Leveraging Vinyl Siding and Foam Sheathing for Optimal Performance and Code Compliance
By Jay H. Crandell, P.E., ARES Consulting, www.aresconsulting.biz & ABTG, www.appliedbuildingtech.com
What do foam sheathing and vinyl siding have in common? More than you might expect. First, they are both from the plastics family of building products that provide durable resistance to the forces of nature and time, including the effects of moisture. When both are used together in accordance with the latest code requirements and industry guidelines, they result in cost-competitive, high-performance, and code-compliant exterior wall coverings that offer many benefits such as durability, aesthetics, and energy efficiency with low up-front embodied carbon impacts and a high return on operational carbon emission savings. You can further explore the multiple benefits of these two complimentary wall covering products here and here.
The purpose of this article, however, is not to discuss benefits. Instead, it has a more practical goal to highlight some key resources to help with code compliance when considering combined applications of vinyl siding (including insulated vinyl siding) and foam plastic insulating sheathing (FPIS) products on wood frame walls. Thus, the focus is on compliance with the relevant building code and energy code requirements for construction of wood frame walls for residential and commercial buildings.
First, we’ll address the International Energy Conservation Code (IECC). Continuous insulation (ci) is a means of compliance included in the code and provides wall performance benefits in any climate zone by creating a continuous layer of insulation over wall framing to reduce the impacts of thermal bridging and enhance the protection of walls and the building interior from the exterior environment. For home construction, the 2021 IECC residential building provisions (and also Chapter 11 of the 2021 International Residential Code (IRC)) prescribe the use of exterior continuous insulation on wood frame walls in Climate Zones 4 through 8 (roughly the northern half of the United States). Typically, an R-5 or R-10 layer of continuous insulation is used for 2x6 or 2x4 walls, respectively. The code also provides prescriptive options to use only ci (no cavity insulation) in all climate zones, ranging from R-10ci in the southern climate zones to R-20ci in the far north.
For wood frame walls in accordance with the 2021 IECC commercial building provisions, the ci R-value options are similar and range from R-3.8ci (on 2x4 walls) in the southern climates up to R-18.8ci in the coldest climates. Depending on the type of insulation material used, the ci thickness may range from ½” to as much as 4 inches. In general, as ci R-value increases, the thermal and moisture performance achieved also increases. It is important to note that the energy code ci requirements can be achieved by a combination of regular vinyl siding and FPIS and also by the use of insulated vinyl siding and FPIS. Insulated vinyl siding R-values typically range from R-2 to R-3.5 and require supplemental use of foam sheathing to achieve the prescribed ci R-values, especially in northern climates. This also applies when using ci to control water vapor as addressed in the building code.
Second, the exterior wall covering assembly using foam sheathing and vinyl siding also must comply with several functional or performance requirements of the 2021 editions of the IRC or the International Building Code (IBC) depending on the type of building project. These include control of water vapor, control of rain water, resistance to wind loads, and cladding installation practices which are addressed below. For matters related to fire safety, the reader is referred to this for vinyl siding and here for foam sheathing products.
Control of Water Vapor – The 2021 IRC Section R702.7 and IBC Section 1404.3 address use of water vapor retarders and ci to control water vapor. Using ci on the exterior side of a wall assembly provides reliable temperature control for protection against condensation and moisture accumulation. This must be coordinated with the use of ci to comply with the energy code as addressed earlier. Following the code and this guide, simply specify an appropriate R-value of FPIS ci (alone or in combination with insulated vinyl siding) based on the climate and select an appropriate interior vapor retarder (or even eliminate it), for maximum inward drying potential and thermal performance. Refer to this wall calculator for greater flexibility in optimizing and coordinating energy code compliance and building code water vapor control compliance. Find more information on the use of FPIS to reliably control water vapor here.
Control of Rain Water – Vinyl siding is a type of rain-screen or vented cladding which provides effective rainwater deflection and drainage when backed with a code-required water-resistive barrier (WRB) in accordance with 2021 IRC Section R703.2 and IBC Section 1403.2. FPIS ci can be used as a durable WRB system behind vinyl siding to eliminate the need for and cost of a separate WRB material. Code-approved FPIS WRB systems are listed here. They are required to pass stringent performance testing to ensure protection a wall assembly from the exterior environment. Coupled with vinyl siding, FPIS WRB systems provide robust protection against water intrusion and entrapped water as a drained rainscreen system. Find more information on water-resistive barrier applications of FPIS here. More information on use of vinyl siding as a rainscreen can be found here. In addition, guidance for flashing and integration with fenestration is addressed here for a complete and continuous system for water and thermal resistance.
Resistance to Wind Load and Related Installation Requirements – All siding must resist wind load and the same goes for sheathing materials like FPIS. Exterior wall covering assemblies specifying vinyl siding over foam sheathing are specifically addressed in Section R703.11.2 of the IRC (and similar guidance will be included in the 2024 edition of the IBC). Basically, the vinyl siding design wind pressure rating must be properly specified and both the vinyl siding and foam sheathing must be installed in accordance with the manufacturer’s installation instructions. For a step-by-step summary of these building code and installation requirements, refer to this quick guide. In addition, an installation video of a representative exterior wall covering assembly can be accessed here.
With the above resources and support of the latest U.S. model building and energy codes, you should consider how to best leverage the aesthetic and functional benefits of using vinyl siding together with foam sheathing. There are a multitude of possible code-compliant solutions for combining these products on wood frame walls for optimal performance and cost.
(Images provided by American Chemistry Council’s Foam Sheathing Committee).
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Mr. Crandell has over 35 years of experience in construction, engineering and innovative building technology research for private sector, government and university clients to help address significant structural, energy and building science challenges. His work has been widely published and used to improve codes, standards and practices for design and construction of buildings. For additional information visit www.aresconsulting.biz
