a window or door. This measurement is listed as the amount of cubic feet of air through one square foot of window area. Heat loss or heat gain, drafts are all prevented by a window with a low AL rating.

  • Condensation Resistance is a measurement from zero to one hundred which is not always on an ENERGY STAR or NFRC label. Condensation will form on the inside of a window when the there is a temperature change between surfaces from hot to cold. Condensation varies by climate and this measure is used to compare the potential of a product to resist condensation, not an absolute value.

U-factors, Solar Heat Gain Coefficients, Visible Transmittance, Air Leakage, as well as Condensation Resistance are important values to evaluate which window system to choose when specifying a high-performance window. The NFRC and ENERGY STAR provide baselines for performance, which evaluate the entire window system.

 

 

Center of Glass
(glass only)

Total Window
(including frame & glass
for a Casement)

Notes

Double Glazing with 11/16" Insulating Glass (IG)

     

Clear IG

     

U-factor

0.49

0.49

Lower U-factor means the product is a better insulator

SHGC

0.78

0.54

Lower SHGC means less heat gain

Visible Light

82%

57%

 

Triple Glazing with 11/16" Low-E Insulating Glass (IG) and Low-E triple glazing panel

     

  U-factor

0.16

0.27

Lower U-factor means the product is a better insulator

  SHGC

0.36

0.24

Lower SHGC means less heat gain

Visible Light

60%

38%

 

Performance values of window glass are increased by high-performance casement window frames. 

(Source: Pella® Windows and Doors)

 

 

Architects usually specify windows through a frame manufacturer. Window frames vary from residential to commercial grades constructed of aluminum, wood, fiberglass, vinyl or combinations of these materials. Most of these products have similar insulating factors. Higher values for performance are easier to acquire through residential windows. High performance frames prevent thermal bridging of heat or cold from the outdoors to penetrate indoors. Frames prevent condensation, air leakage and may significantly increase the performance values of the entire window.

The components of a residential energy-efficient window system include:

  • Frames that reduce heat transfer
  • Multiple panes of glass with air, or gas, such as argon or krypton, which increase the insulation values of the window
  • Special coatings or Low−E glass products, which reflect infrared and ultraviolet light. Infrared light adds heat to the interior and utltraviolet light causes fading of interior finishes.
  • Warm edge spacers made of steel, foam, fiberglass or vinyl to reduce heat flow and prevent condensation

Designers choose the window performance value based on the climate zone of the project's location. Although windows are just one component of an energy-efficient building envelope, by choosing a window with a performance U-Value of 0.35 or less, designers will choose a window which will provide greater energy efficiency. Some triple pane windows with Low-E coating are approaching U-Values of .20, which can translate into substantial energy savings.

It is not always possible to choose the most energy-efficient window system. While commercial window products can also have the recommended NFRC labeled components, many of the commercial systems may not achieve the higher U-value ratings of the residential products. Architects should choose the highest performing window glass product and carefully detail the installation to prevent heat transfer.

Although window manufacturers can replicate the frames of a historic building design, the choice of glass product may be determined by preservation requirements. The National Trust for Historic Preservation allows a wide variety of solutions for replacement windows. Although single-pane, clear glass windows, will allow the designer to replicate a historic profile, the impact on energy budgets may be prohibitively expensive. "We wanted an insulated glass unit, but it had to have the same light reflectance and similar transmittance [as the original plate glass]," said Principal-in-Charge Kevin Harden.  He chose a Low-E insulating glass with argon gas in operable double-hung windows that allow natural ventilation. Through this configuration, the designer was able to maintain the historic façade, updating the envelope to meet current energy standards.

A Vision for the Future

In their book,  Smart Materials and Technologies, Michele Addington and Daniel Schodek call for new materials that would form new boundaries between the outside and inside environments we live in today.6 These authors state that "no other group in the architecture field has embraced smart materials as wholeheartedly as have the designers and engineers responsible for façade and enclosure systems."7

As previously noted, building envelopes (which can protect a building from sun, wind and rain, as well as provide insulation, ventilation and daylight) was a dream of Michael Davies. His vision included dynamic façades that changed with the season, as well as the passage of the sun throughout each day. New windows, walls, roofs and daylight devices will allow designers to customize their designs to climate using the natural resources of the planet, such as fresh air and daylight as energy resources. The goal is to create shelter that energizes human behavior by design.

 

Celeste Novak, AIA, LEED AP, is an architect and writer specializing in green design and community planning.

Endnotes:

1 "Daylighting in Schools: An Investigation into the Relationship between Daylighting and Human Performance," by the Herschong Mahone Group for Pacific Gas & Electric, August 1999. Verified 2002.

2 Joseph Anjali, Ph.D. "The Impact of Light on Outcomes in Healthcare Settings," The Center for Health Design, Concord, CA, August 2006.

3 Figueiro, M. G., Rea, M.S., Rea, A.C. Stevens R.G. (2002) "Daylight and Productivity A Field Study.  Conference Proceedings of the American Council for Energy Efficient Economy (ACEE) Summer Study, Asilomar Ca.

4 Davies, M. (1981) "A Wall for all Seasons,"  RIBA Journal, 88 (2) 55-57.

5http://www.nfrc.org, June 16, 2008

6 Addington, D. Michelle and Schodek, Daniel L.,  Smart Materials and New Technologies: For the architecture and design professions, Architectural Press, Elsevier Ltd., Oxford, 2005, 9.

7 Addington, 166.

 

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Originally published in GreenSource
Originally published in July 2008

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