Windows, Skylights and Title 24
Windows, glazed doors, and skylights (also called fenestration) have a significant impact on energy use in a home. They may account for up to 50 percent of residential space heating loads, and for homes that are air-conditioned, up to 50 percent of the cooling load. The size, orientation, and types of fenestration products can dramatically affect the overall energy performance of a house. Glazing type, orientation, shading and shading devices not only play a major role in the building’s energy use but can affect the operation of the HVAC system and the comfort of the occupants. The Title 24 energy standards recognize three types of fenestration:
- Manufactured windows, glass doors, and skylights. Manufactured windows, glass doors, and skylights are constructed of materials which are factory-cut or otherwise factory-formed with the specific intention of being used to fabricate a fenestration product. A manufactured fenestration product is typically factory-assembled before delivery to a job site. This is the most common type of fenestration in residential construction.
- Site-Built. Site-built fenestration is designed to be field-glazed or assembled in the field using specific factory-cut or otherwise factory-formed framing and glazing units. These are more common in non-residential construction. The glazing contractor may also pre-assemble site-built fenestration at the shop before final installation. Examples of site-built fenestration include storefront systems, curtain walls, and atrium roof systems.
- Field-fabricated. Field-fabricated products are built on site using standard dimensional lumber or other materials not intentionally prepared for use as a fenestration product.
U-factor and SHGC Ratings of Manufactured Fenestration Products
The Title 24 mandatory measures require that both the U-factor and the SHGC of manufactured fenestration products be determined from NFRC’s Certified Product Directory or from the California Energy Commission approved default tables. At the time of inspection, the actual fenestration U-factor and SHGC values as shown on NFRC labels or in the default tables must result in equal or lower overall energy consumption than the values indicated on the compliance documents.
A directory of NFRC certified ratings is available at the NFRC website. The CEC default U-factors are contained in standards table 110.6-A, and the default SHGC values are contained in standards table 110.6-B in the Title 24 residential compliance manual. CEC default values are on the poor side of the performance range for windows. To get credit for advanced window features such as low-e (low-emissivity) coatings and thermal break frames, the window manufacturer must have the window tested, labeled, and certified according to NFRC procedures.
Improved Window Performance
With the 2013 Title 24 energy standards update, the prescriptive maximum u-factor for all fenestration products (windows, glass doors, skylights) has been reduced to 0.32 in all climate zones. The mandatory maximum u-factor for manufactured fenestration products has been reduced to 0.58. If the prescriptive weighted average of fenestration does not exceed 0.58 the project is in compliance. Up to 10 ft2 of fenestration or 0.5% of the conditioned floor area (whichever is greater) is exempt from the maximum u-factor requirement.
Though the 2013 Title 24 standards have lowered the prescriptive fenestration u-factor standard choosing high performance windows that out-perform the prescriptive standard can still earn significant compliance credit through the performance method. In air-conditioning climates, choosing a window with an SHGC lower than 0.25 will reduce the cooling loads compared to the standard design. The magnitude of the impact will vary by climate zone; in mild coastal climates the benefit from reducing window U-factor will be smaller than in cold mountain climates. Computer compliance programs can be a useful tool to compare the impact of different windows and can help the designer determine when an investment in better windows is worthwhile.
Several factors affect window performance. For windows with NFRC ratings, these performance features are accounted for in the U-factor and SHGC ratings:
- Frame materials, design, and configuration (including cross-sectional characteristics). Fenestration is usually framed in wood, aluminum, vinyl, or composites of these. Frame materials such as wood and vinyl are better insulators than metal. Some aluminum framed units have thermal breaks that reduce the conductive heat transfer through the framing element as compared with similar units that have no such conductive thermal barriers.
- Number of panes of glazing, coatings, and fill gases. Dual-glazing offers opportunities for improving performance beyond the dimension of the air space between panes. For example, special materials that reduce emissivity of the surfaces facing the air space, including low-e or other coatings, improve the thermal performance of fenestration products. Fill gases other than dry air such as, carbon dioxide, argon, or krypton also improve thermal performance.
- Gap width (the distance between panes)
- Window type (casement versus double hung)
- Spacer material (the type of material separating multiple panes of glass)
Fixed Shading Devices
Shading of windows is also an important compliance option. Overhangs or sidefins that are attached to the building or shading from the building itself are compliance options for which credit is offered through the performance approach. However, no credit is offered for shading from trees, adjacent buildings, or terrain. Shading devices for which there is credit are those that are a part of the building design. For these, the designer and the builder have control over the measure and can assure that it will be constructed correctly and will perform properly. Non-credit devices are those that the designer has little or no control over, such as the height of a neighboring house or tree. Windows that face south can be effectively shaded by overhangs positioned above the window. The ideal overhang is one that provides shade during the months when the building is likely to be in an air-conditioning mode and allows direct solar gains in the heating months. This can be achieved because during the summer the sun is high as it passes over the south side, while in the winter it is low enabling solar radiation to pass beneath the overhang. Due to the potential effectiveness of south-facing overhangs, a prescriptive compliance option is offered.
Shading is much more difficult on the east and west sides of the house. When the sun strikes these façades it is fairly low in the sky, making overhangs ineffective. Vertical fins can be effective, but they degrade the quality of the view from the window and limit the natural light that can enter. In cooling climates, the best approach is to minimize windows that face east and west. Landscaping features can be considered to increase comfort but cannot be used for compliance credit.
(Excerpted from the CEC Title 24 Residential Compliance Manual)