Elements of the building envelope significantly contribute to the related 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. below. Envelope and other building components are listed in §100.1 of the 2016 Energy Standards and the Reference Appendices (RA).
A. Envelope requirements vary by envelope component and are a function of the type of construction, orientation and space conditions on either side of the envelope surface. Additional envelope component definitions are:
1. Exterior partition or wall is an envelope component (roof, wall, floor, window and others) that separates conditioned space from ambient (outdoor) conditions.
2. Demising partition or wall is an envelope component that separates conditioned space from an unconditioned space.
B. Conditioned space is either directly conditioned or indirectly conditioned (see Section 100.1 for full definition). An indirectly conditioned space has less thermal conductance 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.
C. 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. By comparison, an attic is a space below an uninsulated roof that has insulation on the attic floor, and 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 slab or grade below, and is sometimes used as an air supply for the building when the exterior foundation is sealed to the outside. A plenum can also be the space between the underside of an insulated ceiling and demising partitions or walls separating the attic from the conditioned space below.
D. 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.
E. Floors and roof/ceilings Floors and roof/ceilings do not differentiate between demising and exterior. Thus an exterior roof/ceiling ”is an exterior, or demising partition element, that has a slope less than 60 degrees from horizontal, and that has conditioned space below,” ambient conditions or unconditioned space above. This definition does not include an exterior door or skylight.
F. Roof deck is the surface of an exterior roof that is directly above the roof rafter and below exterior roofing materials.
G. Similarly, an “exterior floor/soffit is a horizontal exterior element, or a horizontal demising element, under conditioned space” and above an unconditioned space or ambient (outdoor) conditions.
H. 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, and so forth), and the outside is cold and dry. Moisture moves from moist to drier conditions and from warm to cold. When the moisture (in vapor form) reaches a point in a wall or roof assembly that has a temperature below the dew point, it will condense into 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 effectiveness.
I. Fenestration (Windows) are considered part of an 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.
Source: California Energy Commission
J. 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 the surroundings than traditional roofing materials. 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 heated.
The Energy Standards specify radiative properties that represent minimum “cool roof performance” qualities for 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 adequate cool roof properties. Excess heat can increase the air-conditioning load of a building, 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 roof surfaces with low-emitting properties.
The Energy 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 Low-sloped roofs receive more solar radiation than steep-sloped roofs in the summer when the sun is higher in the sky.
Example 3-1
Question:
I am a salesperson and represent some roofing products, and many of them are on the U.S. Environmental Protection Agency’s (EPA) ENERGY STAR® list for cool roofing materials. Is this sufficient to meet the Energy 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 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 the CRRC should contact the CRRC. To get started, call toll-free (866) 465-2523 from inside the United States or (510) 485-7176, or email info@coolroofs.org. CRRC staff will walk interested parties through the procedures. Working with, CRRC staff is strongly recommended. In 'addition the CRRC publishes the procedures in "CRRC-1 Program 'Manual," available for free on www.coolroofs.org or by calling CRRC.
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) and is given off – emitted – to the environment in varying amounts depending on the materials and surface types. This emittance is given a unitless 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 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 the 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.
K. 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. Infiltration can occur through holes and cracks in the building envelope and around doors and fenestration framing areas. Ventilation, on the other hand is the intentional replacement of conditioned air with unconditioned air through open windows and skylights or mechanical systems.
Reducing infiltration 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 occupant comfort, reduced moisture intrusion, and fewer air pollutants.
L. Advanced Assemblies
Common strategies for exceeding the minimum energy performance level set by the 2016 Energy Standards include the use of better components such as:
•Higher insulation levels, more
•More efficient fenestration,
•Reducing building infiltration
•Use of roofing products
•Better framing techniques (such as the use of raised-heel trusses that accommodate more insulation)
•Reduced thermal bridging across framing members
•Greater use of nonframed assemblies or panelized systems (such as SIPs and ICFs)
•More efficient heating, cooling and water ‘heating equipment
The Energy Commission encourages the use of energy saving techniques for showing compliance with the Energy Standards. Innovative designs and practices are discussed in Section 3.6.3.4.
M. Advanced Building Design
The design .of a building floor plan, and site design layout all affect energy use. A passive solar building uses elements of the building to heat and cool itself in contrast to relying on mechanical systems to provide the thermal energy needs of the building. 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 and 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 high performing roofing materials (cool roofs) and radiant barriers.
6. Having very low air leakage.
Some measures designed as part of an advanced assembly system may require specific installation procedures or field verification and diagnostic testing to ensure proper performance. Field verification and diagnostic testing are ways to ensure that the energy efficiency features used in compliance calculations is realized as an energy benefit to the occupants.