Using the architectural design and envelope to create low-energy buildings

The architectural design and envelope have major impacts on building energy, lighting, and comfort performance. Envelope features include form, shading, daylighting, choice of materials, sizing, orientation, and glass specification. Too often, an inefficient envelope design must be compensated for with more energy-intensive (and costly) mechanical and lighting systems. Therefore, the envelope design should be the first step in creating low-energy buildings. The mechanical and lighting systems can then provide the remaining (smaller) thermal and lighting comfort needs. Low-energy architecture is not effective if mechanical and lighting systems have to solve problems created by inadequate envelopes.

Each building reduced HVAC energy use through increased insulation levels and tightened envelopes, engineered windows and overhangs, reduced internal gains with daylighting, and an east-west axis with an optimal orientation. Envelope-integrated HVAC technologies, including Trombe walls, natural ventilation, passive solar gains, and cooltowers, met some of the remaining loads.

By using the mechanical system to make up for what cannot be accomplished by architectural features and envelope alone, the mechanical system does not have to correct for an architectural design that is climatically ill conceived. The Zion Visitor Center is a good example of this concept: daylighting was designed into the roof structure through clerestories, the cooling system was integrated into the vertical tower elements, Trombe walls were designed into the south-facing walls, and natural ventilation is provided through operable windows. At BigHorn, a conventional cooling system was not needed because the building had increased envelope insulation and reduced internal gains. All cooling could then be provided through natural ventilation.

3.3.2.1 Watch out for overglazing

Glass has a very strong impact on energy, comfort, and lighting performance, which can be positive or negative depending on many factors. Simulations allow us to optimize glass optical and thermal properties, sizing, orientation, placement, and shading. Additional glass is often added to achieve transparency—a common architectural desire. However, the strong exterior contrast from the sun combined with reflections often prevents the desired transparency, even with lots of glass. Transparency creep is evident at Zion, Oberlin, and CBF. Zion is overglazed on the north façade, Oberlin on the east side, and CBF on the south side. Additional shading would help to reduce cooling requirements caused by excessive glazing at both CBF and Oberlin. The structure outside the south-facing glass at CBF might be refitted with additional shading. A vine trellis on the east side of Oberlin’s atrium was designed to provide summer shading, but was never installed. Before adding shading, a detailed analysis should be conducted that takes into account increased heating loads, reduced PV output, decreased daylighting, and glare problems.

3.3.2.2 Minimize thermal bridging

HVAC loads are affected by the overall thermal performance of the building envelope. Although high levels of thermal insulation are important, two- and three-dimensional heat flows through construction details can also have an important impact; these heat flows are referred to as thermal bridges.

Although designers took care to develop designs that would minimize thermal bridging, four problem areas at the TTF were identified that add to its thermal loads.

• The window frames form a thermal bridge. Figure 3-18 shows a sample of infrared images used to determine that the window and door frames installed were not thermally broken as specified.

Compared to the baseline TTF model, as much as 13.6 MMBtu/yr (3,986 kWh/yr) are lost through the window frames because they are not thermally broken.

Figure 3-18 TTF infrared thermal images showing heat loss through window frames

• There is a thermal bridge where a retaining wall meets the building. Although the impact of this thermal bridge on the annual heating and cooling performance is minimal, multiple incidences of thermal bridging can offset savings from the low-energy envelope. This flaw was built according to plan and should have been identified during the design phase.

• During construction, the foundation insulation was relocated for structural reasons. As a result, 6 in. (15 cm) of insulation was removed, which created a thermal bridge that is approximately 390 ft (119 m) long by 6 in. (15 cm) wide. NREL estimates that an additional 4.3 MMBtu/yr (1,260 kWh/yr) are lost through this thermal bridge. Figure 3-19 shows infrared thermal images that indicate heat loss through the foundation.

>11.7°C

<-8.2°C -5.0

0.0 5.0 10.0

>15.9°C

<-4.0°C 0.0 5.0 10.0 15.0

Figure 3-19 TTF infrared thermal images showing heat loss at the foundation

• The high-bay garage door is a significant thermal bridge. Figure 3-20 shows the temperature distribution of the high-bay roll-up door during a summer morning. Incident morning sun heats this door, which causes summer comfort problems in this section of the high-bay. The interior surface of the east garage door in the high-bay of the TTF can exceed 110°F (38°C) during morning summer hours. As shown in Figure 3-20, the floor-to-ceiling temperature distribution can exceed 30°F (16.7°C) in the high-bay, which results in localized discomfort and additional cooling loads. A high-reflectivity and high-emissivity exterior paint, combined with a low-emissivity interior paint, would reduce the heat absorbed by the door and reduce heat emitted to the space. Reducing heat emitted from this door would improve thermal comfort at the east end of the high-bay. Other solutions include external shading. Although more expensive, replacing the door with a well-sealed R-25 (RSI-4.4) panel garage door would be the optimal solution, and would align with the design intent for the rest of the TTF envelope.

*>110.0°F

*<72.0°F 80.0 90.0 100.0 110.0

Figure 3-20 TTF high-bay insulated roll-up door thermal bridging

Another example of a significant thermal bridge is in a small, unconditioned storeroom at Zion. This storeroom often overheats because its dark brown metal double doors collect heat during the morning.

The internal temperature of the doors radiates to employees in the storeroom (see Figure 3-21). The doors could not be painted white because the exterior color of the building had to follow NPS standards, which is dark brown. To help cool and ventilate this zone, several small fans were installed along the floor vents. Designing the steel door with increased insulation and a radiant barrier would have been a better solution, in line with the design intent used for the rest of the building. A uniform thermal envelope that includes well-insulated storeroom and garage doors can eliminate comfort problems and unnecessary heating and cooling loads.

85.0°F 149.0°F

100 120 140

Figure 3-21 Zion infrared image showing interior surface of storeroom door overheating from solar radiation

Ground heat transfer is another area with complex three-dimensional heat flows. Many questions surround modeling heat transfer through slab-on-grade floors. The common practice of insulating only the perimeter of the slab is justified only if the ground under the center portion of the slab will eventually reach an equilibrium temperature under the influence of the controlled environment above the slab.

Through STEM tests (Subbarao et al. 1989) and thermography, NREL found that more heat is lost through the floor and window frames than models predicted. Based on other work (Deru and Kirkpatrick 2001), researchers suspect that the ground under the center of the slab is not reaching equilibrium. Additional work would be required to determine the benefits and costs of insulating the entire slab. One impact of slab heat transfer losses is comfort. Based on thermal comfort models, a cold floor adversely affects foot temperature which is a critical component for occupant comfort. Insulating floors will result in warmer floors and may result in lower overall building temperatures because people with warmer feet can be more comfortable at lower zone temperatures.

Designing the envelope to minimize thermal bridging is good practice. An otherwise well-insulated envelope can suffer if a single component lacks appropriate thermal properties. Additional consideration is typically needed to ensure that doors are well insulated (especially those that have significant direct gains), window frames are installed with thermal breaks, and the insulation at the interface with ground-coupled surfaces has been appropriately specified. Ensuring insulation details are included in the specifications and then implemented during construction is essential to achieving a thermally uniform envelope.