High Performance Shell – The Gift That Keeps On Giving

In developing a sustainable building, the design team must not overlook the shell. The shell consists of:

Walls                  Roof                      Foundation             Orientation
Floors                Windows                Doors                     Mass    

To maximize sustainable benefits, the air tightness, insulation levels, glazing attributes, roof reflectivity, mass and the orientation of the building to sunlight, wind and the elements must all be factored into the design. Both insulation and air tightness directly improve operating costs by reducing energy waste. Equally important, the shell also helps optimize other building systems. Think of the shell as a water pail. The more it leaks the more you lose. The most efficient equipment only delivers mediocre savings in a leaky shell.

Key factors to be concerned with:

  • Air barrier system--Controls air and humidity leakage into and out of the conditions space. It needs to be tight to prevent leaks and to avoid the build-up of unwanted moisture in building elements.
  • Wall, ceiling and foundation insulation--Controls the flow of heat into and out of the building. Make sure the insulation is at the highest cost-effective level.
  • Glazing--Provides various functions including being the part of the insulation blanket that allows sunlight and radiant heat to come through. Glazing also offers views connecting occupants to the outdoors. Most effective when the heat loss value (U-value), solar heat gain value (SHGC) and visible light transmission (VLT) are correctly balanced for the microclimate and use.
  • Cool roof--Reduces urban heat island effect and energy use through the light-colored, reflective surface. Cool roofs are climate dependent, but work in most parts of North America.
  • Mass--Enables short-term absorption and release of heat to buffer temperatures in the interior space. Morphing into many forms, mass includes concrete floors, sheet rock and masonry walls. Essential to buildings aimed at using solar heat and light.
  • Orientation--Determines the impact—both positive and negative—of elements such as sunlight, wind and noise. Essential to maximizing benefits of daylighting, passive solar heating and  natural ventilation.

Shell Guidelines for Energy Savings

For buildings under 70,000 square feet, New Buildings Institute’s Core Performance Guide provides targets for air tightness, insulation, glazing and cool roof values (see sample chart). These targets support total building savings from 25-35% without the need for independent modeling. For larger buildings or those being modeled, the Core Performance numbers help established a base to be optimized. Mass and orientation, a function of architectural design, need to be custom-developed for each building.

High Performance Shell & Lessons Learned

The only way to construct a truly a sustainable, comfortable and low-energy building is for the design team to have a thorough understanding of the envelope’s fundamental role. Constructing the proper shell is essential to building performance. The good news is that a wide range of materials are available today that can meet energy efficiency criteria in a cost-effective manner.

After determining the building’s design, shape, size and general envelope elements, the details can be modeled to help minimize thermal bridging and optimize practical cost-effective window framing and wall and roof construction. Alternatively, the design team can use the pre-engineered values provided in the Core Performance Guide if the building meets the criteria for that process.

Key Questions to Ask Your Building Envelope Design Team

The focus of design for a building envelope should be as the primary indoor comfort system. Once determined, the HVAC and lighting systems should be applied.

Here are some questions to ask your team:

  • How is a window specified? What quality control is used to ensure performance?
  • How is solar gain addressed on the perimeter?
  • Did an electrical engineer and lighting designer consult about window size specifications and locations to optimize daylighting?
  • Do the HVAC calculations include overall roof and opaque-wall U-values [search on U-values] (R-values) and account for thermal bridging?
  • Is overall window thermal performance being used in HVAC calculations?
  • How thermally efficient are the envelope details? Have thermal bridges been reduced or eliminated?
  • How has envelope air infiltration been specified? How will it be tested and enforced?
  • Are the selected building-shell colors appropriate for the local climate (light reflective finishes in hot climates and darker colors in cooler climates)?
  • Did the architect, owner and facilities maintenance people sign off and accept envelope performance criteria?
  • Has the criteria been linked to the energy performance goals?
  • Is the building design working with local climate or against it?

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