For years, HVAC engineers have been trained to provide “reactive” designs. After the architect determines a building's layout, the HVAC engineer designs a comfort system to fit. The result has been that the HVAC engineer has had little opportunity to influence overall building design to reduce HVAC and energy requirements and increase indoor comfort.
This process has to change if the HVAC building-design industry has any hope of creating a real collaborative integrated design process (IDP), by which heating and cooling loads are lowered as far as possible before passive systems that use the natural local energy around a building (i.e., natural ventilation, daylighting, passive solar heating, solar-gain control, etc.) are designed. HVAC engineers/designers must understand how building skins and passive systems work from the beginning. Also, they must be trained on the IDP, which some feel is an architect's responsibility. (For more on integrated design, see the author's article “High-Performance Buildings Through Integrated Design” in the February 2007 issue of HPAC Engineering.)
THE EDUCATION GAP
One of the biggest challenges facing the building-design industry is the lack of practical building-science/physics education aimed at mechanical engineers, HVAC designers, and some architects. Many advanced building-science courses are available to architects, but virtually no university-level core courses that focus on building-systems design and building science exist for engineers. There are various technical-school/college courses and programs that teach HVAC designers how HVAC systems work, but these classes do not educate beyond rudimentary heating and cooling calculations used to size HVAC systems and plants.
Few HVAC-design engineers have taken a formal university-level course on building science/physics and HVAC design. Additionally, few university-level engineering courses in North America focus on building-system design. Those that are provided are offered as third- or fourth-year “elective”courses. Therefore, many of the people who design the building systems that utilize more than 40 percent of the total energy used in North America have learned their trade through “on-the-job training” alone.
Technical schools provide graduates who are able to size pipes, ducts, and basic “all-air” heating and cooling systems and the science behind them, but are relatively uneducated on the harvesting of natural energy, other types of HVAC systems, building science, building-skin thermal performance, and the life-cycle cost of building systems. HVAC engineers/designers must be able to articulate the consequences of design decisions regarding building-occupant comfort, mechanical-plant first costs, ongoing operation for the life of a building, and environmental impacts. HVAC engineers/designers must attend to this as early as possible in a project's design process.
For the IDP to work properly and result in a truly high-performance, comfortable, low-energy building, entire design teams must understand building physics and passive building design before applying high-tech solutions. The next trick is to model building energy and envelope performance accurately and determine capital-cost trade-offs and life-cycle costs of various building-system options. This requires more effort during the schematic-design and design-development stages, before construction documents are started. The traditional engineering/design fee-management process does not allow for this level of up-front effort.
A recent study1 focused on architects' building-envelope details and their impact on HVAC designers' heating- and cooling-load calculations. Four identically insulated and glazed building models were set up with R-19 wall insulation, but with variations in window frames, wall detailing, and wall construction. The models had the same window areas and center-of-glass thermal performance. The overall wall thermal resistance varied from a low of R-4.8 to R-12.6. (This included adding the R values of the glazing and frames to the wall thermal resistance to get an “overall” R value).
Thermal bridging caused by accepting cheaper/faster construction products and details can degrade wall and roof thermal transmission by more than 25 percent below insulation values. For example, R-20 wall insulation, coupled with poor thermal-bridging details, may provide an actual R-13 to R-14 thermal-transmission barrier.
Three key questions need to be asked when HVAC loads are first calculated: Has thermal bridging been accounted for in the wall and roof R values? Has overall glazing performance been used? What do the architect's envelope air-infiltration-control and performance specifications look like?
After a client's goals are set, one of the first steps that must occur in the IDP is to generate a building energy model using a sophisticated software package. This will give the design team a chance to apply some basic building-envelope and orientation configurations and passive-design elements to reduce building energy loads. This is the point at which daylighting, natural-ventilation paths, and solar-load control designs can be tested and defined.
Once a building's design, shape, size, and general envelope elements are determined, envelope details can be modeled using software to minimize thermal bridging and optimize practical cost-effective window-framing and wall-, and roof-construction details. (Lawrence Berkeley National Laboratory's Therm software is available as a free download at http://windows.lbl.gov/software/therm/therm.html.) This helps reduce building thermal loads and provides accurate thermal-performance values for comfort-system load calculations and designs. Also, it can allow the consideration of radiant-heating and cooling loads inside of a building's perimeter zones and the proper application of ANSI/ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy.
Window and glazing specifications and window load estimations are common weak areas for architects and HVAC designers. Nearly all window suppliers and manufacturers list center-of-glass performance in their catalogs because they have little control about the framing systems in which the windows will be used. A window's true measure is its overall performance, including the derating of thermal performance, which depends on the framing and mullion systems used. Although an architect might specify a high-thermal-performance window with a U value of 0.26, if that sealed window unit is installed in a poor, non-thermally broken frame, the actual overall U value likely will be closer to 0.41 — a 50-percent reduction in the thermal-transmission performance of the entire window area. If an HVAC designer selects perimeter heating and cooling systems using the center-of-glass thermal performance, there will be problems later. Lawrence Berkeley National Laboratory's Window software models the overall thermal performance of a window assembly so thermal load calculations are more accurate. (The software is available as a free download at http://windows.lbl.gov/software/window/window.html.)
The IDP requires an HVAC engineer to work closely with architectural window and glass specifications to ensure they are detailed properly and specified to match heating- and cooling-load calculations. If an architect is listing a center-of-glass U value, as well as a framing system that must meet certain standards, then the overall window systems' thermal performance must be specified to set a standard of acceptance. I commonly require window-system shop drawings to include a software model based on the proposed materials so a comparison can be made between suppliers of different, but potentially equally performing, products.
Another major aspect of building envelopes overlooked by design teams is airtightness. Mechanical systems are commissioned regularly, but most construction specifications do not solicit building-envelope commissioning. The architectural community needs to push for proper envelope-commissioning specifications. Many building air-leakage issues become HVAC issues in short order.
Prior to occupancy, finished buildings must have blower-door and curtain-wall water-penetration tests, and building envelopes must be certified as performing to the standards used for the architectural specifications. How else will a building team know that a building will perform as designed? Therefore, HVAC engineers must specify an airtightness requirement for building enclosures and clearly state that HVAC-system designs are based on specified envelope-performance characteristics.
Good standards for checking and commissioning building envelopes include:2
Canadian General Standards Board Standard 149-GP-2MP, Manual for Thermographic Analysis of Building Enclosures.
ASTM International Standard C1060-90, Standard Practice for Thermographic Inspection of Insulation Installations in Envelope Cavities of Frame Buildings.
ANSI/ASHRAE Standard 101, Application of Infrared Sensing Devices to the Assessment of Building Heat Loss Characteristics.
ASTM International Standard E1186-03, Standard Practices for Air Leakage Site Detection in Building Envelopes and Air Barrier Systems.
ASTM International Standard E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization.
Building envelopes must supply most, if not all, of a building's indoor-comfort needs, as well as lighting during the day, so that minimal additional contributions are needed from active mechanical and electrical systems. On-site generated energy, such as photovoltaic panels, solar water-heating panels, wind generators, geo-exchange systems, and other renewable-energy sources, can be used where additional mechanical and electrical needs are small. The challenge is to create net-zero-energy-use buildings with good indoor environmental quality. This can be accomplished only if the building design team is educated and works in an integrated fashion.
Integrated design will not cost a project more money in the long run, in spite of higher design-team fees used to accomplish a higher-performance building. It has been proved time and again that extra design fees are paid for with first-year building energy savings and that the first costs of an IDP building generally are the same as or lower than those of a conventionally delivered building. The costs of not using the IDP and not designing high-performance building envelopes include:
Ongoing energy costs.
Poorer occupant comfort, more comfort complaints, and worse indoor environmental quality.
More ongoing maintenance for higher-capacity HVAC systems and building-envelope components for leaks and air infiltration.
Building-envelope moisture and thermal problems with concurrent mold issues in envelope cavities.
Shortened building life expectancy and subsequent inability to use the building.
For the IDP to work well, an HVAC designer must have a good foundation in building science and be able to understand how building heating and cooling loads are affected by building-envelope design. A paradigm shift in how HVAC engineers approach the initial design of buildings is required — they no longer can simply “react” to what an architect is designing. Designers must be proactive in developing high-performance envelopes, allowing better indoor comfort and lowering HVAC-system energy use and capital and operating costs.
Der Ananian, J.S., & O'Brien, S.M. (2007). Evaluating energy efficiency using whole-building simulation tools. Journal of Building Enclosure Design, 10-14.
Goncalves, M.D., Gendron, P., & Colantonio, T. (2007, September). Commissioning of exterior building envelopes of large buildings for air leakage and resultant moisture accumulation using infrared thermography and other diagnostic tools. Building Envelope Forum, 9. Retrieved from http://www.buildingenvelopeforum.com/commissioning.htm
A senior mechanical engineer with more than 25 years of building-services-design experience, Geoff McDonell, P.Eng., LEED AP, specializes in low-energy, semipassive building systems and high-performance building envelopes. Over the years, he has provided design, construction, and project-management services for a large number of institutional buildings. He is a technical reviewer for the Association of Professional Engineers and Geoscientists of BC and the author of numerous papers and articles on radiant-cooling, in-slab-radiant, and displacement-ventilation systems and other sustainable-building approaches.
For past HPAC Engineering feature articles, visit www.hpac.com.
For building-science education online, visit the following Websites:
Building-Envelope Design Tips
A building's envelope should be designed as the primary indoor comfort system before “active” HVAC and lighting systems are applied. Following are the top 10 questions an HVAC designer needs to answer about a building envelope:
How is a window specified, and what quality control is in place to ensure its performance?
How is solar gain addressed around the perimeter?
Have an electrical engineer and lighting designer been included in choosing window sizes and locations to optimize daylighting while reducing solar gain?
Are overall roof and opaque-wall U values (R values) that account for thermal bridging being used in HVAC calculations?
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 building-shell colors appropriate for the local climate (light reflective finishes in hot climates and darker colors in heating climates)?
Have the architect, owner, and facilities-maintenance representatives signed off and accepted the building-envelope performance criteria? Have the criteria been tied to the building's energy-performance goals?
Is the building design working with the local climate or against it?