Architectural Record BE - Building Enclosure

Energy Efficient Wood Buildings

Strategies for achieving energy objectives with wood-frame structures
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Continuous Insulation Requirements

Under the prescriptive path of the IECC, walls, floors, and roofs have specific insulation requirements based on framing time and climate zone. For example, in IECC Table C402.1.3, above-grade metal-framed walls in Climate Zones 3 and 4 (except Marine) are required to have R-13 cavity insulation and R-7.5 continuous insulation (ci) applied to one face of the wall. The wood-framed walls at the same location are required to have R-13 cavity insulation and R-3.8 ci or have R-20 cavity insulation with no additional continuous insulation requirements. The option to forego continuous insulation requirements on wood-framed walls with R-20 cavity insulation is acknowledgement that metal studs have a significantly higher thermal conductance than wood studs. The R-20 cavity insulation option allows wood wall frames with 6-inch-deep studs to meet the prescriptive wall requirements with no continuous insulation in Climate Zones 1 through 5 except Group R occupancies in Marine Climate Zone 4. The R-20 wood-framed wall is the only option available in the IECC prescriptive wall path using prescribed R-values that does not require continuous insulation for above-grade walls.

Like the IECC, the perspective path of ASHRAE 90.1 contains an option to reduce or forego continuous insulation in wood-frame walls with the use of 6-inch cavity walls.

Air Leakage Requirements

The requirements in the 2012 IECC for air barrier assemblies and air-leakage control in residential buildings are different than those for commercial buildings. For commercial buildings (including most multi-family buildings), IECC Section C402.5.1 states that, “a continuous air barrier shall be provided throughout the building thermal envelope.” The air barrier can be installed inside, outside, or within the building envelope, and must be continuous and sealed. In Climate Zones 2B, the installation of air barriers is not required for buildings following the commercial requirements of the IECC. Materials must be air impermeable (<0.004 cfm/ft2 @75 Pa), and assemblies must have an average air leakage rate not exceeding 0.04 cfm/ft2 @75 Pa. The completed building must be tested and the air leakage rate of the building envelope cannot exceed 0.40 cfm/ft2 of enclosure area at 75 Pa when tested in accordance to ASTM E779 or equivalent (e.g., the U.S. Army Corps of Engineers [USACE] Standard).

Significant Changes in the 2012 IECC

The 2012 IECC commercial provisions have been found to have an Energy Use Intensity savings of 24.3 percent over the 2006 edition when plug and process loads are neglected. The 2015 IECC is a further reduction of 11.1 percent over the 2012 IECC. Of particular relevance to wood-frame construction, the opaque thermal envelope requirements, which dictate the thermal performance of walls, roofs, and floors when designing a building to the IECC prescriptive approach, have been tightened. Values in Table C402.1.4, if utilizing the prescriptive U-factor alternative, and Table C402.1.3, if utilizing the prescriptive R-value insulation and fenestration criteria, have been made more stringent. If a designer wants to minimize or eliminate the use of continuous insulation in wood-frame buildings, then 2”-by-6” construction may be required in the building envelope in order to accommodate R-20 cavity insulation (depending on the climate zone).

New Opportunities with Mass Timber

Mass timber construction uses large prefabricated wood members such as cross laminated timber (CLT) for wall, floor, and roof construction. Glulam can also be used in beam and column applications. These products, combined with a heightened awareness of wood’s carbon benefits, have focused attention on the possibility of “tall wood” buildings, either made entirely from wood products or a combination of wood and other materials.

From an energy-efficiency perspective, different materials and exposure conditions for taller buildings will dictate different—and likely more rigorous—approaches to heat, air, and moisture control than for buildings up to six stories. However, it is worth noting the unique characteristics of mass timber building systems that lend them to the design of energy-efficient structures.

Like all wood products, CLT panels have good thermal properties. However, their thickness provides both thermal insulation and thermal mass, which is recognized in the IECC. The thermal mass of the building-enclosure elements, as well as that of the interior floors and walls, can improve the energy efficiency of buildings by storing solar heat energy during the day and releasing it at night. This acts to reduce peak utility loads by shifting the time and intensity at which they occur, reduce the building’s overall energy use, and improve occupant comfort. The actual benefits of thermal mass within a building will vary with climate and solar radiation, building type and internal heat gains, building geometry and orientation, and the actual amount and location of thermal mass used, but it is a common strategy in energy-efficient buildings. Thermal mass is typically associated with concrete or masonry buildings; however, heavy timber framing, with products such as CLT, has considerable thermal mass and the associated whole-building energy-efficiency benefits.

Although CLT panels may have some inherent level of airtightness, an additional air barrier membrane is recommended given the possibility that gaps between boards may develop as a result of drying-related shrinkage. The monolithic nature of CLT panels makes it possible to have a single membrane serve as both the water-resistive barrier and continuous air barrier.

 

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Originally published in Engineering News-Record.

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