When 'Going Green' Goes Bad

When the Hollmanns began building their luxury family home among the other million-dollar estates dotting prestigious Miramar Lane, deciding to use an environmentally friendly insulation material seemed like a great way to “go green.” Little did they know that within the span of a few years this decision would land them in court, battling over a property now worth less than their neighbors' garages.

At the center of the controversy is a two-story, 9,000-square-foot residence located in what the U.S. Department of Energy rates as a “hot-humid” climate. The exterior featured a lovely natural stone veneer; the interior boasted expensive hardwood paneling. But when the Hollmanns started discovering fungal growth on the walls, they quickly learned the water intrusion was not stemming from the usual suspects—the building envelope components of exterior walls, roofing, or doors. Rather, it came from within.

Anatomy Of An Envelope

A building envelope is the assembly of materials that separate the exterior environment from the interior of a structure. The envelope serves as the outer shell or the “skin” that protects the building from the elements and facilitates climate control within the same. Water will enter your home; building envelope components are designed and constructed to shed or drain that water to the exterior and away from the skin of the structure. 

In residential construction like the Hollmanns', a masonry veneer cavity wall system typically consists of the following components, listed in order from exterior to interior:  brick or natural stone masonry veneer, air space, weather barrier and/or vapor retarder, load-bearing wall, and interior wall finish. Thermal insulation is used within the wood-framed stud wall. 

Wall Insulation

Thermal insulation is installed inside the exterior stud wall to slow the rate of heat transfer across the exterior wall. Common types of insulation in residential construction include fiberglass batt insulation and cellulose insulation.

Cellulose insulation is made from recycled paper and treated with mold retardants and fireproofing chemicals. It is installed using either dry or wet-spray techniques, with wet-spray being the most common application in residential construction. Like fiberglass batt, cellulose insulation is also low cost, with a fairly high R-value. However, cellulose insulation is “hygroscopic,” meaning that it absorbs and retains moisture. As a result, wet-spray cellulose insulation in particular has two important application limitations:  

1. It requires an experienced installer.

2. It must be allowed to dry properly before it is covered up. 

According to the North American Insulation Manufacturers Association (NAIMA), studies performed under various humidity conditions indicate the shortest dry time for wet cellulose insulation was one month in a dry climate without a vapor retarder in the assembly. Although the exact drying time allowed for the Hollmanns' home is unclear, it definitely was not a year.

Room to Breathe

Making matters worse, the homebuilder did a great job in sealing up the luxury residence—perhaps too great.  As stated earlier, water will enter your home. Architects and engineers accept this and design for it. Masonry veneer, like the natural stone used in the Hollmanns' residence, is considered porous or absorptive, and both liquid water and water vapor will penetrate the masonry into the air space. The masonry cavity wall assembly, therefore, must be designed and constructed to drain this moisture and prevent it from migrating past the air space and into the stud wall where it can damage the insulation and interior finishes. 

Exterior walls should be limited to a single vapor retarder to allow a path for moisture that enters the cavity to escape.  If vapor retarders are installed on both sides of the wall, moisture can be trapped in the wall insulation and prevent the wall cavity from drying.  Further, some commonly used materials will function as vapor retarders when they are not intended to, such as enamel paints and hardwood paneling. 

Peeling Back the Layers

When Nelson Forensics was engaged to determine the cause and origin of the fungal growth found by the Hollmanns, a variety of tests were performed, beginning with high moisture readings at all the interior finishes at the exterior walls, and leading to water spray testing at the exterior, and the eventual removal of exterior stone to inspect the wall cavity. Excavation revealed that the air space was filled with mortar and the mortar was packed against the Tyvek weather barrier. The Tyvek weather barrier was then cut to observe the OSB sheathing, also saturated with water.                               

Lessons Learned

When specifying material components, the project designer or builder must consider the materials' tolerance for common construction defects or imperfections, and the following should be addressed:  

1. Masonry cavity wall moisture intrusion is inevitable and drainage of the cavity wall is an intrinsic design requirement. It is imperative that cavities are kept clean of mortar droppings during construction. 
2. Installation of cellulose insulation between two materials that perform as vapor retarders in an exterior wall is not recommended. 
3. Designers and builders should be thoroughly familiar with both the requirements and limitations of green building materials. Additionally, care should be taken to ensure additional moisture is not added by means of condensation or moisture intrusion.

For the Hollmanns' homebuilder, this was an expensive lesson to learn.  

The above article has been adapted from “Potential Pitfalls of a Green Building Material: A Case Study of Cellulose Insulation,” by Deepak Ahuja, P.E.; B. Matthew Smith, AIA; Gregory G. Schober, P.E.; and Marco A. DeLeon, P.E.

For more information about Nelson Forensics, visit www.nelsonforensics.com.