Exceeding Thermal Performance Goals by Choosing Wood

Understanding how wood can help contribute to significant energy savings in the built environment
 
Sponsored by Think Wood
By Andrew A. Hunt
 
1 AIA LU/HSW; 1 GBCI CE Hour; 1 AIBD P-CE; 0.1 IACET CEU*; SAA 1 Hour of Core Learning

Learning Objectives:

  1. Discuss the thermal advantages of building with wood.
  2. Describe how designing with wood can enable design flexibility to maximize building performance.
  3. Explain common methods for controlling heat and airflow in wood-frame and mass timber buildings.
  4. List common insulation, airtightness, and vapor permeability strategies often employed with wood construction to achieve effective thermal efficiency.

This course is part of the Wood Structures Academy

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The benefits of choosing wood in commercial and nonresidential projects are many. Both light frame and mass timber structural systems offer flexibility in design options. They also are economical and relatively easy to construct, providing ease of use on the job site. Yet one important benefit that should not be overlooked is the thermal performance that wood can provide. Thermal performance contributes to a range of important goals for most projects, including energy efficiency, comfort, durability, code compliance, structural integrity, and sustainable outcomes. Designing with wood not only meets performance requirements for commercial and nonresidential buildings—it can also exceed goals.

Common Ground High School

Photo: David Sundberg + Gray Organschi Architecture

Common Ground High School

Designing with wood offers architects the flexibility to design projects with increased insulation. From a thermal perspective, wood-frame building enclosures are inherently more efficient than steel-frame, concrete, or masonry construction.1 This advantage stems from the insulating qualities of the structural elements of wood, including studs, columns, beams, and floors, but also the fact that wood stud walls are easy to insulate. Wood has the bonus benefit of allowing less thermal bridging, as the relative R-value of wood is higher than steel and masonry.2

The Natural Thermal Properties of Wood

Wood naturally provides insulation benefits due to its inherent structure. Air pockets within timber’s cellular structure create a barrier to heat and cold, similar to air pockets found in most types of insulation. The air pockets reduce the thermal conductivity of wood, slowing heat transfer and helping to reduce thermal bridging. Thermal conductivity, or the ease with which heat energy flows through the material, declines as the density of the wood decreases.3 This means that modern softwoods have a thermal performance edge over traditional hardwoods.

thermal resistance

Source: CLT Handbook, U.S. Edition, Chapter 10

The thermal resistance of typical softwood varies depending on thickness and moisture content.

As an organic material, other aspects of wood can alter thermal performance. Grain direction can significantly impact thermal conductivity. For example, thermal conductivity of pine in the direction of the grain is 0.22 W/moC (which is equivalent to degrees Celsius/W) and perpendicular to the grain 0.14 W/moC.4 This is important to know, as dimensional lumber in studs will have less thermal conductivity through the wall and greater heat transfer between the floor and ceiling.

Moisture also plays a part in wood’s thermal performance. Increasing moisture in wood increases thermal conductivity, so kiln-dried wood or fully aged and dried wood will allow less heat transfer than “green” or wetted wood. Keeping wood at the optimum moisture level is important to ensure performance goals are met and maintained over time.

When compared to steel or masonry, wood has a higher thermal resistivity, which improves energy-efficiency performance, especially when used in wall assemblies. Wood studs have nearly four times the thermal resistance of steel studs or masonry blocks. A nominal 2-by-4 stud has an R-value of 4.38, whereas a 3½-inch cold-rolled steel stud has an R-value of 1.34, and an 8-inch nominal concrete masonry unit (CMU) has an R-value of 1.11. The 2018 International Energy Conservation Code (IECC) recognizes this difference and requires continuous insulation on steel and CMU wall assemblies.5

Steel and masonry materials for thermal properties of wood

Steel and masonry materials are both energy intensive to produce and do not offer the same inherent thermal properties as wood.

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Originally published in Architectural Record

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