Understanding the Building Envelope

Why is it so important?

Showing a typical building envelope.

Building Envelope

What it is a building envelope?

A building envelope is comprised of the components that make up the shell of the building. The components separate the exterior from the interior of the building, and are designed to meet or exceed the needs of the specific application. The building envelope may also be described as what separates the interior areas that are temperature controlled (conditioned) space from exterior unheated (unconditioned) space. To break it down any area that is heated or air conditioned is considered a conditioned area where as any area that isn't would be considered an unconditioned area. The building envelope must be designed with regard to climate, ventilation, and energy consumption within the building.

The many functions of the building envelope can be separated into three categories:

  • Support (to resist and transfer mechanical loads)
  • Control (the flow of matter and energy of all types)
  • Finish (to meet human desires on the inside and outside)

The control function is at the core of good performance, and in practice focuses, in order of importance, on rain, air, heat, and vapor control.

Building envelopes are often characterized as either "tight" or "loose." A tight envelope is accurately constructed to allow as little air leakage as possible. This requires more insulation, caulk, sealants, and energy-efficient windows to acquire a tight shell for the building. A loose envelope on the other hand allows air to flow a lot more freely from the exterior of the building to the interior. This type of envelope is either by design or by poor construction abilities.

A tight building envelope allows for a high level of control over indoor air quality, temperature, humidity levels, and energy consumption. This leads to fewer drafts and a more comfortable building for its occupants, which often results in less waste in heating and cooling costs. Tight envelopes also have a lower chance of producing mold or mildew from moisture infiltration, this can help prolong the life of the building components. The downside to a tigher building envelope is it requires more extensive mechanical ventilation systems because it limits how much natural ventilation can occur.

A loose building envelope allows more of a natural air transfer to occur, which improves indoor air quality which can remove the need for mechanical ventilation. These types of building envelopes make the building more drafty and uncomfortable, it also makes the building harder to regulate temperature levels. This creates a higher chance of mold or mildew, and higher quantities of heated or cooled air are able to escape through leaks in the loose building envelope. This will increase energy bills along with negatively impacting the environment by releasing more greenhouse gases.

Air Control

Controlling air flow is key to controlling energy consumption, ensuring indoor air quality, avoiding condensation, and to providing comfort. Control of air movement includes flow through the enclosure or through components of the building envelope itself, as well as into and out of the interior space. Therefore, air control includes the control of windwashing (cold air passing through the insulation) and convective loops which are air movements within a wall or ceiling that may result in 10% to 20% of heat loss alone.

Physically the envelope components include the roof, ceiling, walls, doors, windows, and their related barriers and insulation. The dimensions, compatibility of materials, performance, fabrication process and details, connections and interactions are the main factors that determine the effectiveness and durability of the building enclosure system.

Typical measures of the effectiveness of a building envelope include physical protection from weather and climate (comfort), indoor air quality (hygiene and public health), durability and energy efficiency. In order to achieve these goals, all building enclosure systems must include a solid structure, a drainage place, an air barrier, a thermal barrier, and may include a vapor barrier. Moisture control is essential in all climates, but cold climates and hot-humid climates are especially demanding.

Thermal Envelope

Also known as a heat flow control layer, a thermal envelope is part of a building envelope but may be in a different location such as in a ceiling. The difference can be illustrated by understanding that an insulated attic floor is the primary thermal control layer between the inside of the building and the exterior while the entire roof (from the shingles to the paint on the ceiling) makes up the building envelope.

Thermal cameras are a good way of seeing how well your building envelope is working. At Reichel Insulation we provide thermal imaging as part of our estimating process if you're interested in seeing how well your home is performing feel free to contact us today for a free estimate.

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