A ToolBase TechNote
This TechNote is intended to provide guidance for builders considering building more energy efficient homes. Homeowners working with a custom home builder or serving as a general contractor on a home construction project may also find this information useful.
When to start
As with most projects, it is best if planning starts early. However, with the goal of improving a home's energy efficiency, a home builder can start at any time: before a home design has been created, with a set of blueprints, or even after construction has been started.
The Design Phase
Starting with a building lot, various techniques can be employed to maximize a home's energy efficiency such as taking advantage of solar energy and topographic features using a compact layout, and designing mechanical systems for efficiency.
One example is passive solar design which uses the orientation of a home to take advantage of solar energy in winter when the sun is low in the sky and uses architectural features such as overhangs to provide shading to avoid unwanted solar heat gain during the summer when the sun is high overhead. See more information about passive solar design.
Often overlooked, but an important design consideration affecting the total energy used by the home, is the size of the home. Recent statistics show that new homes on average use more energy than older homes, partially due to larger home size, increased use of air-conditioning, and the widespread use of numerous electronics. While home size will likely be determined by factors other than energy efficiency, consider whether the same function can be delivered in a smaller package. Smaller homes, because there is less space to heat and cool and less wall area for energy loss, use less energy for heating and cooling than larger homes. For example, in many climates a screened porch can add practical living space without the cost of adding air conditioned space. Designing ideas for efficient and functional spaces can be found in the book, The Not So Big House.
Lastly, designing a home so that distribution systems for mechanical systems (e.g., ductwork and hot water pipe runs) can be minimized and kept in conditioned space will improve energy efficiency and comfort. Builders can incorporate dropped ceilings, soffits, coffered ceilings, and other architectural features to serve as duct and pipe chases.
Wall/Ceiling/Foundation Insulation System
Once the basic house design is determined, the next consideration is the type of wall, roof, and foundation system to be usedand the R-value that will be achieved. Because R-value represents resistance to heat flow, the higher the R-value, the better a wall's efficiency. High R-values can be achieved with any type of construction: standard "stick-built" or alternative wall construction methods such as structural insulated panels, insulating concrete forms, or straw bale construction. Minimum R-values may be specified in local energy codes; recommended values for efficient homes can be obtained from the U.S. Department of Energy's Insulation Fact Sheet.
Windows
Windows, which have a much smaller R-value than walls, can have a large impact on the energy efficiency of a building. For this reason, one step towards efficiency is to minimize window area. Next, choose windows that are appropriate for the climate and house design. The Efficient Windows Collaborative offers advice and cost calculators for selecting windows appropriate for your climate. The ENERGY STAR label is featured on windows that meet specific efficiency criteria for a particular climate. Therefore, looking for the ENERGY STAR label is an easy way to select energy efficient windows.
There are several factors to consider when choosing windows such as frame material, glass coatings (such as low-e), gas-fill between the panes, overall U-value, solar heat gain coefficient (SHGC), and UV and visible light transmittance. Generally, look for windows with the lowest U-value for any climate. In cold climates, focus on finding a low U-value window and if windows face south and have summertime shading (e.g., an overhang or deciduous trees), consider using a high SHGC window. In hot sunny climates that use a lot of cooling, look for a low SHGC. After the above factors are determined, look for windows with the highest visible light transmittance (VT). The National Fenestration Rating Council rates most windows, and rated windows feature a sticker that provides information about the window including U-value and SHGC.
Air sealing
Keeping air from leaking in and out of a building can dramatically reduce energy needs. Air infiltration which occurs naturally through small gaps and cracks between a wall and foundation, around windows and doors, and through utility penetrations between conditioned and unconditioned space, can be a big source for energy loss. Air infiltration can draw in humidity during the cooling season, and create uncomfortable drafts during the heating season. To improve comfort and reduce energy use created by air infiltration, builders seal numerous areas of the home during the framing and finishing processes. Figure 1 shows key points of a home that should be sealed against air infiltration.
Ventilation
Once the home is sealed to prevent air leaks, it is necessary to provide controlled ventilation. Ventilation helps promote good indoor air quality and prevent the accumulation of moisture in the indoor air. While it may seem counterproductive to prevent air infiltration and then purposefully bring in fresh air, it makes good sense. One problem with relying on natural air infiltration is that it is often driven by wind and the difference in temperature between indoors and out. There is more air infiltration during the most extreme outdoor conditions and little air infiltration in mild weather. Another problem in relying on air infiltration alone is that air may be forced through building cavities which can lead to condensation in walls (potentially leading to moisture problems) and the admittance of dust and dirt from the wall cavity into the home.
To mitigate the energy penalty of mechanical ventilation, builders in severe cold climates will use heat recovery ventilation systems that temper the incoming fresh air with the exhaust (conditioned) air stream. This type of system can recover 70% or more of the energy in the indoor air before it is exhausted. In hot humid climates, energy recovery ventilators-which recover heat and moisture in air-can be beneficial.
Mechanical systems
Once the building shell has been tightly built and well insulated, it is time to focus on mechanical systems. Mechanical systems encompass heating, cooling, and hot water systems. The choice to invest in high efficiency equipment will likely be based on climate and local equipment pricing. For example, in a cooling-dominated climate, it may make more sense to invest in higher efficiency cooling equipment and use standard efficiency heating equipment.
There are federal minimum efficiency standards for all mechanical equipment with many options available for higher efficiency equipment. Each equipment type is rated by a different factor such as AFUE, COP, HSPF, SEER, EER, and EF. There are many types of equipment for heating, cooling and water heating, and without getting bogged down by acronyms and equipment types, the easiest way to select efficient mechanical equipment is to choose equipment labeled with the ENERGY STAR. ENERGY STAR-labeled equipment meets high efficiency performance criteria set by the program.
A list of the most energy efficient equipment available can be found at the American Council for Energy Efficient Economy's website.
Heating and Cooling System Sizing
An important element of ensuring efficient operation of mechanical equipment is to size equipment properly. Seek HVAC contractors that use industry standards for equipment sizing rather than "rule-of-thumb" approach. Oversized heating equipment turns on and off frequently because it heats the home very quickly. This cycling results in less efficient operation (equipment operates most efficiently when it is humming along) and increased wear and tear on the system. The industry standard for calculating heating and cooling loads on a house is the Air Conditioning Contractor's Association's (ACCA) Manual J. The software version of this calculation method is called Right-J. Make sure the contractor has as many details about the house for use in the calculation (e.g., window U-values and SHGC, wall insulation values, house orientation, and floor plan).
Mechanical Systems: Ductwork
Another critical aspect to the efficiency of heating and cooling is the design and installation of ducts. You wouldn't install a radiator that leaked energy (i.e., in the form of hot water) all over the attic, so why install ductwork that leaks?
Ductwork, especially when contained in the attic, is a big source of energy loss. The reason is that ductwork is typically insulated to much lower R-values than ceilings or walls and faces a much higher temperature difference than a typical indoor-outdoor temperature difference. As a result, when a home is being heated with delivery air of 130ºF, the other side of that R-6 insulation might be 15ºF! Under these conditions, consider an example in which ducts in an attic are insulated to R-6 and the ceiling is insulated to R-38. In this case, while the heat is on, the amount of energy being lost from just 20 linear feet of 12-inch ductwork in the attic is greater than the amount of energy lost from over 1000 ft2 of ceiling area!
To mitigate duct energy losses, many builders will design ductwork into conditioned space. The book, A Builder's Guide to Placement of Ducts and HVAC Equipment in Conditioned Spaces gives tips and techniques for bringing ducts into conditioned space.
Lighting and appliances
In very energy efficient homes, the amount of annual energy use from lights, appliances, and "plug loads" can be equal to the energy needed for heating, cooling, and water heating. While a builder cannot affect a homeowner's personal energy use habits, she can install highly efficient lighting and appliances and add controls that can minimize these energy loads.
Fluorescent lighting uses about ¼ the energy of incandescent lighting and provides the same amount of light. Many people are convinced they don't like fluorescent lighting. However, this negative bias is usually a function of the older cool-white fluorescent lights. There are a variety of fluorescent bulbs (lamps) that closely approximate the warm-white light of incandescent lamps. A lamp having a color temperature (CT) of about 2700 to 2800 K and a Color Rendering Index (CRI) of more than 80 will not be discernable from incandescent lighting to most homeowners. Another option in fluorescent lighting is the permanent (pin-type) fluorescent fixtures. Attractive fixture options are increasingly becoming available (see www.energystar.gov for a list), but still tend to be difficult to find.
Major appliances such as the clothes washer, dishwasher, and refrigerator can have a big impact on a home's energy consumption and hot water usage. Energy efficient dishwashers use about half as much hot water as their less efficient counterparts; efficient clothes washers use less than half; and the most energy efficient refrigerators use about three-quarters as much energy as less efficient ones.
Scoring Efficiency
You may design and build an efficient homebut how can a person in the market tell, especially since many efficiency features are hidden behind walls and tucked into mechanical closets? A home energy rating can provide the information you want to relay to your customer. The Home Energy Rating System (HERS) evaluates homes on a scale of 1 to 100. A HERS 80 meets model energy code minimum and every point above 80 indicates that the house performs 5% better than code. A home that scores an 86 or better can be labeled as an ENERGY STAR home under the ENERGY STAR New Homes program.
To obtain a home energy rating, a certified HERS rater performs air tightness testing of the home with a device called a blower door and may perform air tightness testing of ductwork. Entering these diagnostic test results along with the home plans and specifications into a computer program produces the home energy rating. To find a home energy rater in your area, visit www.energystar.gov.
There are numerous resources and programs designed to help you improve home energy efficiency. Information about technologies can be found on the ToolBase website. The EnergyValue Housing Award (EVHA) program website describes energy saving practices of the EVHA-winning builders, and all applicants receive an evaluation of their company's energy efficiency practices. Check out EnergyValue Housing Award Guide: How to Build and Profit with Energy Efficiency in New Home Construction for information on the basics of energy efficient construction, tried-and-true ideas from successful builders, and a list of resources for those wishing to learn more. Other programs, such as Building America work with builders to incorporate energy efficient systems. Manufacturer and utility programs work with builders to meet criteria set by the programs. For example, the Environments for Living program reviews house plans and backs an energy cost guarantee for homes that meet the program criteria.