(Written by Jack Milarch – NMHBA EVP/CEO – Originally published in the September 2009 Housing Journal)
Essential Terms for a Discussion of Energy Efficient Building
For the last couple months a number of NMHBA member volunteers and staff have participated in our state’s periodic code change process. It’s a huge project. We have been meeting about 10 hours per week, and sometimes more. Our building codes change about every three years, and our project consists of reviewing and “New Mexico-izing” the 2009 model building codes. The priority subject this time around is energy efficiency. Everything else is taking a back seat to this. Governor Richardson has decreed that our new building codes be 20% more efficient than the 2006 base building codes. And that isn’t just a distant fantasy. My observation is that our meetings are being carefully monitored and managed to be sure that result, or better, is achieved.
What does this mean to the average contractor and the crews who work on the job site? Nearly every aspect of construction will be impacted. Many field observers believe the learning curve will be steep. For early adopters of “green” building techniques, this won’t be as much of a problem. Unfortunately Construction Industries Division staff anticipates that for the majority of contractors and crews the changes will mean lots of “correction” notices and “red” tags. If your business is directly involved in the construction process I believe you can help avoid the pain of the change by learning as much as you can about highly energy efficient construction and passing relevant information along to your employees.
An understanding of a handful of the relevant and commonly used terms can go a long way in all this. Below is my group of important terms and concepts gleaned from hours of code change discussions, in no particular order. I’m sure those who are deeply involved in this type of construction could add pages of items to my list, so please forgive me if it seems to you that this list seems overly elementary.
L.E.E.D. — Leadership in Energy and Environmental Design is a program of the U.S. Green Building Council to provide standards for environmentally sustainable construction. LEED as a construction standard is used for both home building and commercial building, including “certification” of the finished building.
Build Green New Mexico – Another standard for “green” construction based on the work of National Association of Home Builders, applying to residential construction. Both L.E.E.D. and Build Green New Mexico standards are allowed to qualify a new home for New Mexico’s Sustainable Building Tax Credit, which is reportedly the most generous tax credit for energy efficient homes in the nation.
Thermal Envelope — The insulated assemblies surrounding conditioned (heated/cooled) space within a house. The attic or basement may or may not be included in this area depending on the placement of the insulated assemblies. Recent code discussions related to this involve air duct placement and duct insulation details depending on whether the duct is within the thermal envelope.
Thermal Bypass — The unintended transfer of heat through insulated assemblies such as walls. This loss of insulating ability frequently occurs due to holes drilled into framing wood or gaps between the framing and insulation. Infrared camera photos dramatically display this problem. A newly mandated code inspection (thermal bypass) will be looking for such holes and gaps. A pre-drywall blower door test is intended to expose such defects during construction.
Blower Door Test – A fan-based test changing the home’s inside pressure relative to ambient outside pressure, to detect air leakage from outside the thermal envelope by measuring air loss. Results are given in air changes per hour.
Duct Blaster Test – Similar to above, but specifically tests duct and mechanical equipment for unintended air leakage. Older homes routinely fail this test massively. Results are given in percentage of air loss.
Heel Truss — A truss designed to allow full depth insulation out to and over the outside top plate. Traditionally designed roof trusses can hinder the home’s energy saving performance because they fail to leave enough space at the eaves for full insulation which can be a technical violation of the energy codes.
H.E.R.S. (HERS) – Home Energy Rating System of the Residential Energy Services Network (RESNET). A HERS Rater works to review the home design and construction details to identify its energy characteristics and anticipated performance. Details such as insulation levels, window efficiency, wall-to-window ratios, and the heating and cooling system efficiencies are reviewed and graded. Performance testing, such as a blower test for air infiltration and duct leakage can be part of the rating. Data gathered by the HERS rater is entered into a RESNET accredited computer program and translated into a rating index. A HERS Index of 100 means the home meets the requirements of the (currently 2006 edition) International Energy Conservation Code (IECC). The City of Santa Fe requires a new home to show a HERS index of 70. Some “zero energy” homes reportedly show HERS index numbers in negative territory.
REScheck – Free DIY software designed to demonstrate and check compliance with the International Energy Conservation Code (IECC). Intended for prescriptive design, it does not require a blower door test or duct blaster test to demonstrate compliance with the IECC. A little practice with this “light” version of rating software can help familiarize you with the concepts of home energy rating. Download REScheck at: http://www.energycodes.gov/rescheck/download.stm
Energy Star — An international standard for energy efficient consumer products, including new homes. It was first created as a U.S. Department of Energy program in 1992. Products carrying the Energy Star logo, such as computer products and peripherals and kitchen appliances, are expected to save 20%-30% on average. The residential construction program generally certifies a home is at least 15% more energy efficient than the 2004 IECC. Builders are finding that an Energy Star label on their new homes is a meaningful attraction to buyers.
Performance Based Design – One of two “paths” for home design and construction allowed by our new building code. Performance Based Design means a building with components specifically designed to work as a part of an overall design, in contrast to a home built to “prescriptive” code. A very simplistic illustration of these two concepts could be seen in the nail pattern for a framed sheer panel. The “prescriptive” design would call out the familiar one-size-fits-all pattern of nailing following the code book example, and a building inspector would look for nails spaced like the example shown in the code book. In contrast a performance based nail pattern might call out a different nail pattern on each individual sheer panel based on careful design anticipation of the seismic loads expected to be on that part of the building. The inspector would need to check the design calculations for that particular sheer panel to see if the pattern is right. For anyone who has used the old NM Model Energy Code Trade-off worksheets, you will recognize that performance based building is a similar concept. Highly energy efficient construction is often accomplished through performance based design. How our inspectors react to this idea remains to be seen. Our regulators are discussing the idea of adding “maxi” type rules for construction. An example might be giving us a maximum tonnage allowable for builder installed air conditioning equipment.
Prescriptive Design – This describes most construction done today. This is the concept where every detail is done following the code book, with strict adherence to tables containing specific minimums for wall and attic insulation, framing, windows and doors, floors, slab, etc. In this instance, the composite energy value is obtained by simply adding up each preset value to reach the required result.
Sustainable Building Tax Credit – Referring to a law passed by the NM Legislature in 2005, these NM tax credits are designed to reward builders who construct residential buildings to at least the LEED for Homes or Build Green NM “Silver” levels. The program was designed to encourage the spread of highly energy efficient building techniques and required support skills, and it seems to be working as planned.
“R” Value and “U” Value – The “R” value is a measurement of heat loss retardation characteristics of a building component. For example increasing the thickness of an insulating material increases the “R” value. The “U” value is the inverse of “R” value. “U” value describes how well a building element conducts heat. It measures the rate of heat transfer through a building element over a given area, under standardized conditions, and therefore lower “U” value numbers mean higher levels of energy conservation. Both “R” and “U” values are commonly used in building component discussions.
Net Metering – This is the term used for recognizing the value of homeowner generated power which is fed back into the power grid. Under current Public Regulatory Commission rules a homeowner receives retail credit for at least a portion of the electricity their solar photo-voltaic systems generate. Their electricity meters record in “both directions”, allowing a no-cost method of effectively banking excess electricity production for future credit.