• Energy & Power

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 energyefficient. It is now even possible to purchase a sustainably harvested wood.

Some of the available and popular energyefficient 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 , 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 is a major threat to the structure of a house, no matter what the climate. In cold , 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 , 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 .

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 with gaskets and 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.