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Home sweet home: keep it simple with energy-efficient home construction.

The importance and need of continued improvement in energy efficiency in the transportation, industry, and buildings sectors is well known. Therefore, the increased interest in green homes on the part of the American public is significant and noteworthy since conserving energy is fundamental to making any building green. Unfortunately, many people have the misconception that an energy-efficient house requires far-out designs, exotic materials, and/or complex assembly techniques. In fact, building an energy-efficient home can be readily accomplished by incorporating simple and straightforward construction techniques using standard materials and based on sound engineering design principles and practices. Examples of these techniques (all of which are code approved) include advanced framing, raised-heel trusses, airtight drywall approach, frost-protected shallow foundations, and optimizing window design.

Advanced framing

The advanced framing technique is based on the concept of minimizing the use of wood members that are not needed for structural support in framing the exterior of the house. Energy efficiency is increased (insulation replaces the wood components deleted) and the amount of wood used in reduced. Elements of advanced framing include nominal 5 X 15-cm (2 X 6-in.) studs located on 61-cm (24-in.) centers; two-stud corners; single member wood headers; elimination of unneeded trimmer (jack) and cripple studs in rough openings; elimination of headers in non-support walls, windows in alignment with regular studs, single top plate (trusses need to be stacked directly above the studs); drywall clips (instead of wood backer components); and rigid insulation exterior sheathing (eliminates thermal bridging).

Raised-heel trusses

Regular trusses (and rafters) prevent an adequate depth of ceiling insulation from being installed in the attic areas adjacent to the top plate. The effect of this missing insulation is a substantial reduction in the overall R value of the installed ceiling insulation (typically in the range of 25 percent). Raised-heel (energy) trusses are designed such that their bottom and top chords extend well beyond the top plate. As a consequence, these trusses enable a full depth of ceiling insulation to be installed adjacent to and over the top plate. Raised-heel trusses not only significantly reduce heat movement through the ceiling but also result in wide roof overhangs. The wide overhang has two additional advantages: 1) it reduces solar gain in the summer by providing considerable shading of south-facing windows (but does not substantially reduce solar gain in the winter due to limited shading of these same windows), and 2) it provides additional protection from rain for exterior walls, doors, and windows.

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Airtight drywall approach

Air leakage is the uncontrolled air movement into and out of houses via the building envelope. Limiting and controlling air leakage is important for energy efficiency because uncontrolled air leakage can represent up to 50 percent of the annual heat gain and loss in a house. In addition, air leakage is the major mechanism by which moisture is transported into building shell cavities. Up to 95 percent of the moisture movement into house walls and ceilings is due to moisture "hitchhiking" as part of the air leaking into these spaces (as opposed to vapor diffusion through surface covering materials). Other negative effects of uncontrolled air leakage include drafts and cold spots, low indoor relative humidity in the heating season, and high indoor relative humidity in the cooling season. Therefore, houses should be built tight and ventilated right (using controlled mechanical ventilation systems).

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The airtight drywall approach is a simple and reliable method for effective air sealing of houses. Advantages include low material and labor costs, specialized materials and subcontractors are not required, and the drywall panels can be glued to the face of wall studs and ceiling joists. The airtight drywall approach is based on the fact that drywall (and its taped joints) and wood (lumber) are excellent air barriers. Therefore, the interior finish of drywall on walls and ceilings is used in combination with structural building components and sealants (caulks, foams, and adhesives) to form a continuous air barrier and achieve a tight house. The airtight drywall approach method of air sealing is completed in three stages--before, during, and after drywall installation--and the process and procedures are easy to learn and perform.

Frost-protected shallow foundations

Protecting footings for houses and other buildings from damage due to frost heaves is typically accomplished by installing the footings below the frost line. For homes without basements, the construction options usually considered are 1) a separate footing, foundation curtain wall, and concrete slab or 2) a separate footing, foundation curtain wall, and crawlspace. Both of these options involve considerable material, equipment, and labor resources and costs which increase in relation to the required frost-free depth. Excessive heat loss and humidity problems (in the case of crawlspaces) are also common.

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Frost-protected shallow foundations provide a third alternative. In this construction technique, rigid insulation is used to provide frost protection without the need of deep footings. The equired footing depth is only 30 cm (12 in.). The building footing, foundation, and floor are constructed at the same time and consist of an integral monolithic slab thickened at the edges. Frost-protected foundations are a well-proven energy-efficient, cost-effective technique.

Optimizing window design

Windows have a major effect on the energy efficiency of a home and also perform a number of other functions. Therefore optimizing window design requires careful consideration of and compromise among factors not directly related to energy efficiency such as code requirements, views, and aesthetics. In terms of energy efficiency, window design should take into account heat loss and gain during both heating and cooling seasons, natural lighting, and ventilation. High-quality, low-e, gas-filled, double-pane windows with low air-leakage rates have become standard stock items and should always be selected.

South-facing windows make effective passive solar collectors, and the house design should include an appropriate (but not excessive) number of these windows. The number of north-facing windows should be limited (to reduce heat loss during the heating season) and the number of west- and east-facing windows should also be limited to a lesser extend (to reduce heat gain in the summer). Movable interior window coverings such as shades and blinds can also be valuable in reducing heat loss and gain. When all window functions have been considered in total, a typical optimal design might have the following representative specifications: 1) total window area - 12 percent of the floor area and 2) percent of total window area by compass direction: south 40 percent, east 30 percent, west 20 percent, and north 10 percent.

What about costs?

Some of these techniques will result in slightly higher initial costs whereas others will have lower initial costs. However, when these methods are incorporated into houses that are designed and constructed based on the "building as a system" approach, any higher initial costs related to energy efficiency can typically be offset by subsequent lower initial costs for other components. As one example, the furnace and air conditioning units and their distribution systems can be substantially downsized in an energy-efficient house which results in considerable savings. Obviously, the annual costs of an energy-efficient house will be much lower for the entire life of the structure!

It is also important to realize that an emphasis solely on initial costs or payback periods fails to take into account the many other benefits of energy-efficient homes. At the societal level, an energy-efficient house has considerable value in that energy and material resources are conserved and pollution is reduced. Major benefits to homeowners and occupants are homes that are comfortable, safe, healthy, durable, quiet, and affordable in addition to being energy efficient. How so? I can personally attest to this.

Since 2000, my wife and I have lived in this type of house (which I designed and built) and have experienced and enjoyed all of the attributes listed. An additional bonus: annual heating costs to heat our moderately-sized [201 [m.sup.2] (2,165 f[t.sup.2)], all-electric home located in northern Ohio have averaged $350, and there are no cooling costs (air conditioning is not required).

ASABE member Allen Zimmerman is a professor at The Ohio State University, 1328 Dover Road, Wooster, OH 4469; 330-264-3911, fax 330-262-7634, zimmerman.7@osu.edu.
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Author:Zimmerman, Allen
Publication:Resource: Engineering & Technology for a Sustainable World
Date:May 1, 2006
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