Energy Efficiency

Designing and building an energy-efficient home that conforms to the many considerations faced by home builders can be a challenge. However, any house style can be made to require relatively minimal amounts of energy to heat and cool, and be comfortable and healthy. It's easier now to get your architect and builder to use improved designs and construction methods. Even though there are many different design options available, they all have several things in common: a high R-value, tightly sealed thermal envelope; controlled ventilation; and lower than usual heating and cooling bills.

Some designs are more expensive to build than others, but none of them need to be extremely expensive to construct. Recent technological improvements in building elements and construction techniques, and heating, ventilation, and cooling systems, allow most modern energy saving ideas to be seamlessly integrated into any type of house design without sacrificing comfort, health, or aesthetics. The following is a discussion of the major elements of energy-efficient home design and construction systems.

The 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, windows, doors, finishes, weather-stripping, and air/vapor retarders. Specific items to consider in these areas are described below.

Wall and Roof Assemblies

There are several alternatives to the conventional "stick" (wood stud) framed wall and roof construction now available and growing in popularity. They include:

Optimum Value Engineering (OVE)

This is a method of using wood only where it does the most work, thus reducing costly wood use and saving space for insulation. However, workmanship must be of the highest order since there is very little room for construction errors.

Structural Insulated Panels (SIP)

These are generally plywood or oriented strand board (OSB) sheets laminated to a core of foam board. The foam may be 4 to 8 inches thick. Since the SIP acts as both the framing and the insulation, construction is much faster than OVE or it's older counterpart "stick-framing." The quality of construction is often superior too since there are fewer places for workers to make mistakes.

Insulating Concrete Forms (ICF)

These often 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. This is the fastest and least likely technique to have construction mistakes. Such buildings are also very strong and easily exceed code requirements for tornado or hurricane prone areas.

Insulation

An energy-efficient house has much higher insulation R-values than required by most local building codes. For example, a typical house in New York State might have haphazardly installed R-11 fiberglass insulation in the exterior walls and R-19 in the ceiling, and the floors and foundation walls may not be insulated. A similar, but well-designed and constructed house's insulation levels would be in the range of R-20 to R-30 in the walls (including the foundation) and R-50 and R-70 in the ceilings. Carefully applied fiberglass batt or roll, wet-spray cellulose, or foam insulations will fill wall cavities completely.

Air / Vapor Retarders

These are two things that sometimes can do the same job. How to design and install them depends a great deal on the climate and what method of construction is chosen. No matter where you are building, water vapor condensation is a major threat to the structure of a house. In cold climates, pressure differences can drive warm, moist indoor air into exterior walls and attics. It condenses as it cools. The same can be said for very Southern climates, just in reverse. As the humid outdoor air enters the walls to find cooler wall cavities it condenses into liquid water. This is the main reason why some of the old buildings in the South that have been retrofitted with air conditioners now have mold and rotten wood problems.

Regardless of your climate, it is important to minimize water vapor migration by using a carefully designed thermal envelope and sound construction practices. Any water vapor that does manage to get into the walls or attics must be allowed to get out again. Some construction methods and climates lend themselves to allowing the vapor to flow towards the outdoors. Others are better suited to letting it flow towards the interior so that the house ventilation system can deal with it.

The Airtight Drywall Approach and the Simple CS system are other methods to control air and water vapor movement in a residential building. These systems rely on the nearly airtight installation of sheet materials such as drywall or gypsum board on the interior as the main barrier, and carefully sealed foam board and/or plywood on the exterior.

Foundations and Slabs

Foundation walls and slabs should be at least as well insulated as the living space walls. Uninsulated 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 that supply heat as a by-product, such as domestic hot water heaters, washers, dryers, and freezers, are often located in basements. By carefully insulating the foundation walls and floor of the basement, these appliances can assist in the heating of the house.

Windows

The typical home loses over 25% of its heat through windows. Since even modern windows insulate less than a wall, in general an energy-efficient home in heating dominated climates should have few windows on the north, east, and west exposures. A rule-of-thumb is that window area should not exceed 8-9% of the floor area, unless your 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% of the floor area is recommended. In cooling dominated climates, its important to select east, west, and south facing windows with low solar heat gain coefficients (these block solar heat gain). A properly designed roof overhang for south-facing windows is important to avoid overheating in the summer in most areas of the continental United States. At the very least, Energy Star rated windows or their equivalents, should be specified according to the Energy Star regional climatic guidelines.