Log walls are typically made from trees that have been cut into logs that have not been milled into conventional lumber. Logs used for walls, roofs, and floor systems may be milled or laminated by the manufacturer or supplier to meet specific dimensions and fitting and finishing conditions.
Log homes are an alternative construction type used in some parts of California. Log home companies promote the aesthetic qualities of solid wood construction and can package the logs and deliver them directly to a building site. Some companies provide log wall, roof, and floor systems with special insulating channels or other techniques to minimize the effect of air infiltration between log members and to increase the thermal benefit of the logs.
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Log walls do not have framing members like conventional wood stud walls. Section 150.0(c)3 says that opaque nonframed assemblies need to have an overall maximum U-factor of 0.102, which is equivalent to a 2x4 R-13 wood-framed assembly. Per JA4 Table 4.3.11, any log wall 8 inches or more in diameter would meet this requirement, but less than 8 inches would not. |
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In prescriptive compliance, log walls must meet the same thermal requirements as other construction types. The prescriptive requirements for mass walls are less stringent than the criteria for wood-framed walls. Reduced insulation is allowed because the effects of the thermal mass (interior and exterior) can compensate for less insulation. Footnotes 5 and 6 to Table 150.1-A define the prescriptive mass wall as having heat capacity (HC) 7.0 Btu/°F-ft² or more, depending on whether the insulation is interior or exterior. |
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For performance compliance, consult the compliance software vendor’s documentation for any unique modeling requirements for mass walls using values from the Joint Reference Appendices. |
The thermal performance of log walls is shown in JA4 Table 4.3.11. The U-factor ranges from 0.132 for a 6-inch wall to 0.053 for a 16-inch wall. The U-factor of an 8-inch wall is 0.102, which complies with the mandatory U-factor requirements. U-factors for other log wall constructions (not shown in JA4) would have to be approved by the Energy Commission through the exceptional methods process.
Log walls have a heat capacity that exceeds conventional construction, as seen in JA4 Table 4.3.11 (Thermal Properties of Log Home Walls) (Table 3-15). The thermal mass effects of log home construction can be accounted for within the performance approach.
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U-Factor |
Heat Capacity (HC) |
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Log Diameter |
A | ||
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6” |
1 |
0.132 |
5.19 |
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8” |
2 |
0.102 |
6.92 |
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10” |
3 |
0.083 |
8.65 |
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12” |
4 |
0.070 |
10.37 |
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14” |
5 |
0.060 |
12.10 |
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16” |
6 |
0.053 |
13.83 |
Air Infiltration. Air infiltration between log walls can be considerably different among manufacturers depending upon the construction technique used. For compliance, infiltration is always assumed to be equivalent to a wood-frame building. The builder should consider using a blower door test to find and seal leaks through the exterior walls.
Straw bale construction is a building method that uses bales of straw (commonly wheat, rice, rye, and oat straw) as structural and insulating elements of the building. Straw bale construction is regulated within the CBC, and specific guidelines are established for moisture content, bale density, seismic bracing, weather protection, and other structural requirements.
The Energy Standards have determined specific thermal properties for straw bale walls and thermal mass benefits associated with this type of construction. The performance compliance approach can be used to model the heat capacity characteristics of straw bales.
Straw bales that are 22 inches by 16 inches are assumed to have a thermal resistance of R-30, when stacked so the walls are either 22 inches wide or 16 inches wide. The minimum density of load bearing walls is 7.0 lb/ft3, and this value or the actual density may be used for modeling straw bale walls in the performance approach. Specific heat is set to 0.32 Btu/lb-°F. Volumetric heat capacity (used in some computer programs) is calculated as density times specific heat. At a density of 7 lb/ft³, for example, the volumetric heat capacity of the straw bale is 2.24, and 6.34 Btu/ft³-°F for the entire wall assembly. See JA4 Table 4.3.12
The minimum dimension of the straw bales when placed in the walls must be 22 inches by 16 inches, and there are no restrictions on how the bales are stacked. Due to the higher resistance to heat flow across the grain of the straw, a bale laid on edge with a nominal 16-inch horizontal thickness has the same R-Value (R-30) as a bale laid flat.
The nature of straw bale construction provides an effective air barrier. For compliance, infiltration is assumed to be equivalent to framed walls.
Structural insulated panels (SIPS) are a nonframed advanced construction ‘system that consists of rigid foam insulation sandwiched between two sheets of board. The board can be sheet metal, plywood, cement, or oriented strand board (OSB), and the foam can be expanded polystyrene foam (EPS), extruded polystyrene foam (XPS) or polyurethane (PUR), or polyisocyanurate (ISO) foam.
SIPs combine several components of conventional building, such as studs and joists, insulation, vapor barrier, and air barrier. They can be used for many different applications, such as exterior walls, roofs, floors, and foundation systems. Little or no structural framing penetrates the insulation layer. Panels are typically manufactured at a factory and shipped to the job site in assemblies that can be as large as 8 ft by 24 ft.
Figure 3-53: Methods of Joining SIPS Panels
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SIPS U-Factors for Compliance |
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In the field, the SIPS panels are joined in one of three ways, as shown in Figure 3-53: 1. Single or double 2x splines 2. I-joists 3. With OSB splines. The choice of these options affects thermal performance and structural capacity. The 2x and I-joist spline types fit in a recess of the foam core, between the two layers of plywood or OSB. Joint Appendix JA4 Table 4.2.3 contains U-factors for roof/ceiling assemblies, JA4 Table 4.3.2 has U-factors for SIPS wall assemblies, and JA4 Table 4.4.3 has U-factors for SIPS floor constructions. U-factors used for compliance must be taken from these tables or by using Commission-approved performance compliance software. | |
Insulating concrete forms (ICFs) are a ‘system of formwork for concrete that stays in place as permanent building insulation and can be used for cast-in-place reinforced above- and below-grade concrete walls, floors, and roofs. They are interlocking modular units that can be dry-stacked (without mortar) and filled with concrete as a single concrete masonry unit (CMU). ICFs lock together externally and have internal metal or plastic ties to hold the outer layer(s) of insulation to create a concrete form and are manufactured from several materials, including expanded and extruded polystyrene foam, polyurethane foam, cement-bonded wood fiber, and cement-bonded polystyrene beads.
Three factors contribute to the energy efficiency of buildings using an ICF wall:
1. Continuous rigid insulation on both sides of a high-mass core
2. Elimination of thermal bridging from wood framing components
3. A high degree of airtightness inherent to this method of construction.
Climate zones with large daily temperature fluctuations have the greatest potential to benefit from the time lag and temperature dampening effects of these high-mass envelope systems. However, this combination of mass and insulation is beneficial in almost all climates, with the possible exception of mild coastal climate zones.
There are three basic types of ICFs:
1. Flat wall - A flat wall ICF results in a wall with a consistent and continuous thickness of concrete.
2. Waffle-grid - A waffle-grid ICF creates a concrete waffle pattern, an uninterrupted grid, with some concrete sections thicker than others.
3. Screen-grid - A screen-grid ICF consists of a discrete post-and-beam structure with the concrete completely encapsulated by the foam insulation, except at the intersection of posts and beams.
The insulating panels for all three ICF types are most commonly made from expanded polystyrene (EPS) and extruded polystyrene (XPS) rigid insulation boards. Insulating panels also are made from polyurethane (PUR), composites of cement and EPS, and composites of cement and shredded wood fiber, although these tend to be proprietary materials developed by the ICF manufacturer.
Plastic or metal cross-ties, consisting of two flanges and a web, separate the insulating panels and provide structural integrity during the pouring of concrete, resulting in uniform wall thickness. A variety of wall thicknesses can be obtained by changing the length of the web. The area of attachment of the cross-ties to the insulating form provides a secure connection surface located at standard spacing for mechanical attachment of finished materials to the interior and exterior of the wall. ICFs can be used to construct load-bearing and nonload-bearing walls and above- and below-grade walls, and can be designed to structurally perform in any seismic zone.
The ICF system is modular and stackable with interlocking edges. The materials can be delivered as preassembled blocks or as planks that require the flanges and web to be assembled during construction. The forms vary in height from 12” - 24” and are either 4’ or 8’ long. Vertical panels come in similar modules but are stacked vertically. ICF panels are typically available with core thickness ranging from 4” to 12”.
The thermal aspects of ICFs are represented in Joint Appendix JA4 Table 4.3.13.
