Elements of an Energy-Efficient House
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Introduction
You have much to consider when designing and building a new energy-efficient house, and it can be a challenge. However, recent technological improvements in building elements and construction techniques also allow most modern energy-saving ideas to be seamlessly integrated into house designs while improving comfort, health, or aesthetics. And even though some energy-efficient features are expensive, there are others that many home buyers can afford.
While design costs, options, and styles vary, most energy-efficient homes have some basic elements in common: a well-constructed and tightly sealed thermal envelope; controlled ventilation; properly sized, high-efficiency heating and cooling systems; and energy-efficient doors, windows, and appliances.
Thermal Envelope
A thermal envelope is everything about the house that serves to shield the living space from the outdoors. It includes the wall and roof assemblies, insulation, air/vapor retarders, windows, and weatherstripping and caulking.
Wall and Roof Assemblies
Most builders use traditional wood frame construction. Wood framing is a ?tried and true? construction technique that uses a potentially renewable resource—wood—to provide a structurally sound, long-lasting house. With proper construction and attention to details, the conventional wood-framed home can be very energy-efficient. It is now even possible to purchase a sustainably harvested wood.
Some of the available and popular energy-efficient construction methods include the following:
- Optimum Value Engineering (OVE). This method uses wood only where it is most effective, thus reducing costly wood use and saving space for insulation. The amount of lumber has been determined to be structurally sound through both laboratory and field tests. However, the builder must be familiar with this type of construction to ensure a structurally sound house.
- Structural Insulated Panels (SIPs). These sheets are generally made of plywood or oriented-strand board (OSB) that is laminated to foam board. The foam may be 4 to 8 inches thick. Because the SIP acts as both the framing and the insulation, construction is much faster than OVE or stick framing. The quality of construction is often superior because there are fewer places for workers to make mistakes.
- Insulating Concrete Forms (ICF. Houses constructed in this manner consist of two layers of extruded foam board (one inside the house and one outside the house) that act as the form for a steel-reinforced concrete center. It's the fastest technique and least likely to have construction mistakes. Such buildings are also very strong and easily exceed code requirements for areas prone to tornadoes or hurricanes.
Insulation
An energy-efficient house has much higher insulation R-values than required by most local building codes. An R-value is the ability of a material to resist heat transfer, and the lower the value, the faster the heat loss. For example, a typical house in New York might have insulation of R-11 in the exterior walls and R-19 in the ceiling, while the floors and foundation walls may not be insulated. A similar, but well-designed and constructed house will have insulation levels that range from R-20 to R-30 in the walls and from R-50 to R-70 in the ceilings. Carefully applied fiberglass batt or rolls, wet-spray cellulose, or foam insulation will fill wall cavities completely.
Foundation walls and slabs should be as well insulated as the living space walls. Poorly insulated foundations have a negative impact on home energy use and comfort, especially if the family uses the lower parts of the house as a living space. Also, appliances—such as domestic hot water heaters, washers, dryers, and freezers—that supply heat as a byproduct are often located in the basement. By carefully insulating the foundation walls and floor of the basement, these appliances can assist in heating the house.
While most new houses have good insulation levels, it is often poorly installed. In general, gaps and compaction of insulation reduce its effectiveness.
Air/Vapor Retarders
Water vapor condensation is a major threat to the structure of a house, no matter what the climate. In cold climates, pressure differences can drive warm, moist indoor air into exterior walls and attics. The air condenses as it cools. The same can be said for southern climates, just in reverse. As the humid outdoor air enters the walls and encounters cooler wall cavities, it condenses into liquid water. This is the main reason why some buildings in the South have problems with mold and rotten wood after they're retrofitted with air conditioners.
A vapor retarder is a material or structural element that can be used to inhibit the movement of water vapor, while an air retarder can inhibit airflow, into and out of a house's envelope. How to design and install vapor retarders depends a great deal on the climate and on the chosen construction method. However, any water vapor that does manage to get into the walls or attics must be allowed to escape.
Regardless of climate, water vapor migration should be minimized by using a carefully designed thermal envelope and sound construction practices. Systems that control air and water vapor movement in homes rely on the nearly airtight installation of sheet materials on the interior as the main barrier.
The Airtight Drywall Approach (ADA) uses the drywall already being installed along caulking to create a continuous air retarder. In addition, seams where foundation, sill plate, floor joist header, and subfloor meet are also carefully sealed with appropriate caulk or gasket material.
Consult your local building codes official on the best vapor retarder method to use in your area.
Windows
The typical home loses more than 25 percent of its heat through windows. Even modern windows insulate less than a wall. Therefore, an energy-efficient house in a heating-dominated climate should, in general, have few windows on its northern, eastern, and western sides. Total window area should also not exceed 8 to 9 percent of the floor area for those rooms, unless the designer is experienced in passive solar techniques. If this is the case, then increasing window area on the southern side of the house to about 12 percent of the floor area is recommended. This is often called solar tempering.
A properly designed roof overhang for south-facing windows will help prevent overheating in the summer. North, east, and west windows should have low Solar Heat Gain Coefficients (SHGC). South windows with properly sized overhangs should have a high SHGC to allow winter sun (and heat) to enter the house. The overhang blocks the high summer sun (and heat). If properly sized overhangs are not possible, a low SHGC glass should be selected for the south windows.
At the very least, you should use windows (and doors) with an Energy Star? label, which are twice as energy efficient as those produced 10 years ago, according to regional, climatic guidelines (note: houses with any kind of solar tempering have other guidelines). The best windows are awning and casement styles because these often close tighter than sliding types. In all climates, window glass facing south without overhangs can cause a problem on the cooling side that far exceeds the benefit from the winter solar gains.
Weatherstripping and Caulking
You should seal air leaks everywhere in a home's thermal envelope to reduce energy loss. Good air sealing alone may reduce utility costs by as much as 50 percent when compared to other houses of the same type and age. You can accomplish most air sealing by using two materials: caulking and weatherstripping. Caulking can be used to seal areas of potential air leakage into or out of a house. And weatherstripping can be used to seal gaps around windows and exterior doors.
Controlled Ventilation
Since an energy-efficient house is tightly sealed, it needs to be ventilated in a controlled manner. Controlled, mechanical ventilation prevents health risks from indoor air pollution, promotes a more comfortable atmosphere, and reduces air moisture infiltration, thus reducing the likelihood of structural damage.
Furnaces, water heaters, clothes dryers, and bathroom and kitchen exhaust fans expel air from the house, making it easier to depressurize an airtight house if all else is ignored. But natural-draft appliances may be back-drafted by exhaust fans, which can lead to a lethal buildup of toxic gases in the house. For this reason, sealed-combustion heating appliances, which use only outside air for combustion and vent combustion gases directly to the outdoors, are very important for ventilation energy efficiency and safety.
Heat recovery ventilators (HRV) or energy recovery ventilators (ERV) are growing in use for controlled ventilation in airtight homes. These ventilators can salvage about 70 percent of the energy from the stale exhaust air and transfer that energy to the fresh air entering by way of a heat exchanger inside the device. They can be attached to the central forced air system or may have their own duct system.
Other ventilation devices, such as through-the-wall or ?trickle? vents, may be used in conjunction with an exhaust fan. They are, however, more expensive to operate and possibly more uncomfortable to use because they have no energy recovery features to precondition the incoming air. Uncomfortable incoming air can be a serious problem in northern climates and can create moisture problems in humid climates. Therefore, this ventilation strategy is only for arid climates. Other systems pull outside air in with a small outside duct on the return side of the furnace.
Heating and Cooling Systems
Specifying the correct sizes for heating and cooling systems in airtight, energy-efficient homes can be tricky. Rule-of-thumb sizing is often inaccurate, resulting in wasteful operation. Conscientious builders and heating, ventilation, and air-conditioning contractors size heating and cooling equipment based on careful consideration of the thermal envelope characteristics.
Generally, energy-efficient homes require relatively small heating systems, typically less than 50,000 Btu/hour even for very cold climates. Some require nothing more than sunshine as the primary source of heat along with auxiliary heat from radiant in-floor heating, a standard gas-fired water heater, a small boiler, a furnace, or electric heat pump. Any common appliance that gives off ?waste? heat can also contribute significantly to the heating requirements for such houses.
If an air conditioner is required, it's often a small unit and sufficient for all but the warmest climates. Sometimes only a large fan and the cooler evening air are needed to make the house comfortable. The house is closed up in the morning and stays cool until the next evening.
Smaller-capacity heating and cooling systems are usually less expensive to buy and operate. This helps recover the costs of purchasing more insulation, and other energy-efficient products, such as windows and appliances. Always look for the EnergyGuide label on heating and cooling equipment. The label will rate how efficient it is as compared to others available on the market.
In climates where summer cooling requirements dominate, light-colored materials and coatings (paint) on the exterior siding and roof can help reduce cooling requirements by up to 15 percent. Carefully selected and placed vegetation in any climate also contributes to reduced cooling and heating loads.
Energy-Efficient Appliances
Appliances with relatively high operating efficiencies are usually more expensive to purchase. However, higher efficiency appliances provide a measure of insurance against increases in energy prices, emit less air pollution, and are attractive selling points when the home is resold.
Home buyers should invest in high-efficiency appliances?such as water heaters, clothes washers and dryers, dishwashers, and refrigerators?especially if these appliances will be used a great deal. Because all major appliances must have an EnergyGuide label, read the label carefully to make sure you buy the most efficient appliance. To help you choose wisely, major appliances with an Energy Star? label exceed the federal government's minimum efficiency standards by a large percentage.
Energy-efficient lighting helps keep energy bills down by producing less heat and reducing cooling requirements. Fluorescent lighting, both conventional tube and compact, is generally the most energy-efficient for most home applications.
Advantages and Disadvantages
Houses that incorporate all of the above elements of energy efficiency have many advantages. They feel more comfortable because the additional insulation keeps the interior wall at a more comfortable and stable temperature. The indoor humidity is also better controlled, and drafts are reduced. A tightly sealed air/vapor retarder reduces the likelihood of moisture and air seeping through the walls. They are also very quiet because the extra insulation and tight construction helps to keep exterior noise out better.
But these houses also have some potential disadvantages. They may cost more and take longer to build than a conventional home if there's a lack of builder familiarity with new construction techniques and products available on the market. Even though the house's structure may differ only slightly from conventional homes, the builder and contractors may be unwilling to deviate from what they've always done before. They may need more training if they have no experience with these systems.
Building and Buying
Before you start a home-building project, the building site and its climate should be carefully evaluated to determine the optimum design and orientation for the house. There are energy-related computer software programs that can help with these evaluations. The design should accommodate appropriate insulation levels, moisture dynamics, and aesthetics. Decisions regarding appropriate windows, doors, and heating, cooling, and ventilating appliances are central to an efficient design. Also the cost, ease of construction, the builder's limitations, and local building code compliance should be competently evaluated. Some plans are relatively simple and inexpensive to construct, while others can be extremely complex and, thus, expensive.
An increasing number of builders are participating in the federal government's Building America and Energy Star? Homes programs, as well as local home energy rating programs, all of which promote the construction of energy-efficient houses. Many of these builders construct energy-efficient homes to differentiate themselves from their competitors. Construction costs can vary significantly depending on the materials, construction techniques, contractor profit margin, experience, and the type of heating, cooling, and ventilation system chosen.
Because energy-efficient homes require less money to operate, many lenders now offer energy-efficient mortgages (EEMs). EEMs typically have lower points and allow for the stretching of debt-to-income ratios. State and local government energy offices can be contacted for information on region-specific financing.
In the end, your energy-efficient house will provide you with superior comfort and lower operating costs, not to mention a higher real estate market value.
