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What Is a Building Envelope?

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  • Written By: B. Turner
  • Edited By: Bronwyn Harris
  • Last Modified Date: 15 October 2014
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A building envelope includes all the components that make up the shell or skin of the building. These components separate the exterior of the building from the interior, and are designed by the project architect or engineers to meet the needs of each individual application. The building envelope may also be defined as the components that separate conditioned areas from unconditioned space. Exterior or unheated living spaces are not included inside the envelope, while any living space that is equipped with heat or air conditioning would be included. The building envelope must be carefully designed with regard to climate, ventilation, and energy consumption within the structure.

There are four basic functions of the building envelope. These include adding structural support, controlling moisture and humidity, regulating temperature, and controlling air pressure changes. By serving these different functions, the envelope also affects ventilation and energy use within the building.

The envelope is made up of all of the exterior components of the building, including walls, roofing, foundations, windows, and doors. Finish materials like siding and decorative items are not usually considered a part of the envelope. Insulation, building paper, and other components aimed at controlling moisture and airflow are typically included in the building envelope design.

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Building envelopes are often characterized as "tight" or "loose." A tight envelope is precisely constructed to allow relatively few air leaks. This often requires significant quantities of insulation, caulk, sealants, and energy-efficient windows to create a tight shell for the building. Loosely-constructed envelopes allow air to flow more freely from the exterior to interior spaces. A loose envelope may be created by design, or may be the result of poor construction techniques.

Many experts debate the benefits of tight versus loose building envelopes. A tight envelope allows for a high level of control over indoor air quality, energy consumption, temperature, and humidity levels. It leads to fewer drafts and a more comfortable environment for occupants, and often results in less waste in heating and cooling costs. Tightly-designed envelopes also reduce the likelihood of mold or mildew caused by moisture infiltration, which may prolong the life of building components. At the same time, tighter buildings also limit how much natural ventilation can occur, which leads to more extensive mechanical ventilation requirements.

A loosely-constructed building envelope allows natural air transfers to occur, which improves indoor air quality and often eliminates the need for mechanical ventilation. At the same time these looser buildings tend to be more drafty and uncomfortable, and can make it difficult to regulate temperature levels. There is an increased chance of moisture-related mold, and higher quantities of heated or cooled air are able to escape through leaks in the envelope. This can increase energy bills and negatively impact the environment by increasing greenhouse gas levels.

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submariner
Post 3

I would just like to point out that the cost effectiveness of a tight building envelope in new construction increases as the square footage increases. After a certain point, the square footage of a building increases relatively faster than the buildings volume. Because of this, the energy use per square foot begins to decrease significantly as the building square footage increases. This allows the builder to afford things like demand controlled ventilation systems that increase ventilation automatically as carbon dioxide levels rise.

chicada
Post 2

@Georgesplane- I would agree that a tight building envelope would be the best type of building envelope for the money. Besides the windows, the building materials that go into making a tight building envelope are not that expensive.

On a new build, it is cost effective to make an envelope tighter when the costs of energy are taken into consideration. In most cases, the payback time for the investments in constructing a tight building are in the three to five year range. Depending on the climate, the windows usually make an ROI in five to seven years.

Furthermore, over 40% of the thermal losses in a building are attributed to the building envelope. In a hot or cold climate, the building envelope is the best area to find cost effective energy efficiency improvements. The market value of an efficient building is much better than one that is inefficient, and as code standards continually improve, this extra market value will surely increase.

Georgesplane
Post 1

What a great article. As a building inspector, I have to say that I think a tight envelope is the best for any type of residential or commercial building. It does require more ventilation, but a building with a tight envelope can have mechanically or manually controlled natural ventilation.

A tight building envelope is an energy efficient building envelope. In almost every case, it is better to have the operator and occupants of the building control the ventilation. The only time that I can think of where the expense of a tight building envelope is not justified is in the housing of livestock, or an equipment shed that houses mechanical machinery resistant to the climate.

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