Elements of the building envelope significantly contribute to its energy efficiency. Several features are important to note when a method is chosen to demonstrate compliance. Components of the building envelope include walls, floors, the roof and/or ceiling, and fenestration. Details for compliance of fenestration are addressed in Section 3.5, Fenestration.
A. Walls and Space(s) Surrounding Occupancy Uses
B. Envelope and other building component definitions are listed in §100.1 of the 2013 Standards, and the Reference Appendices.
C. Envelope requirements vary by envelope component and are a function of their type of construction, their orientation and the space conditions on either side of the envelope surface. Additional envelope component definitions are as follows:
D. An exterior partition or wall is an envelope component (roof, wall, floor, window etc.) that separates conditioned space from ambient (outdoor) conditions.
E. A demising partition or wall is an envelope component that separates conditioned space from an unconditioned space.
F. A conditioned space is either directly conditioned or indirectly conditioned (see Section 100.1 for full definition). An indirectly conditioned space has less thermal resistance to a directly conditioned space than to the outside. An unconditioned space is enclosed space within a building that is not directly conditioned, or indirectly conditioned.
G. A plenum is a space below an insulated roof and above an uninsulated ceiling. It is an indirectly conditioned space as there is less thermal resistance to the directly conditioned space below than to the ambient air outside. In comparison, an attic below an uninsulated roof and having insulation on the attic floor is an unconditioned space because there is less thermal resistance to the outside than across the insulated ceiling to the conditioned space below. A plenum can also be the space between the underside of a raised floor and the crawl space ground, and is sometimes used as an air supply for the building when the exterior foundation is sealed to the outside.
H. Sloping surfaces are considered either a wall or a roof, depending on the slope (see Figure 3-1). If the surface has a slope of less than 60° from horizontal, it is considered a roof; a slope of 60° or more is a wall. This definition extends to fenestration products, including windows in walls and any skylight types in roofs.
I. Floors and roof/ceilings do not differentiate between demising and exterior. Thus an exterior roof/ceiling ”is an exterior partition, or a demising partition, that has a slope less than 60 degrees from horizontal, that has conditioned space below,” ambient conditions or unconditioned space above “and that is not an exterior door or skylight.”
J. Similarly an “exterior floor/soffit is a horizontal exterior partition, or a horizontal demising partition, under conditioned space” and above an unconditioned space or above ambient (outdoor) conditions.
A. A vapor retarder or barrier is a special covering over framing and insulation or covering the ground of a crawl space that protects the assembly components from possible damage due to moisture condensation. During cold weather, the inside of the house is warm and moist (from breathing, showers, etc.) and the outside is cold and dry. Moisture moves from more to less and from warm to cold. When the moisture (in vapor form) reaches a point in the wall or roof assembly that has a temperature below the dew point, it will condense into liquid water. Water build up can cause structural damage, create mold that may contribute to indoor air quality problems and can cause the insulation to lose its effectiveness.
B. Fenestration or Windows are considered part of the exterior wall because the slope is typically over 60°. Where the slope of fenestration is less than 60°, the glazing indicated as a window is considered a skylight.
C. Roofing Products (Cool Roof)
Roofing products with a high solar reflectance and thermal emittance are referred to as “cool roofs.” These roofing types absorb less solar heat and give off more heat to their surroundings than traditional roofing material. These roofs are cooler and thus help reduce air conditioning loads by reflecting and emitting energy from the sun. Roof radiative properties are rated and listed by the Cool Roof Rating Council (CRRC) (www.coolroofs.org/).
In general, light-colored high reflectance surfaces reflect solar energy (visible light, invisible infrared and ultraviolet radiation) and stay cooler than darker surfaces that absorb the sun’s energy and become hot.
The standards specify radiative properties that represent minimum “cool roof performance” qualities of roofing products:
1. Solar reflectance—the fraction of solar energy that is reflected by the roof surface
2. Thermal emittance—the fraction of thermal energy that is emitted from the roof surface
Both solar reflectance and thermal emittance are measured from 0 to 1; the higher the value, the "cooler" the roof. There are numerous roofing materials in a wide range of colors that have relatively good cool roof properties. Excess heat can increase the building’s air conditioning load resulting in increased air conditioning energy needed for maintaining occupant comfort. High-emitting roof surfaces reject absorbed heat quickly (upward and out of the building) than darker roof surfaces with low-emitting properties.
The standards prescribe cool roof radiative properties for low-sloped and steep-sloped roofs (§150.1(c)11). A low-sloped roof is defined as a surface with a pitch less than or equal to 2:12 (9.5 degrees from the horizon), while a steep-sloped roof is a surface with a pitch greater than 2:12 (9.5 degrees from the horizon). Because solar heat gain is based on the sun’s angle of incidence on a surface, low-sloped roofs receive more solar radiation than steep-sloped roofs in the summer when the sun is high in the sky.
Example 3-1
Question
I am a salesperson and represent some roofing products, and many of them are on the EPA’s Energy Star list for cool roofing materials. Is this sufficient to meet Standards?
Answer
No. Energy Star has different requirements for reflectance and NO requirements for emittance. Per Section10-113 of the Energy Building Regulations, the Cool Roof Rating Council (www.coolroofs.org) is the only entity currently recognized by the Energy Commission to determine what qualifies as a cool roof.
Example 3-2
Question How does a product get CRRC cool roof certification?
Answer
Any party wishing to have a product or products certified by CRRC should contact CRRC to get started call toll-free (866) 465-2523 from inside the US or (510) 485-7176, or email info@coolroofs.org. CRRC staff will walk interested parties through the procedures. In 'addition, CRRC publishes the procedures in "CRRC-1 Program 'Manual," available for free on www.coolroofs.org or by calling CRRC. However, working with CRRC staff is strongly recommended.
Example 3-3
Question
I understand reflectance, but what is emittance?
Answer
Even a material that reflects the sun’s energy will still absorb some of that energy as heat; there are no perfectly reflecting materials being used for roofing. That absorbed heat undergoes a physical change (an increase in wavelength, for readers who remember physics) and is given off – emitted – to the environment in varying amounts by various materials and surface types. This emittance is given a unit-less value between 0 and 1, and this value represents a comparison (ratio) between what a given material or surface emits and what a perfect blackbody emitter (again, recall physics) would emit at the same temperature.
A
higher emittance value means more energy is released from the material or
surface; scientists refer to this emitted energy as thermal radiation (as
compared to the energy from the sun, solar radiation, with shorter wavelength).
Emittance is a measure of the relative efficiency with which a material,
surface, or body can cool itself by radiation. Lower-emitting materials become
relatively hotter for not being able to get rid of the energy, which is heat.
Roof materials with low emittance therefore hold onto more solar energy as heat,
get hotter than high-emittance roofs, and with help from the laws of physics,
offer greater opportunity for that held heat to be given off downward into
the building through conduction. More heat in the building increases the
need for air conditioning
for comfort. A cool roof system that reflects solar
radiation (has high reflectance) and emits thermal radiation well (has high
emittance) will result in a cooler roof and a cooler building with lower
air-conditioning costs.
D. Air Leakage
Infiltration is the unintentional replacement of conditioned air with unconditioned air through leaks or cracks in the building envelope. It is a major component of heating and cooling loads. Air leakage can occur through holes and cracks in the building envelope and around doors and fenestration framing areas. Ventilation is the intentional replacement of conditioned air with unconditioned air through open windows and skylights or mechanical ventilation.
Reducing air leakage in the building envelope can result in significant energy savings, especially in climates with more severe winter and summer conditions. It also can result in improved building comfort, reduced moisture intrusion, and fewer air pollutants.
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E. Advanced Assemblies
Common strategies for exceeding the minimum energy performance level set by the 2013 Standards include the use of better components such as: higher insulation levels, more efficient fenestration, reducing the building’s air leakage, using radiant barriers, “cool” roofing products, better framing techniques that accommodate more insulation (raised-heel truss) and reduce thermal bridging across framing members, greater use of non-framed assemblies or panelized systems (SIPs and ICFs), and more efficient heating, cooling and water heating equipment. The Energy Commission encourages the use of energy savings techniques for showing compliance with the standards. Innovative designs and practices are discussed in the “Advanced Assembly System” section.
F. Advanced Building Design
The building’s design, its floor plan and site design layout impact energy use. A passive solar designed building uses elements of the building to help heat and cool itself as opposed to relying on mechanical systems to provide the building's thermal energy needs. Passive solar strategies encompass several advanced high performance envelope techniques, such as:
1. Carefully choosing the size, type and placement of fenestration and shading
2. Providing and controlling fresh air ventilation during the day or night
3. Having internal and external thermal mass components that help store useful heat and cooling energy
4. Having highly insulated envelope assemblies
5. Using radiative energy performing roofing materials (cool roofs) and radiant barriers
6. Having very low air leakage
Some measures included as part of an Advanced Assembly System may require specific installation procedures, or field verification and diagnostic testing to ensure their proper performance. Field verification and diagnostic testing is a way to ensure that the energy efficiency that is used in compliance calculations is actually realized as an energy benefit by the homeowner.