Understanding the effects energy codes are having on design is quickly becoming a top priority for architects, developers, and general contractor firms.
R-value requirements for building envelope components are determined by climate zone and are also impacted by building occupancy types, wall types, and the compliance path chosen.
Climate zone. It is first necessary to determine in what climate zone the project is located. Today there are eight climate zones for the entire U.S., a vast improvement over 1989 when there were 38. Tables in the code identify the correct climate zone for every U.S. county and territory.
Building type. Section 502 of the code defines residential and commercial building types. Residential buildings include detached one- and two-family dwellings and multiple single-family dwellings (townhouses) as well as Group R-2, R-3, and R-4 buildings three stories or less in height above grade plane. The last phrase is sometimes called “light commercial.” In the energy code world, it is considered residential, primarily because the equipment and appliances used and exterior walls more closely match those used or found in a single-family house than in an office building. Commercial buildings refer to all buildings not included in the definition of “residential buildings.”
Wall type. While the IECC has reduced the number of climate zones, R-value tables have become more complex. The commercial table lists multiple variations within each envelope component, except for walls below grade, where only one type is listed. Three types of roofs, for example, four types of above-grade walls, and two types of floors make for many permutations.
Compliance Path. There are three paths to compliance: prescriptive, performance, and whole building analysis. In the prescriptive path, building design and components need to meet R-values listed in the tables. The prescriptive path is stringent and offers little flexibility, and does not take into account much individualized proposed building information, other than categories given in the tables. The performance path uses established software to measure compliance, providing less stringent requirements and more flexibility to trade off requirements. In the performance path, the designer is allowed to “build” a description of the structure, including specific information on project location, wall and window areas, orientation, the location of thermal mass relative to the exterior insulation layer, etc. If the envisioned envelope fails to meet the code requirements, it becomes a matter of adjusting the various components. To be sure, the performance path is more complex than the prescriptive path, which architects have traditionally followed. But with software becoming easier to use and the design flexibility it offers in meeting more stringent code requirements, the performance path may be more readily embraced in coming years. While not explicitly referenced in the IECC, the ASHRAE standard on which the IECC is based, specifically names two computer programs that are designed for use by the average architect.
Another path to compliance uses the whole building analysis. While this method requires even more complex software, tradeoffs are allowed among envelope components, HVAC systems, and lighting. For example, the whole building path may allow lower R-value wall material in exchange for pre-heating incoming “fresh” air. While not widely used today, the whole building path may be the main method in future years as it analyzes a building’s total energy use, rather than compliance of individual components.
IECC and Continuous Insulation
National model energy codes are advancing the way designers approach commercial and residential exterior walls, some by focusing on continuous insulation (CI) systems, which provide an uninterrupted insulation layer over an entire wall, not just in the wall cavities. The 2012 code has created more opportunities for the use of CI on both new residential and commercial walls. Any time the insulating layer is interrupted, the effective R-value of the insulation is reduced. Increases in thermal performance have resulted in CI figuring more and more in meeting both prescriptive requirements and overall building R-value targets.
Continuous insulation is not currently defined specifically in the ICC family of International Building Codes, but it is defined in ASHRAE 90.1 as:
Continuous insulation (c.i.): Insulation that is continuous across all structural members without thermal bridges other than fasteners and service openings. It is installed on the interior or exterior or is integral to any opaque surface of the building envelope.
Because CI combines rigid insulating foam and structural sheathing into an easily fabricated product that provides uninterrupted exterior insulation, air sealing, and a solid nailing surface for exterior finishes, many find it helps meet new code requirements in less time and with less incremental cost.
CI is found in various places in the 2012 IECC. Table C 402.2, for example, is a prescriptive table that lists all opaque (non-window) wall R-value requirements. Where CI is listed in this table, it is mandatory.
The table pictured below is an R-value summary for the 2012 code. CI requirements are scattered throughout the table. Note that EPS (expanded polystyrene) R value = 3.8 /inch, while XPS (extruded polystyrene) R value = 5 /inch.
2012 IECC R-Value Summary
Footnotes are worth noting. In the commercial table, the footnotes specify: for SI: 1 inch = 25.4 mm. CI = Continuous insulation. NR = No Requirement. In the residential table, the following footnotes are important to consider:
Footnote C. “15/19” means R-15 continuous insulation on the interior or exterior of the home or R-19 cavity insulation at the interior of the basement wall. “15/19” shall be permitted to be met with R-13 cavity insulation on the interior of the basement wall plus R-5 continuous insulation on the interior or exterior of the home. “10/13” means R-10 continuous insulation on the interior or exterior of the home or R-13 cavity insulation at the interior of the basement wall.
Footnote H. First value is cavity insulation, second is continuous insulation or insulated siding, so “13+5” means R-13 cavity insulation plus R-5 continuous insulation or insulated siding. If structural sheathing covers 40 percent or less of the exterior, continuous insulation R-value shall be permitted to be reduced by no more than R-3 in the locations where structural sheathing is used– to maintain a consistent total sheathing thickness.
R-values are on the increase. For example, in terms of increased R-values, Zone 4 steps up basements for residential and commercial properties. Zone 5 steps up residential basements and commercial wood, and Zone 6 steps up R-values in residential wood frame and commercial wood, and all other occupancy types.
It is important to note the definition of mass walls. According to Section C402.2.3 of the code, mass walls weigh at least 35 lb/ft2 of wall surface area, or 25 lbs./ft2 of wall surface area if material weight is ≤ 120 lb./ft3. Masonry veneers are not mass walls. The Residential Code in Section R402.2.5 says that mass walls are above-grade walls of concrete block, concrete, insulated concrete form (ICF), masonry cavity, brick (other than brick veneer), earth (adobe, compressed earth block, rammed earth) and solid timber/logs. Masonry veneers are not mass walls. They are usually isolated from the interior conditioned air. Architects should not use “Effective” or “Thermal Mass adjusted” R-values for walls and/or products when checking compliance in prescriptive tables. For example, consider the R-value minimum for mass wall vs. wood frame in a non-residential occupancy such as a strip shopping center or fast food restaurant. In every climate zone the mass wall R-value is less than for the frame wall in the same climate zone. Therefore, it is safe to conclude that thermal mass benefits are already included in the tables.
While staying up to date on the dizzying amount of building code changes can become challenging, it is becoming clear that having the right product for the right job can save developers cost on the front end and provide building owners with long term savings that come in the form of reduced energy costs.
A special thanks to Oldcastle APG Masonry and Architectural Record for supplying us with this article.
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