Back to Basics: Factors to Consider When Designing a Building Envelope

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By Claire McIntyre, AIA

 

McIntyre & McIntyre, Inc.

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Of the many factors that must be considered in the design of a building envelope there are a few basic truths of science with which architects, engineers and contractors must deal. When ignored, these cause the most problems with waterproofing.      

 

 

A building envelope is comprised of the walls, roof and floor foundation (including below-grade walls and slabs-on-grade). Different exposure factors for each of these lead to different designs and materials being selected to waterproof each separate area.

 

BASIC TRUTHS OF SCIENCE

 

In order to create a weatherproof building envelope the design professional and contractor must not forget the fundamental laws of physics and thermodynamics. No matter what new materials are developed, there are some basic truths that have always been and always will be actions that must be accommodated in building envelope design and construction. Some of them are as follows:

 

Water will seek the lowest level. If it happens to “bottom out” at a window or door head or sill or at the base of a wall cavity, then it must be directed back to the exterior before it can come in contact with the interior wall components.

 

Wind creates differential pressures. These pressures can drive rain through a seemingly waterproof exterior wall and into a building interior. Air travels from high pressure to low pressure, and that pressure can drive quite a lot of water through very small cracks in an exterior veneer.

 

Air molecules move from warm to cold. Warm air holds more moisture than cold. Condensation forms when warm, moist air is cooled to the dew point (the temperature at which the air cannot hold the water molecules). The dew point temperature is dependent on the relative humidity of the air. Areas of low humidity will have a lower dew point temperature than areas of high humidity. The arid desert air must cool to a much lower temperature to condense moisture than humid air of a tropical climate.    

 

Water travels by capillary action. Wind can send rain into cracks in a masonry or plaster veneer. Skin friction between the sides of crack and the water molecules allows the moisture to be wicked into the material and into the wall.

 

Materials expand and contract with changes in temperature. Some materials are more stable than others, however. These differentials in temperature movements can stress the skins of walls and roofs resulting in small microscopic cracks or large openings. Even small cracks in masonry or plaster walls will allow water intrusion from capillary action or rain being blown directly into the wall.

 

Allowances must be made for these factors to design and construct an exterior veneer wall that will function properly. Graphic A (below) illustrates these truths. (Source: National Institute of Building Sciences Whole Building Design Guide website article by Daniel J. Lemieux, AIA and Paul E. Totten, PE, Wiss, Janney, Elstner, Associates Inc.  http://www.wbdg.org/design/env_wall.php)

 

  

                                               

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Graphic A Illustrates some basics truths of science.

  

 

 

 

 

 

 

 

 

 

 

 

 

DOUBLE INDEMNITY

 

Okay, so we’ve established that water will get through the exterior veneer and into the wall cavity, whether due to movement cracks, blowing wind and capillary action or construction error. Now what? The answer is to provide a secondary weather resistant barrier as a drainage plane to the flashing that directs water back out. 

 

The exterior veneer is the first line of defense for walls. It should not be relied upon as the only protection from the elements. Photo 1 (below) illustrates what not to do. In the “old days” this secondary wall membrane was often building paper (asphalt impregnated sheets) lapped shingle style to shed water. New products and code regulations have made the selection process more complicated.

  

 

 

 

 

 

 

 

 

 

 

Photo 1 :  Construction Don''t!  No Secondary Weather Barrier and open joints in sheathing are an open invitation for water damage.  Notice the lack of flashing at base of EIFS and top of stone.  And look at all that mortar in the cavity space!  This building leaked like a sieve.

 

 

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Once in the cavity space, water can come in contact with the sheathing layer (the outside face of the wall behind the veneer) and travel to the insulation, the interior wall layer and interior base molding. Water reduces the effectiveness of insulation. If any of these materials are organic (such as the paper layer on gypsum board, plywood sheathing, wood base, wood studs) mold can develop. 

 

We all have heard a lot about mold in the last several years. This has become a particular problem as codes required buildings and homes to be made “tighter” in an effort toward better energy efficiency. Blocking air infiltration is great for energy efficiency. However, if these air barriers also act as a vapor barrier, then there can be trouble.

 

Although effective as air barriers, vapor barriers can trap moisture in the wall. All vapor barriers are air barriers but all air barriers are not vapor barriers. Water molecules are larger than water vapor molecules therefore a particular material can be an effective air barrier yet not block moisture vapor. All of this must be considered when selecting and installing a secondary weather resistant membrane to provide double protection.

 

FLASH, FLASH, FLASH

 

Somehow flashing has been forgotten by some design professionals and contractors. This critical element must be installed to direct water outward. Photo 2 (below) shows a shelf angle above a window head without flashing installed.  

 

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Photo 2:  Construction Don''t.  No flashing on shelf angle above window head.  More mortar clog.  Water ran down this sheathing face to window head and into building. 

 

 

 

 

 

 

 

Flashings begin at the exterior wall sheathing six to eight inches above the base of walls, heads of windows and doors and above shelf angles and extend across the cavity space. The flashing top must be overlapped by the secondary drainage membrane. The outside edge of the flashing material must extend under the exterior veneer. Weep holes must be provided through the exterior veneer at regular intervals to allow water to drain. And, please, joint sealants are not a substitute for flashings. 

 

EIFS (Exterior Insulation & Finish System) has received much notoriety from involvement in water intrusion and mold lawsuits. The problem is not inherent with the EIFS, but with the lack of flashings and drainage. These errors occur with all types of materials not just EIFS. Water will get in and it must be directed out by the flashings.

 

SELECT & INSTALL WISELY

 

The design professional and contractor must be fully informed in order to make a proper selection regarding the installation of secondary weather resistant membranes and flashing materials. Know the manufacturer’s intended use of the material and any product restrictions. And follow the manufacturer’s instructions for installation.

 

 

 

BASICS FOR OWNERS

 

Owners, know your design professional and your contractor. An experienced architect, engineer and contractor can mean the difference between a successful project and a headache. 

 

The difference is in the details and the attention to the basics.  Let’s all get back to basics. Happy building!

 

Claire McIntyre holds a B.S. in Architectural Engineering from the University of Texas at Austin and is also a licensed architect and real estate broker. She has been professionally involved in the design and construction industry for over 30 years. Claire was one of the first to qualify by examination as a Certified Construction Specifier by the Construction Specifications Institute.

 

McIntyre & McIntyre, Inc., her Austin, Texas firm with her husband/partner, provides architectural and engineering (structural, mechanical, electrical and plumbing) design services for commercial buildings. Their practice has included work on schools, hospitals, parking garages, office buildings, apartment buildings, high rise construction and expert witness testimony.

 

Claire can be reached at 512-699-9912 or cmcintyre@austin.rr.com.