3.1 Chapter Overview

3.1.1 What’s New for 2016

The 2016 Building Energy Efficiency Standards (Energy Standards) include several important changes to the building envelope component requirements, as described below:

A. Revised the minimum mandatory requirements for insulation (§120.7) that apply to

wall and roof/ceiling insulation:

a. Metal building.

b. Demising walls.

c. Placement of insulation in the roof/ceiling to limit heat loss and gain from conditioned to unconditioned spaces.

B. Revised prescriptive requirements of §140.3 that apply to:

a. Metal building roofs, and wood-framed and other roofs.

b. Metal-framed walls and wood-framed walls.

C. Requirements for additions, alterations, and repairs (§141.0) that apply to:

a. Mandatory insulation requirements for alterations.

b. Maximum U-factor and shading requirements for fenestration in alterations.

c. Roof/ceiling insulation tradeoff for aged solar reflectance of roofing being replaced, recovered, or recoated.

d. Window films.

D. Requirements for additions and alterations applying to covered processes, §141.1.

3.1.1.1 Mandatory Measures

§120.7

When compliance is being demonstrated with either the prescriptive or performance compliance paths, there are mandatory measures that must always be met. If the prescriptive compliance approach is used, the prescriptive efficiency levels are required; if the performance approach is used, the prescriptive requirements establish the baseline for comparison for the proposed building.

The minimum mandatory levels are sometimes superseded by more stringent prescriptive or performance approach requirements. For example, the mandatory measures specify a weighted average U-factor of a metal framed wall insulation to be U-0.151 but, if compliance is being demonstrated with the prescriptive approach for a nonresidential building, Table 140.3-B of the Energy Standards is used to establish the minimum wall thermal compliance level, which in some cases exceeds the mandatory insulation requirements.

3.1.1.2 Prescriptive Approach

Energy Standards Table 140.3-B, C, and D

The prescriptive requirements are the simplest way to comply with the building envelope requirements but offer little flexibility. If each and every prescriptive requirement is met, the building envelope complies with the Energy Standards.

The prescriptive compliance approach consists of specific requirements for each envelope component, in which minimum mandatory levels of insulation must be met. Prescriptive requirements apply to:

• roofs and ceilings

• exterior roofing products

• exterior walls

• demising walls

• floors and soffits

• fenestration products

Envelope requirements vary by climate zone and occupancy type. The prescriptive requirements are located in §140.3 which includes Table 140.3-B for all nonresidential buildings; Table 140.3-C for high-rise residential buildings and hotel/motel buildings; and Table 140.3-D for relocatable public school buildings.

3.1.1.3 Performance Approach

§140.1

The performance approach is a more sophisticated compliance method that offers design flexibility. It may be used for:

• Envelope-only compliance

• Envelope and lighting compliance

• Envelope and partial lighting compliance (where some tenant spaces are not yet defined)

• Envelope and mechanical compliance

• Envelope, lighting and mechanical compliance

The performance approach allows for energy tradeoffs between building features, such as increasing envelope insulation levels or improving window performance to allow more lighting power or a less efficient space-conditioning system. Under this method, energy use of the building is modeled by compliance software approved by the Energy Commission. See Section 3.5 and Chapter 11 for a more complete discussion of the performance approach.

3.1.2 Envelope Definitions and Features

Elements of the building envelope contribute significantly to the energy efficiency of the building and its design intent. Several features are important to note when a method is chosen to demonstrate compliance. Components of the building shell include the walls, floor, the roof or ceiling, doors, and fenestration. Details for fenestration compliance for windows, skylights and doors are addressed in Section 3.

3.1.2.1 Walls and Space(s) Surrounding Occupancy Uses

Envelope and other building component definitions are 'listed in §100.1.

A. Envelope requirements vary by envelope component and are a function of their type of construction, and the space conditions on either side of the envelope surface.

B. An exterior partition is an envelope component (roof, wall, floor, window etc.) that separates conditioned space from ambient (outdoor) conditions. A demising partition is an envelope component that separates conditioned space from an enclosed unconditioned space.

C. A conditioned space is either directly conditioned or indirectly conditioned (See §100.1 for full definition). Indirectly conditioned space is influenced more by directly adjacent conditioned space than it is by ambient (outdoor) conditions. An unconditioned space is an enclosed space within a building that is not directly conditioned or indirectly conditioned.

Example - A plenum space below an insulated roof and above an uninsulated ceiling 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 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.

D. An exterior wall is considered separate from a demising wall or demising partition and has more stringent thermal requirements.

E. 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 skylights in roofs.

F. 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.”

G. 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.

H. Windows are considered part of the wall because the slope is more than 60°. Where the slope is less than 60°, the glazing indicated as a window is considered a skylight.

Figure 3-1: Slope of a Wall or Window (Roof or Skylight Slope Is Less Than 60°)

3be_slope-wall-window-roof-skylit_r1

3.1.2.2 Roofing Products (Cool Roof)

Roofing products with a high solar reflectance and thermal emittance are referred to as “cool roofs.” This roofing type absorbs less solar heat and gives off more heat to the surroundings than traditional roofing material. The roofs are cooler and 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) (http://www.coolroofs.org).

Light-colored high reflectance surfaces reflect solar energy (visible light, invisible infrared and ultraviolet radiation) and stay cooler than darker surfaces that absorb more of the sun’s energy.

Thermal emittance refers to the ability of heat to escape from a surface once heat energy is absorbed. 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 range of colors that have relatively good cool roof properties. Surfaces with low emittance (usually shiny metallic surfaces) contribute to the transmission of heat into components under the roof surface. Excess heat can increase the air-conditioning load of a building resulting in increased energy needed for maintaining occupant comfort. High-emitting roof surfaces reject absorbed heat faster (upward and out of the building) than roof surfaces with low-emitting properties that are usually darker.

The Energy Standards prescribe cool roof radiative properties differently for low-sloped and steep-sloped roofs (§140.3(a)1A). 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 heat solar 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.

The Energy Standards specify radiative properties that represent minimum “cool roof performance” qualities of roofing products. Performance values are established based on “initial” testing of the roofing product and for the “aged” value, which accounts for the effects of weathering due to climate conditions:

• Solar reflectance: The fraction of solar energy that is reflected by the roof surface.

• Thermal emittance: The fraction of thermal energy that is emitted from the roof surface

• Solar reflectance index (SRI): The relative surface temperature of a surface with respect to standard white (SRI=100) and standard black (SRI=0) under the standard solar and ambient conditions. This combined metric is a function of both solar reflectance and thermal emittance. The same SRI can be achieved if the roofing product has a higher solar reflectance but a lower thermal emittance. The SRI metric is a prescriptive alternative to reflectance and emittance requirements and is not used with the performance compliance method.

3.1.2.3 Infiltration and Air Leakage

Infiltration is the unintentional replacement of conditioned air with unconditioned air through leaks or cracks in the building envelope. Poor construction detailing at interfacing points of different construction materials, particularly in extreme climates, can significantly affect heating and cooling loads. Air leakage can occur through holes and cracks in the building envelope, and around doors and fenestration areas. Ventilation is the intentional replacement of conditioned air with unconditioned air through open windows or mechanical ventilation.

Reducing air leakage in the building envelope can result in significant energy savings, especially in climates with more severe winters and summers. It can also result in improved building comfort, reduced moisture intrusion, and fewer air pollutants.

An air barrier that inhibits air leakage is critical to good building design and is a prescriptive requirement (See Section 3.2.1.2).

3.1.2.4 Thermal Properties of Opaque Envelope Components

Typical opaque envelope assemblies are made up of a variety of components, such as wood or metal framing, masonry or concrete, insulation, various membranes for moisture and/or fire protection, and may have a variety of interior and exterior sheathings even before the final exterior façade is placed. Correctly calculating assembly U-factors is critical to the selection of equipment to meet the heating and cooling loads of the building. Performance compliance software automatically calculates the thermal effects of component layers making up the envelope assembly, but software programs may use different user input hierarchies. The Reference Appendices, Joint Appendix JA4, provide detailed thermal data for many wall, roof/ceiling, and floor assemblies. However, this reference cannot cover every possible permutation of materials, thickness, and so forth, that might be used in a building; thus, the Energy Commission has incorporated a program for calculating material properties of typical envelope assemblies that may not be found from the JA4 reference data into the public domain software CBECC-COM.

Key terms of assembly thermal performance are:

A. Btu (British thermal unit): The amount of heat required to raise the temperature of 1 pound. of water 1^°F.

B. Btuh or Btu/hr (British thermal unit per hour): The rate of heat flow during an hour. The term is used to rate the output of heating or cooling equipment or the load that equipment must be capable of handling; that is, the capacity needed for satisfactory operation under stated conditions.

C. R or R-value (thermal resistance): The ability of a material or combination of materials to retard heat flow. As the resistance increases, the heat flow is reduced. The higher the “R-value”, the greater the insulating value. R-value is the reciprocal of the conductance, “C-value.”

R-value = hr x ft² x ^oF/Btu

R = inches of thickness/k

D. U or U-factor (thermal transmittance or coefficient of heat transmission): The rate of heat transfer across an envelope assembly per degree of temperature difference on either side of the envelope component. U-factor is a function of the materials and related thickness. U-factor includes air film resistances on inside and outside surfaces. U-factor applies to heat flow through an assembly or system, whereas “C” has the same dimensional units and applies to individual materials. The lower the “U” the higher the insulating value.

U-factor = Btu/(hr x ft² x ºF)

E. k or k-value (thermal conductivity): The property of a material to conduct heat in the number of Btu that pass through a homogeneous material 1 inch thick and 1 square foot in area in an hour with a temperature difference of 1°F between the two surfaces. The lower the “k” the greater the insulating value.

k = Btu x in/(hr x ft² x °F)

F. C or C-value (thermal conductance): The number of Btu that pass through a material of any thickness and 1 square foot in area in an hour with a temperature difference of 1°F between the two surfaces. The time rate of heat flow through unit area of a body induced by a unit temperature difference between the body surfaces. The C-value does not include the air film resistances on each side of the assembly. The term is applied usually to homogeneous materials but may be used with heterogeneous materials such as concrete block. If “k” is known, the “C” can be determined by dividing “k” by inches of thickness. The lower the “C”, the greater the insulating value.

C = Btu/(hr. x ft² x °F) or C = k/inches of thickness

G. HC (heat capacity – thermal mass): The ability to store heat in units of Btu/ft² and is a property of specific heat, density, and thickness of a given envelope component. High thermal mass building components, such as tilt-up concrete walls, can store heat and release stored heat later in the day or night. The thermal storage capability of high mass walls, floors, and roof/ceilings can slow heat transfer and shift heating and cooling energy affecting building loads throughout a 24-hour period, depending on the design, location, and occupancy use of a building.