Specifying polyiso for continuous insulation in walls

ATL Wall 0005
All images courtesy Polyisocyanurate Insulation Manufacturers Association

By Jared O. Blum
Architects, specifiers, and building owners are striving to advance the way commercial and residential building envelopes are developed, in response to more stringent policies for energy conservation. Continuous insulation (ci) is prominently featured as a solution because it is an effective means of addressing these challenges.

In American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1-2010, Energy Standard for Buildings Except Low-rise Residential Buildings, the ci concept is defined as:

insulation that is continuous across all structural members without thermal bridges other than fasteners and service openings. It is installed on the interior, exterior, or is integral to any opaque surface of the building envelope.

This approach is not new to insulation—it has been commonly employed for many years on various types of low-slope roofing systems. However, use of truly continuous insulation within building walls has lagged behind its energy-saving potential.

The situation is changing through the emphasis of higher-performing wall assemblies. This article focuses on the application of ci to building walls. Like any construction material, ci must be properly specified to ensure its intended performance and appropriate use. In this regard, this article addresses five topics to consider when specifying ci for walls:

  • function and versatility;
  • materials;
  • modern energy code requirements;
  • building code requirements; and
  • construction detailing.
Polyiso Figure 1
Light frame and mass wall systems with continuous insulation (ci) for code-compliant residential and commercial building construction.

Function and versatility
As shown in Figure 1, ci can be used with various wall structural systems and cladding materials, such as:

  • cement board;
  • portland cement stucco;
  • wood lap;
  • brick veneer;
  • stone; and
  • vinyl siding.

In all these applications, its primary function is to provide code-compliant or better energy conservation performance. Additionally, properly qualified and installed ci products can serve other important functions for exterior wall assemblies, including air barriers and water-resistive barriers (WRBs). When laminated to structural materials, ci can even provide structural functions such as wall bracing. It is important to refer to the insulation manufacturer’s data for code-approved capabilities.

Polyiso and continuous insulation
Various foam plastic insulating sheathing materials and other types of products are available to address ci applications on walls. The most common foam plastic insulating sheathing materials include expanded polystyrene (EPS), extruded polystyrene (XPS), and polyisocyanurate (polyiso) foam. Each product type has different thermal properties (affecting the required thickness), costs, and capabilities, as shown in Figure 2.

Regarding polyiso, it is a closed-cell, rigid foam board insulation used primarily on the roofs and walls of offices, healthcare facilities, warehouses, retail, and industrial manufacturing facilities and educational institutions. One of North America’s most widely-used and cost-effective insulation products, it offers excellent fire performance. As roof insulation, it meets Factory Mutual (FM) 4450, Approval Standard for Class 1 Insulated Steel Roof Decks, and Underwriters Laboratories (UL) 1256, Fire Test of Roof Deck Constructions. As a wall component, it meets ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials.

Polyiso Figure 2
Examples of R-value per inch for common types of continuous insulation (foam sheathing). Refer to manufacturer data for specific R-values.

As Figure 2 illustrates, polyisocyanurate continuous insulation has at least a 20 to 80 per cent greater R-value per inch than other common types of continuous insulation. When compared to fiberglass insulation blown into walls, polyiso’s R-value is 87 per cent higher and when compared to XPS, polyiso’s thermal performance is 20 per cent higher. This means more energy savings and/or more manageable wall thicknesses. With thinner, energy-efficient walls using polyiso continuous insulation, there will be more usable floor area within the footprint of the building. Also, cladding materials are more easily installed.

Modern energy code requirements
Continuous insulation provides one of the most thermally efficient ways of complying with modern energy codes. It mitigates avoidable heat loss due to thermal bridging in walls that are not continuously insulated. Building codes require structures to meet certain R-values to achieve a specific level of efficiency. The climate zone plays a big role in determining what the minimum R-value has to be for a specific region.

According to ASHRAE 90.1’s Table B1-2, Canada is in Climate Zones 6, 7, and 8, with some example solutions using continuous insulation illustrated in Figure 3. The country’s National Energy Code for Buildings (NECB) defines climate zones differently in increments of 1000 heating degree days (18 C [64 F] basis). For this and other reasons, solutions may vary for a given project location depending on the climate zone and which code is used.

Building code requirements
When using continuous insulation to meet or exceed the applicable energy code, other matters of building code compliance should also be considered.

Water-resistive barrier
Many ci products can be used as a WRB behind cladding—providing water resistance and thermal performance in one product. It is important to refer to the manufacturer’s installation instructions and code-compliance data. Alternatively, water-resistive barriers can be separately applied to walls with ci.

Wind pressure resistance
For code compliance guidance on wind pressure resistance of foam sheathing materials, one should refer to the manufacturer’s installation instructions and design data. In some applications, wind pressure resistance is only a matter of temporary construction concern because the product is encompassed or restrained by other materials designed to resist wind pressure.

In other cases, the foam sheathing material may be required to resist wind loads. For example, in their 2015 editions, the U.S. model building codes now reference American National Standards Institute/Structural Building Components Association (ANSI/SBCA) FS 100, Standard Requirements for Wind Pressure Resistance of Foam Plastic Insulating Sheathing. Depending on the foam sheathing type and thickness used, its wind pressure capability may actually exceed the capacity of the supporting framing. Thus, wind pressure resistance is a matter that must apply to all components in any given wall assembly for a complete solution.

Cladding (siding) attachment
Various proprietary and standard fasteners and connection strategies can be used for attachment and support of cladding materials when installed over ci. Several standardized solutions for attaching siding and furring over foam sheathing up to 102

Polyiso_Figure 3

mm (4 in.) thick have been added to the 2015 editions of the U.S. model building codes. The design professional and cladding installer should consult the cladding manufacturer’s installation requirements to co-ordinate requirements. For example, minimum siding fastener size and penetration into framing should be maintained; longer fasteners may be required.

Fire performance
Foam plastics are held to a comprehensive set of fire-performance requirements that, in some cases, exceed those applied to other common construction materials. The requirements include various types of fire tests and criteria to address flame spread, smoke development, and ignition protection.

Foamed plastics used as part of a non-load bearing exterior wall must comply with the full scale fire test Underwriters Laboratories of Canada (CAN/ULC) S134, Standard Method of Fire Test of Exterior Wall Assemblies. Load-bearing walls using foam plastics must comply with CAN/ULC S102, Standard Method of Test for Surface Burning Characteristics of Building Materials and Assemblies.

Moisture vapour retarders
It is important to ensure ci is specified with moisture vapour retarders in such a way vapour is properly managed. Diffusion is managed by control of vapour permeance and surface temperatures of the material layers comprising an assembly. For example, the National Building Code of Canada (NBC) has provisions in Part 9, Section 9.25.5 addressing the use of low-perm materials on the exterior side of a wall assembly by providing insulation ratios to control surface temperatures in combination with the use of a Class I (vapour-impermeable) or Class II (vapour-semi-impermeable) interior vapour retarder.

As with any building assembly, a hygrothermal analysis may be performed to justify alternative designs and address special conditions for moisture vapour control. For example, a design using a ‘smart vapour retarder’ may provide an appropriate level of moisture diffusion control while also improving drying potential. Finally, adequate control over indoor relative humidity (RH) and minimization of air-leakage by use of a continuous air barrier system is important to a completely integrated and successful approach to moisture vapour control for any wall assembly.

Construction detailing
It is important to provide workable and complete construction details for walls with ci to ensure a constructible and functional assembly relating to many of the previously discussed topics. Construction details to consider include:

  • envelope component attachments;
  • integration of flashing and WRB;
  • integration of furring (if used) around wall penetrations and flashing;
  • attachment of cladding to wall framing through ci or to furring;
  • details for cladding attachments through ci at inside and outside corners; and
  • installation detailing per NFPA 285 tested assembly when required.

Useful detailing concepts can be found from various sources online. For proprietary cladding or exterior wallcovering systems that include continuous insulation, the specific manufacturer’s installation details and instructions should be consulted.

JBlumFour.5.16.12Jared O. Blum is the president of the Polyisocyanurate Insulation Manufacturers Association (PIMA), which is the North American trade association representing manufacturers of polyiso foam insulation. He can be reached via e-mail at joblum@pima.org.

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One comment on “Specifying polyiso for continuous insulation in walls”

  1. My dad is making plans to insulate our basement and asked me to help him out. I’ve never done insulation before, but want to learn more about it so that I can be a good help. This gave me some great insights on it and I can’t wait to help my dad out this weekend.

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