Understanding the Role of
the Building Envelope

A building envelope is a term used to define the components of a building that separate the interior environment of a house from the exterior environment. People often think of the building envelope as being the walls and roof of a structure. Although a building envelope can include these components, the building envelope is not necessarily defined by these components. For example, the interior wall between a house and an attached garage is typically insulated and is considered to be a component of the building envelope. The exterior walls of the attached garage in this example are not considered part of the building envelope because they do not separate the interior environment from the exterior environment.

A roof is considered part of the building envelope when there is no attic, such as a roof over a cathedral ceiling where the drywall is attached to the underside of the rafters. When there is an attic between the roof and an insulated ceiling, the insulated ceiling separates the interior environment from the exterior environment and is considered a component of the building envelope. Typical components of the building envelope include a building wrap, exterior sheathing, insulation, and a vapor barrier.

The building envelope plays a vital role in the energy efficiency of a building. One of the keys to an energy-efficient house is minimizing openings such as cracks and gaps in the building envelope. Any openings in the building envelope allow inside air to escape to the outside. The conditioned flow of air out of the house will be balanced by unconditioned air intruding into the house. During the winter, the intruding air needs to be heated.

The building envelope also helps to stabilize humidity inside a house, preventing undesirable changes in moisture that can cause damage. When intruding air enters the house at 32 degrees with 50 percent relative humidity, the relative humidity will decrease to 12.5 percent when the air is heated to 70 degrees.

Figure 1: Blocking at ceiling joists to prevent air intrusion below a knee wall

This dry air will cause the wood framing and the components of the house to dry out and shrink, especially across the grain. When wood trim, wood doors, and wood flooring dry out, they shrink to expose paint or stain lines. Wood flooring can dry out and squeak during the cold winter months. Low humidity can also cause cracks and gaps to appear in wood flooring.

Framing can shrink enough to cause cracking in drywall. Cracks in drywall above wood headers (lintels) are the most common cracks in a house. When this wood framing assembly shrinks from drying out, it can crack the drywall at the top corners of the door frame, especially at the taped joints.

A well-built building envelope will maintain a fairly constant temperature throughout the house. When proper attention is not given to the building envelope, problems such as cold spots and frozen pipes can result.

Figure 2: Continuous sheathing behind eyebrow to prevent air intrusion

Construction of a "knee wall" or "eyebrow roof" is one area where special attention must be given to the building envelope. Figures 1 and 2 provide details on preventing air intrusion through the building envelope at knee walls and eyebrow roofs when constructed on the second-floor level of a two-story house. Blocking is installed between the floor joists to prevent air intrusion at the floor joists below knee walls. Sheathing is installed continuously behind the eyebrow roof to prevent air intrusion through the exterior wall.

Sometimes fiberglass batts are used to close this space without sheathing or blocking. Batts of insulation will not seal the area and can be displaced by the wind. Even worse, loose blown cellulose, when used at this detail, will be eroded by wind.

Freezing of interior pipes can occur when the knee wall or eyebrow roof is symmetrical on both sides of the house and when the building envelope is not properly constructed in these areas. Wind can enter through the improperly constructed building envelope on one side of the house; pass through the house along the cavities created by the floor joists, second-story floor sheathing, and the first-story drywall ceiling; and exit through the improperly constructed building envelope on the other side of the house. If waterlines run through any of these cavities, the house is only one cold, windy day away from having a frozen pipe burst.

If there are open areas in your building envelope, the best way to seal the envelope is to use blocking for closure and to apply spray foam insulation. The spray foam provides an insulation factor as well as seals the cavity to prevent air intrusion.

The referenced details are from the Journal of Light Construction Field Guide, A Manual of Best Practice, Volume 2 To learn more about Donan Engineering and the services we provide, please visit our website at www.donan.com.

About the Author:
Donan Engineering Co., Inc., a family-owned company founded in 1947, provides full-service forensic engineering and fire investigation services. For over 60 years, Donan Engineering has been providing unbiased answers to our client's complicated questions. Click to continue...

This article was reproduced with permission from Kevin Abell of Donan Engineering Co., Inc.

   
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