Wednesday, August 4, 2010

Sealing the Envelope

Home Energy magazine. Source: Flicker
Sealing the envelope of the home is at the core of weatherization and home energy science. Before moving on to more advanced measures in green-building, air sealing and insulating the thermal boundary of a home must first be achieved. The US Department of Energy, through its Energy Efficiency and Renewable Energy office, recently published some very useful information regarding these essential weatherization tactics on their Energy Saver's blog (shown below). 

Weatherization and energy efficiency in homes and other buildings has become a significant area of study and concern as energy rates continue to soar, and the depletion of natural resources and degradation of the environment remains considerably problematic. Weatherization programs are in place throughout many states, in both private and public sectors, and retrofit installers, crew chiefs, and energy auditors are rapidly becoming fast-track career fields in the home-building trades.--D.A DeMers

Your Home

Air leakage, or infiltration, occurs when outside air enters a house uncontrollably through cracks and openings. Properly air sealing such cracks and openings in your home can significantly reduce heating and cooling costs, improve building durability, and create a healthier indoor environment.

It is unwise to rely on air leakage for ventilation because it can't be controlled. During cold or windy weather, too much air may enter the house. When it's warmer and less windy, not enough air may enter. Air infiltration also can contribute to problems with moisture control. Moldy and dusty air can enter a leaky house through such areas as attics or foundations. This air in the house could cause health problems.

The recommended strategy in both new and old homes is to reduce air leakage as much as possible and to provide controlled ventilation as needed.

For more information from the Department of Energy, see the following resources:
Note that air sealing alone can't replace the need for proper insulation throughout your home, which is needed to reduce heat flow.


In addition to the DOE's air sealing article above, here are some insights I've included on insulation issues for specific climates, as well as other weatherization related situations:

Climate Map of the US. Source: Google Maps

Cold Climates

In cold conditions, the main aim is to reduce heat flow out of the building. The components of the building envelope - windows, doors, roofs, walls, and air infiltration barriers are all important sources of heat loss; in an otherwise well insulated home, windows will then become an important source of heat transfer. Heat loss can be reduced by good weatherization, bulk insulation, and minimizing the amount of the building's non-insulative glazing. Some insulated glazing systems can greatly increase R-values, the industry's measure of thermal resistance.

Hot Climates

In hot conditions, the greatest source of heat energy is solar radiation. This can enter buildings directly through windows or it can heat the building shell to a higher temperature than the ambient air, increasing the heat transfer through the building envelope. The process is called solar heat gain, a measure of heat transmittance via sunlight in a home. 

Central Air Conditioners. Source: GNU 
Solar gain can be reduced by adequate shading from the sun, light colored roofing, heat-reflective paints and coatings and various types of insulation. Specially coated glazing can reduce SHG to around 10%. Thermal insulation is material specifically designed to reduce the flow of heat by limiting conduction, convection, or both.

Radiant barriers are materials which reflect radiation and therefore reduce the flow of heat from radiation sources. Good insulators are not necessarily good radiant barriers, and vice versa. Metal, for instance, is an excellent reflector and poor insulator. Radiant barriers are highly effective for attic spaces in hot climates. In this application, they are much more effective in hot climates than cold climates. For downward heat flow, convection is weak and radiation dominates heat transfer across an air space. Likewise, radiant barriers must face an adequate air-gap to be effective for the rest of the envelope.

If air-conditioning is utilized in a home in a hot, humid climate, then it is particularly important to seal the building envelope. Dehumidification of humid air infiltration can waste significant energy. On the other hand, some building designs are based on effective cross-ventilation instead of air-conditioning to provide convective cooling from prevailing breezes.

Building Envelope

The thermal envelope defines the conditioned or living space in a house. The attic, basement, or any porches may or may not be included in this area. Reducing airflow from inside to outside, or "tightening the envelope," can help to reduce convective heat transfer significantly.

The less natural airflow into a building, the more mechanical ventilation will be required to support human comfort. High humidity can be a significant issue associated with lack of airflow, causing condensation, rotting construction materials, and encouraging microbial growth such as mold and bacteria. Moisture can also drastically reduce the effectiveness of insulation by creating a thermal bypass.

Thermal Bypass

Thermal bypasses or "bridges" are points in the building envelope that allow heat conduction to occur. Since heat flows through the path of least resistance, thermal bridges can cause reduced energy efficiency. A thermal bypass is created when materials create a continuous path across a temperature difference, in which the heat flow is not interrupted by thermal insulation. Common building materials that are poor insulators include glass and metal.

A building design may have limited capacity for insulation in some areas of the structure. A common construction design is based on stud walls, in which thermal bypasses are common in wood or steel studs and joists, which are typically fastened with metal. Notable areas that most commonly lack sufficient insulation are the corners of buildings, kneewall in attic spaces, chimney chase-ways and voided areas where insulation has been removed or displaced to make room for system infrastructure, such as electrical boxes, plumbing, fire alarm equipment. Utilizing proper insulating materials for such spots or limiting the cross-sections can minimize the bypass.




Insulation Matthew Bisanz
There are essentially two types of building insulation - Bulk Insulation and Reflective Insulation. Most buildings use a combination of both types to make up a total building insulation system. The type of insulation used is matched to create maximum resistance to each of the three forms of building heat transfer - Conduction, Convection, and Radiation.

Bulk insulators block conductive heat transfer and convective flow either into or out of a building. The denser a material is, the better it will conduct heat. Because air has such low density, air is a very poor conductor and therefore makes a good insulator. 

Insulation materials to resist conductive heat transfer, or radiant barriers (see above), use air spaces between fibers, inside foam or plastic bubbles and in building cavities like the attic. This is beneficial in an actively cooled or heated building, but can be a liability in a passively cooled building where proper provisions for cooling by ventilation or radiation are needed.

Some radiant barriers are particular to specific spectral frequencies and will, for example, reduce the flow of infra-red radiation in comparison to other wavelengths.

Likewise, low-emissivity windows will transmit light and short-wave infra-red energy into a building but reflect back the long-wave infra-red radiation generated by interior furnishings. Similarly, special heat-reflective paints are able to reflect more heat than visible light, or vice-versa.

Thermal emissivity values probably best reflect the effectiveness of radiant barriers. Some manufacturers give an equivalent R-value for these products. However, these numbers are sometimes difficult to decipher, since R-value testing measures total heat loss in a lab and does not necessarily include situations involving radiation, conduction, or convection.--D.A DeMers (in collaboration with documented sources).

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