Page 58 - Zero Net Energy Case Study Buildings-Volume 3

CASE STUDY NO. 13
LA ESCUELITA EDUCATION CENTER
(Below and Bottom of oppo- site page) Glazing types used in the Classroom Building
to maximize daylighting or
to minimize solar heat gain, depending on location. (Courtesy SVA Architects)
Building Envelope
The building enclosure is designed to maximize daylighting and to minimize solar gain. As a baseline design, the insulating characteristics of the opaque parts of the envelope were estab- lished as R-19 for the walls and R-30 for the roofs, which meets the requirements of the Cali- fornia energy code. The buildings use standard metal stud framing but because of the relatively mild climate and extra cost required, a layer of continuous insulation over the exterior metal stud wall construction to prevent thermal bridging was omitted. The result was a reduction in the effec- tive R-value of the wall to approximately R-6, a contributing factor to the relatively high heating load measured in the winter months. (See the chart on the bottom of p. 55 and the discussion on p. 60.)
The innovative design feature of the building envelope, however, was the introduction of a tech- nique of increasing the thermal mass of the walls beyond normal construction. This was a neces- sary development because of the District’s policy of no mechanical cooling and the clear need for comfort cooling at certain times of the year. The CHPS criteria also required that ASHRAE Stan- dard 55 (thermal comfort) be met. Thermal mass would provide a damping effect to this peak cooling load and allow as well a night ventilation strategy to precool the building during those sequences of weather events. (This operational strategy is discussed further below in Heating, Ventilating and Cooling Systems.)
The technique was to introduce two layers of cementitious board on the inside surface of the walls, covered with 1” of plaster to form 2” of a thermal mass material. (See the figure on the facing page.) To provide both acoustic treatment for the room and to allow tackable surfaces for students and teacher without covering the heat transfer surface of the thermal mass, tackboard was designed to be installed at certain locations using “standoff” hardware from the wall surfac- es. This acoustic treatment was necessary as well to meet a CHPS criterion for acoustic quality. (Due to a construction error, this standoff hardware was not actually installed.)
Another design feature of the building envelope was the selection of three different types of glazing for the windows, depending on the location and function of the openings in the specific location. The partial elevations of the classroom building shown on the facing page illustrate the locations of these three glazing types.
Glazing intended to maximize daylighting has a high visible light transmission (70% for glass type T2) and was used on the south-facing high windows and north-facing windows generally. Glazing in south-facing view windows has a low solar heat gain coefficient (SHGC = 0.30 for glass type T1). An even lower SHGC was specified in relatively unprotected south facade loca- tions (SHGC = .24 for glass type T3). The low SHGC was specified because of concerns about solar heat gain overwhelming the thermal mass cooling strategy.
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Zero Net Energy Case Study Buildings: Volume 3