What type of CI works best?
Several kinds of insulation, sometimes in combination with other building materials, can be used to achieve the desired CI performance. One of those is sprayed polyurethane foam (SPF). Four kinds of rigid or semi-rigid products are more commonly employed (noted with their R-value per inch):
- mineral fiber, R-4.0;
- expanded polystyrene (EPS), R-4.0;
- XPS, R-5.0; and
- foil-faced polyisocyanurate (polyiso), R-6.5 per inch.
In high-performance buildings that have better insulation, 100 mm (4 in.) or more of these rigid materials might be incorporated into the enclosure. However, a benefit of walls with CI is the elimination of thermal bridging enables them to be thinner than equivalently insulated walls without CI. This is good news, as the 2012 IECC calls for R-20 or R-13+5 for Climate Zones 3, 4, and 5; for Climate Zones 6, 7, and 8, the minimum is R-20+5 or R-15+10.
The insulation types noted above can meet these goals, and all are relatively inexpensive building materials. They also offer good long-term thermal resistance and the ability to reduce operating costs.
Mineral fiber
Also called ‘mineral-wool’ and ‘stone-wool’ insulation, mineral fiber is noncombustible and fire-resistant due toits high melting temperature, allowing fire ratings of one to two hours. It is resistant to water and moisture, which helps it retain R-value when wet. Chemically inert, this UV-stable insulation will not rot, cause corrosion, or support microbiological growth.
Most products made with mineral fiber for building applications are from natural and recycled feedstocks, and do not require fluorocarbons in manufacture. In typical applications, mineral wool allows for draining water and absorbing sound for acoustical properties in the envelope.
For nonresidential projects, this insulation is a medium-density or high-density semi-rigid board; it can be foil-faced, and works in cavity wall and rainscreen applications. Its fire and moisture performance make mineral fiber a good choice for wet cavity walls, as well as metal cladding systems or open-joint rainscreens.
Polyisocyanurate
Polyiso has recorded the highest R-value per inch compared to other rigid-foam-board insulation materials, according to Polyisocyanurate Insulation Manufacturers Association (PIMA); its R-value increases with board thickness. However, recent testing has shown losses of R-value over time, in cold temperatures, or both.
Some polyiso products have a higher level of inherent fire resistance, and the material has been used in various assemblies passing the following fire tests:
- ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials;
- NFPA 285; and
- NFPA 286/UL 1715, Standard Methods of Fire Tests for Evaluating Contribution of Wall and Ceiling Interior Finish to Room Fire Growth.
Polyiso’s foam core is moisture-resistant with some water absorption potential, and the boards are stable and compatible with most construction sealants and adhesives. A facer is applied in manufacturing, and may be used as a moisture drainage plane.
Foil-faced polyiso insulation is commonly used in masonry and rainscreen cavity walls, where its high R-value per inch tends to reduce the cavity depth needed. Polyiso experiences some change in R-value in cold weather, which may be considered when calculating CI performance.
Expanded polystyrene
Known for high R-value per unit cost, EPS is cost-effective, dimensionally stable, and commonly used for ground contact and below-grade uses, as it does not retain water. Faced boards also function as vapor retardant, though, when used as sheathing, EPS should be laminated or used with an air-and moisture-barrier layer. A versatile rigid insulation, EPS is useful in foundation applications and is typical in EIFS façades and integrated assemblies such as insulated concrete forms (ICFs) and structural insulated panels (SIPs).
Extruded polystyrene
The use of XPS takes advantage of its closed-cell structure and water resistance, offering a cost-effective choice for improved R-value. It is also recyclable—another benefit for green building projects. Specifiers often use plastic-faced versions, which can serve as vapor retarders. Like EPS, XPS is combustible, and its performance is affected over time by UV light, according to Slone. XPS may also absorb more moisture over time than other insulation boards.
For these three reasons, XPS is generally unsuitable for open-joint applications such as metal panels, terra cotta, or high-pressure laminates. Instead, XPS works well for barrier walls and closed-joint rainscreen, as well as most cavity-wall drainage systems.
Conclusion
As shown, there are benefits and drawbacks to each insulating material. For an EIFS project, EPS is ideally compatible. For brick veneer, the higher R-values of polyiso and XPS allow for thinner wall sections, meaning smaller shelf angles and lintels that help reduce thermal bridging.
While rigid-foam insulation boards have varying performance capacities, they all have excellent R-values per unit cost. All the materials noted can meet the core goals of a continuous insulation layer, and all are relatively inexpensive. Looking at first costs alone, insulation is a valuable performance element. In a CMU masonry wall with an installed cost of $370/m2 ($34.30/sf), the CI layer accounts for only about seven percent of that total. For a steel-frame assembly, the first cost is just over eight percent. By comparing their functional characteristics, specifiers can determine which insulation works best for a given CI application.
The material choice is one step in the complex yet critical process of wrapping the building in a blanket of continuous insulation. By adding to this process such variables as cladding choice, fire-safety rules, and building operations and life-cycle needs, the specifier team can make the best choice possible to meet the highest building-efficiency standards.
Brian Chang is director of product and channel management for Sto Corp. in Atlanta, where he is responsible for overseeing and leading the product management team of the company’s overall product lines to include profitability, market segmentation, positioning, and new product development. Chang comes to Sto from LG Hausys (LG’s Building Materials Division), where he served as senior product manager, responsible for their composite countertop division. He holds an MBA from Georgia State University. Chang can be reached via e-mail by contacting bchang@stocorp.com.
