College Planning & Management

OCT 2012

College Planning & Management is the information resource for professionals serving the college and university market. Covering facilities, security, technology and business.

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Page 25 of 83

REBORN ON THE BAYOU installation of new information technology systems — all while meeting USGBC LEED Silver certifi cation criteria and City of Houston building code requirements and fi nding generally more sustainable solu- tions for all the building's needs. In addition to those concerns, the project must address the existing mass wall with no drainage plane, the removal of existing coatings, and the existing interior plaster fi nish. The cherry on the top of this challeng- ing sundae is Houston's notoriously hot, humid climate, making moisture control a top priority. This article will address that issue, including the various considerations, challenges, and proposed solutions. Vapor Permeable or Impermeable Air Barrier? As previously mentioned, moisture con- trol is a priority. The question was whether to use a vapor permeable or impermeable air barrier. Vapor will move from the warm side of the wall to the cold side of the wall, and the greater the temperature differen- tial, the greater the vapor drive. In addi- tion, the more humid the air is, the greater the vapor drive. Houston's hot, humid climate provides a perfect storm for vapor drive from the outside to the inside. In order to fi nd the best approach for combating this problem, it was essential to perform a dew-point analysis. When the air reaches its dew-point temperature, it can no longer hold water and that water condensates. For this project varying sce- narios were analyzed, including variations for locating the air barrier, insulation, and outside air temperature. The existing wall is comprised of a 4-in. cast stone veneer on an 8-in. hollow clay tile with no air space (see Figure A). To pro- vide opportunity for thermal insulation, space for data and electrical conduit, and to provide a fi nished interior wall surface, a stud wall furring assembly at the interior is proposed (see Figure B). In all cases in the analysis, the most substantial jump in temperature was between the interior FIG. A FIG. B furred-out assembly and the insulation. The potential for condensation was always more within the existing masonry mass wall (exterior side of the assembly). Where a non-permeable vapor retarder was placed on the interior side of the 12-in. masonry mass wall, the dew point temperature and the temperature of the wall were within 1°F of each other (at an outside temperature of 85°F), thus posing the most risk of creating condensation in the wall. All the dew-point analyses were run with a constant interior temperature of 72°F with a relative humidity (RH) of 50 percent and exterior RH of 85 percent. 26 COLLEGE PLANNING & MANAGEMENT / OCTOBER 2012 The calculations were run with exterior temperatures of 100°F and 85°F. When the outside air temperature was increased, the dew-point temperature was close to the wall temperature, creating a higher poten- tial for condensation in the wall. Because this project involves an existing, aged structure rather than new construction where conditions can be more tightly controlled, the vapor permeable air barrier is a more forgiving approach, al- lowing the wall to breathe. In addition, the dew-point analyses showed a greater risk for condensation in the exterior wall with a vapor impermeable air barrier. WWW.PLANNING4EDUCATION.COM FIGURES COURTESY OF BRW ARCHITECTS, INC.

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