Energy Efficiency: Some Building Envelope Basics

The building envelope is comprised of the foundation, floors, walls, roof, windows, exterior doors, and other components separating the "inside" from the "outside" of a building. This "shell" is an integral part of the energy system. The ability of the foundation, walls, and roof to isolate occupants from outddor conditions significantly impacts the operation of HVAC systems. Windows, doors, skylights, and other apertures impact interior lighting and ventilation conditions in addition to heating and cooling demands.

The building envelope, lighting, and HVAC systems should work together to optimize energy efficiency and provide a comfortable, pleasant environment for employees and customers. For example, high efficiency lighting reduces air conditioning costs since less heat is generated in the building by the high efficiency lighting.

If the building enevelope is "tight", the outside temperature fluctuations will have less impact on the building HVAC systems which work to maintain indoor temperatures. In Southern California, however, buildings can also take advantage of typically mild ambient conditions. In fact, for many commercial buildings the HVAC systems primarily work to remove interior heat (from occupants, lighting, and equipment) from the building. For these buildings a well-insulated "tight" envelope will actually increase HVAC energy consumption.

Building Envelope and Energy Use

The building envelope can waste energy in four basic ways:

Air Leakage. Conditioned (heated or cooled) air leaking out of the building, or outside air leaking in, through surface cracks and openings around windows and doors can dramatically increase the amount of energy needed to operate HVAC systems in order to maintain building comfort.

Poor Insulation. Heat transfer into the building in summer, and out of the building in winter, through window glass or poorly insulated walls, ceilings and doors can also increase the amount of energy needed to operate HVAC systems.

Ventilation and Humidity. The building envelope plays an important role in the ventilation of the building, that is the movement of air and moisture in and out of the building. The enevelope should be designed in conjunction with the ventilation system to prevent excess condensation on windows and walls that can cause mildew and rot problems.

Excess or Insufficient Sunlight. Good envelope design can allow the sun's light and heat in when desired, and deflect them when unwanted. The building enevelope influences the artificial lighting systems and HVAC equipment sizing needed to maintain comfort in the building.

Reducing Energy Use

There are several inexpensive ways to reduce energy usage due to building envelope problems. The ideas detailed below fall into the general strategies of:

Reducing air infiltration.

Reducing heat transmission.

Controlling humidity.

Controlling sunlight.

No-Cost Oppportunities

Isolate Unused Areas

Closing HVAC dampers and sealing exterior windows can isolate spaces that are not occupied by people and may not need space conditioning. Isolating unused areas of the building results in a reduction of the total conditioned space and the energy used by the HVAC system.

Door and Window Frames

Tightening window and door frames will ensure that they are closely attached to the building and minimize unwanted air leaks. Automatic door closers should be periodically adjusted to ensure that they close the door completely and rapidly.

Low-Cost Oppportunities

Seal Cracks

Cracks may occur in foundations, walls, roofs, and around apertures (windows, doors, and skylights) as well as flue chases, pipes, and conduits leading into the building. Normal use, thermal expansion and contraction, material aging, foundation settling, or seismic damage can cause these cracks. Sealant caulking will eventually dry out and separate from building material surfaces. A variety of glass fiber, expanding foam, silicone, and acrylic products can be used to repair cracks or restore damaged seals in the building envelope.

Install Air Conditioner Covers

Placing exterior insulating covers on window-mounted air conditioners during winter months will prevent air leakage both through and around the unit.

Install Weather-stripping

Installing rubber blade or brush-type door sweeps on the bottoms of exterior doors, or rubber weather-strips on thresholds, will limit airflow beneath the door. Rubber strips can also be used between double doors. Weather-stripping can be added around doorframes with metal or plastic "V" strips or adhesive-backed foam strips. Operable windows, particularly double-hung windows, may also benefit from the installation of thin foam weather-stripping.

Window Repairs

Broken or cracked windows should be repaired as soon as possible. Heavy guage transparent tape can be used as an interim measure.

Shading Devices

Instaling interior operable shading devices (blinds and shades) gives building occupants the ability to deflect direct sunlight when it is undesirable. This will reduce unwanted glare and temperature buildup in the building

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