Module02

Building Envelopes

Environmental Impact:

Embodied energy

Embodied energy is the amount of energy used to extract, produce, and distribute a material to the location of use. In general, the embodied energy of building materials contributes about 15 to 20 percent to the energy used by a building over a 50-year period[i]. This percentage is expected to increase as buildings become more energy efficient. The units for embodied energy may be expressed as Mega Joules (MJ) or BTU per unit of weight (kg or lb) or per unit area (square meter or square foot).
The embodied energy for an enclosure system is the summation of the embodied energy values of each individual material used in the system or the assembly. Although the use of different methodologies and data sets for calculating embodied energy can result in different measurements, it still provides a good indication of resource depletion and energy utilization in the construction of a building.

Strategies to for reducing Embodied Energy:

• · Utilizing salvaged materials and materials with high recycled content reduces embodied energy, construction waste, air pollution, and saves water.
• Investigating the embodied energy values of materials prior to construction can identify material choices with lower embodied energy. (see Embodied Energy Values Table)
• Reducing construction waste will lower embodied energy.
• Utilizing regional construction materials will decrease the transportation distances and will result in reduction of embodied energy.
• Utilizing efficient methods of processing materials can reduce embodied energy. As an example, using dry manufacturing process for producing cement (feeding dry grounded raw materials to kiln) is 50% more efficient than the wet process.References:
  “Building Strategies Architecture 2030.” http://www.architecture2030.org/regional_solutions/materials.html.

Carbon Footprint

Carbon footprint refers to the amount of carbon dioxide and other greenhouse gas emissions associated with an activity, a process or a product. Building materials cause greenhouse emissions throughout their life cycles as they require energy for their production and disposal. Building materials with high embodied energy generate considerable amounts of greenhouse gas emissions. Steel generates 26% more greenhouse gases and concrete 31% more, than wood[i]. Reducing a material’s carbon footprint will minimize embodied energy and mitigate environmental impact.

References:
European Commission- Joint research Centre – Institute for Environment and Sustainability, “Life Cycle Thinking and Assessment.” http://lct.jrc.ec.europa.eu/

Maureen Puettmann, WoodLife, “Carbon Footprint of Renewable and Nonrenewable Materials.” October, 18, 2009. http://www.corrim.org/presentations/2009/Puettmann_WEI.pdf

Materials Thermal Properties

Designing efficient building envelopes requires an understanding of the thermal performance of utilized materials. Reducing the envelope energy loss is critical, particularly where significant heating or cooling is required to provide thermal comfort. In general, thermal transmittance and thermal resistance are the two most important properties for understanding the conductive performance of building materials.

Thermal Resistance or R value

Thermal resistance is the capacity of a material to resist heat transfer through conduction, convection, and radiation. It is a measure of insulation value, and is largely a function of the number and size of cavity spaces in a material. Thermal resistance is expressed as R-value which has units of ft2.°F.h/Btu (square feet-Fahrenheit hour per Btu), m2. K/W (square meter- Kelvin per watt), or m².°C/W (square meter- Celsius per watt). To show the thermal resistance of a material, the symbol R is usually placed before the numerical value, as in R20. The R-value for building assemblies composed of various elements can be found by the summation of all individual element R-values.

Thermal Transmittance or U value

Thermal transmittance is the rate of heat loss or heat transfer through a material and is expressed by the U-value. Thermal transmittance measures the capacity of a material with a certain thickness to transmit heat through conduction, convection and radiation. The units of U- value are Btu/h.°F.ft² (Btu per hour per square foot per degree Fahrenheit) or W/m²K (Watts per square meter per Kelvin). The U-value of a building assembly, such as a wall system, is calculated from the reciprocal of the combined thermal resistances of the materials in the wall assembly (U=1/R).

Thermal Conductivity

Thermal conductivity is the ability of a material to transmit heat. Conductivity is a result of direct molecular interaction for transferring heat when there is a temperature difference in the material. Thermal conductivity is express by k or λ and is measured as Btu/ hr°F. ft (Btu per hour per foot per degree Fahrenheit) or W/m·K (Watts per meter per Kelvin). Thermal Conductivity and Thermal transmittance are closely related. However, Thermal conductivity does not take into account heat transfer due to radiation and convection, or the material thickness.

Thermal Bridging

Heat loss through surfaces
A thermal bridge is a part of the building envelope at which heat transfer occurs at a significantly higher rate than the surrounding areas of the envelope. Thermal bridging and air leakages happen when relatively highly thermal conductive materials such as steel and concrete are used without airtight insulation in transitional spaces thus, creating vulnerable locations for heat loss or heat transfer. These locations include periphery of windows and doors, and connection areas between the envelope components and structural elements.
Thermal, bridges are more critical in cold climates or in hot and humid climates when the indoor-outdoor temperature differences are the greatest. Without adequate insulation, the cool air and the warm air meet through thermal bridge on the surface of the envelope creating condensation and moisture built up. Moisture decreases the envelope’s R-value, creates water leakage and deteriorates the building envelope. Thermal bridging is best addressed during the design process for new buildings. For existing buildings with mandatory energy saving ordinances, various testing methods can determine the exact location and to some extent the magnitude of thermal bridges. The U.S. ENERGY STAR Qualified Homes Thermal Bypass Inspection Checklist includes two methods of infrared thermography (thermal imaging) and blower door test inspections.

References:

Energy Star, “Energy Star Qualified Homes Thermal Bypass Inspection Checklist.” June,2008. http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/ThermalBypass_Inspection_Checklist.pdf

U.S Department of Energy. “Energy Efficiency and Renewable Energy EERE.” http://www.energysavers.gov/your_home/energy_audits/index.cfm/mytopic=11190

Enclosure Materials:

• Wood
• Concrete
• Metal
• Brick
• Glass

Insulation Materials:

• Rigid Insulation
• Structural Insulated Panels
• Vacuum Insulation Panels
• Synthetic Insulation
• Sound Insulation
• Alternative Thermal Materials

Shading Devices:

• Horizontal Shading devices
• Vertical Shading devices

Photo-voltaic Facades

Climate Control Systems

Degree Days

Degree Days is a quantitative index used to estimate the energy demand of a building for heating and cooling. This index helps designers to relate the daily outdoor temperatures to the amount of energy required for space conditioning in order to provide thermal comfort. Degree days measure the difference between the average daily temperatures and a base temperature representing human comfort.

Heating Degree Days (HDD)

Heating Degree Days (HDD) indicates how much (in degrees), and for how long (in days), outside air temperature drops below a specific base temperature of 65°F (18°C)[i]. In other words, it shows the number of total degrees that the average temperature has been below the base temperature. HDD is used for estimating the building’s heating requirement and can be accessed from a wide range of resources that gather data from weather stations across the country.

Cooling Degree Days (CDD)

Cooling Degree Days (CDD) designates how much (in degrees), and for how long (in days), outside air temperature has been above a specific base temperature of 65°F (18°C). CDD indicates the number of total degrees that the average temperature has been above the base temperature. CDD is used to estimate the building’s cooling requirement and is available from a wide range of resources that gather data from weather stations across the country.

Reference:

Markuss Kottek , Jurgen Grieser, Beck Christoph, ,Franz Rubel and Bruno Rudolf, Map of the Köppen-Geiger climate classification updated, Meteorologische Zeitschrift, Publisher E. Schweizerbart’sche Verlagsbuchhandlung, Volume 15, No. 3, June 2006 , pp. 259-263(5)

Occupancy Loads

Building occupancy codes categorize buildings based on the occupancy type, building function and operations. The codes dictate a range of requirements for the occupant safety, thermal comfort, and expected resource and energy usage of the building. Awareness of theses code categories is an important step in providing an accurate estimate of the building’s energy demand and designing the environmental control systems properly.

Occupancy Types:

The most commonly used building occupancy classification code in the United States is based on the International Building Code (IBC). IBC classifies buildings into the following groups:

• Assembly (Group A) – places used for people gathering for entertainment, worship, and eating or drinking. Examples: churches, restaurants (with 50 or more possible occupants), theaters, and stadiums.
• Business (Group B) – places where services are provided. Examples: banks, insurance agencies, government buildings (including police and fire stations), and doctor’s offices.
• Educational (Group E) – schools and day care centers up to the 12th grade.
• Factory (Group F) – places where goods are manufactured or repaired (unless considered “High-Hazard”). Examples: factories and dry cleaners.
• High-Hazard (Group H) – places involving production or storage of highly flammable or toxic materials. Include places handling explosives and/or highly toxic materials (such as fireworks, hydrogen peroxide, and cyanide).
• Institutional (Group I) – places where people are physically unable to leave without assistance. Examples: hospitals, nursing homes, and prisons.
• Mercantile (Group M) – places where goods are displayed and sold. Examples: grocery stores, department stores, and gas stations.
• Residential (Group R) – places providing accommodations for overnight stay (excluding Institutional). Examples: houses, apartment buildings, hotels, and motels.
• Storage (Group S) – places where items are stored (unless considered High-Hazard). Examples: warehouses and parking garages.
• Utility and Miscellaneous (Group U) – others. Examples: water towers, barns, towers.

Reference:
International Code Council, “2009 International Building Code.” http://www.iccsafe.org/GR/Pages/adoptions.asp

Building thermal load

Calculations are conducted to estimate the total annual energy requirements to condition a building. Load calculations assist designers to determine both the type and size of the environmental systems. Thermal loads refer to the quantity of heat that must be added or removed from a space to meet the occupancy needs, thermal comfort, and air quality requirements. It takes into account both external and internal loads. External loads include the impact of solar radiation, wind, precipitation, humidity, thermal bridges, heat and moisture infiltration on the building enclosure. Internal Loads include heat generated from electrical appliances, lighting systems, and the waste heat and moisture caused by the various activity levels of the building occupants.

Building loads are often accessed from various software applications. Most software allow the designers to compile typical design load data based on particular climatic conditions, occupancy classification and schedules, and building enclosure type to accurately simulate the building and predict its energy consumption. Most simulation software utilize building loads to model the building’s behavior at the early design and planning stage.

References:
ASHRAE Design Guide Special Project 200, EPA, USGBC, BOMAI:” Indoor Air Quality –Best Practice for Design, Construction and Commissioning,” 2009
U.S. Department of Energy Building Energy, Energy Efficiency and Renewable Energy, “Building Energy Software Tools Directory”,
http://apps1.eere.energy.gov/buildings/tools_directory/subjects_sub.cfm
U.S. Department of Energy Building Energy, Energy Efficiency and Renewable Energy, “EnergyPlus Energy Simulation Software”, http://apps1.eere.energy.gov/buildings/energyplus/cfm/weather_data.cfm
NOAA National Weather Service, Climate Prediction Center, “Degree Days Statistics”,
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/cdus/degree_days/

Thermal Comfort

One of the main goals of building design is to provide comfortable indoor spaces for living and working. All indoor environments should be designed and controlled to assure thermal comfort, air quality, safety and health for the occupants.

Heat Forms

A significant portion of energy consumption and discomfort in buildings is a result of unwanted heat exchange through the envelope. The analysis of heat exchange through building envelopes requires an understanding of heat forms and heat exchange mechanisms. The following sections present some essential concepts for explaining the interaction of the building skin with the exterior environment.

Latent Heat

Latent heat is the amount of required energy to change the physical state of a material (i.e. solid, liquid, or gas) without changing its temperature. In other words, latent heat is the amount of energy absorbed or given off when a material changes its state. As this form of heat does not cause a change in temperature it cannot be measured with a thermometer.

Sensible Heat

Sensible heat is the amount of heat energy required to change a material’s temperature without changing its physical state. As sensible heat results in changing the material temperature, it can be sensed or measured by a thermometer.