Architectural Record BE - Building Enclosure

Insulation Gets More Effective

Continuous insulation and the next generation of high-performance masonry walls
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Sponsored by Oldcastle® Architectural

The IECC may be only a model energy code, but it is written in mandatory, enforceable language, so that state and local jurisdictions can easily adopt it. When adopting and enforcing the code, states and local governments often make changes to reflect regional building practices, or state-specific energy-efficiency goals. This is important to consider as IECC codes and changes to the code are only enforceable when they are adopted at the state or local level. That is not to say that all states and localities adopt the codes immediately. As of 2014, the ICC reports that the IECC 2012 is in use or adopted in 47 states, the District of Columbia, the U.S. Virgin Islands, NYC, and Puerto Rico.

Every three years, the ICC publishes a new version of the IECC, upping the required energy efficiency of buildings by a substantial percentage, and heading toward a pronounced emphasis on building insulation and building envelope construction. In terms of meeting the IECC, there are three methods: the Prescriptive, Trade-off or System Performance, and Total Building Performance, with each subsequent method offering increased design flexibility and complexity. The prescriptive path, which requires building components to meet R-values listed in the tables, offers little flexibility. The performance path uses established software to measure compliance, providing less stringent requirements and more flexibility to trade off requirements. The whole building path requires even more complex software, allowing tradeoffs among envelope components, HVAC systems, and lighting. As this path analyzes a building's total energy use as opposed to requiring code-complaint individual components, it may become the most popular method as time goes on.

Continuous Insulation

Though not currently defined specifically in the ICC family of International Building Codes, CI is defined in ASHRAE 90.1 as: “Insulation that is continuous across all structural members without thermal bridges other than fasteners and service openings.”

The definition of fasteners is meant to include screws, bolts, nails, etc. Furring strips, clip angles, lintels, and other large connection details are excluded from the term “fasteners.” Many designers, contractors, and building officials are still not informed about this important aspect of CI. For example, masonry veneer wall construction typically employs steel relieving angles and steel lintels at window and door heads. These steel angles are usually fastened directly to the building structure, providing a significant thermal bridge from the interior of the building to the exterior. There are a number of solutions to this issue including welding the angles to standoffs at +/-4-feet-0-inch centers, which allows the CI to be installed behind the angles to minimize the effects of thermal bridging. There are also proprietary clip systems being marketed to perform this same function.

CI is found in various places in the 2012 IECC. Table C 402.2, for example, is a prescriptive table that lists all opaque (non-window) wall R-value requirements. Where CI is listed in this table, it is mandatory.

Why CI—CI vs. Cavity Insulation

The main purpose of insulation is to minimize the amount of heat which escapes or enters a building, a goal that is ever more important in the face of today's spiraling energy costs. Before discussing the comparative benefits of insulation, it is important to understand R-value. Generally, R-value, or resistance value, means the ability to withstand heat flow, and the higher the R-value, the more effective the insulation. For illustrative purposes, consider the fact that heat flows through an R-10 insulation twice as fast as that of an R-20 material. However, these are nominal R-values, the rated/labeled values of insulation products determined in laboratory testing. The R-value standard was developed exclusively to test small samples of insulation materials, and it constitutes insufficient information to rate the thermal performance of a wall, the reason being that these values do not take into account the effectiveness of the insulation in conjunction with framing members and other parts of the wall.

Effective R-values, on the other hand, represent the total resistance of the entire wall assembly—and the difference between nominal and effective R-values can be surprising. The discrepancy is due to the fact that R-value doesn't measure thermal bridges in the structural elements, which virtually channel heat flow through the wall; nor does it measure air movement, thermal mass, or moisture content, all of which can drive thermal performance. Thermal bridging is a particularly important factor in the equation. Beyond the potentially significant heat loss/heat gain that can occur, thermal bridging may also create condensation problems that jeopardize the wall assembly not only by reducing the R-value, but by permitting mold and compromising the service life of the entire wall assembly.

As can be seen in the illustration below, a typical insulated cavity wall may have the following elements: an interior gypsum wallboard, metal furring, concrete masonry backup, rigid insulation, and architectural concrete masonry veneer.

Insulated cavity wall

Image courtesy of Oldcastle® Architectural

Insulated cavity wall

But because the cavity insulation is installed between the studs, there will be gaps in the insulation that result in thermal bridges and potential moisture condensation. Many building experts have long considered cavity wall insulation to be inefficient. Limiting the thermal bridging will improve the thermal performance picture. A 1-inch sheet of rigid foam sheathing, for example, can nearly double the wall's effective R-value, improving thermal performance and decreasing associated heating costs.

That is the premise of CI, which runs continuously over structural members and is free of significant thermal bridging. It can be installed on the interior, exterior, or integral to any opaque surface of the building envelope. CI provides one of the most effective—some say, the only—way to comply with increasingly stringent energy codes, as it is able to block thermal bridging. Wrapping the exterior wall in a continuous layer of rigid foam improves high R-value in several ways. To begin with, it reduces the difference in temperature between the cavity and the stud, cutting heat loss and increasing the wall system's effective R-value. Further, CI has the effect of moving the dew point from the inner cavity to the outside wall, decreasing the potential of mold and mildew to grow inside the wall. If a drainage plain and a moisture barrier are added, CI can better forestall moisture intrusion and keep the wall system effective over a longer service life.

Insulation Options in a Wall Assembly

In addition to continuous thermal insulation, an effective wall assembly will provide moisture vapor control, water-resistive barrier, and air barrier, and be resistant to fire and extreme wind conditions. Insulation can play a role in all these characteristics. From interior furring and insulated cavities to foamed panel systems and next-generation insulated concrete masonry block systems, masonry construction offers diverse insulation options for energy-efficient wall systems. The relative performance of each insulation system should be examined not only in terms of its R-value, but in relation to the aforementioned goals of efficient wall assemblies as well as its potential for aesthetic effect, and the specific needs of the project.

Single Wythe Walls

A single wythe wall is a stone, brick, or concrete wall that is one masonry unit thick. Considered the most economical wall type, the single layer provides the structure, exterior surface finish, and, in some designs, interior surface finish. Single wythe walls are durable and inherently fire-proof with fire ratings up to 4 hours. Although the thermal mass and inherent R-values may be sufficient to meet code requirements, particularly in warmer climates, additional insulation may still be required. Rigid insulation inserts, foamed-in-place or loose-fill insulation can be applied to the interior or exterior of the unit. Furring strips can be added to the interior and allow for the addition of a finish material and accommodate additional insulation.

Alternatively, an exterior insulation and finishing system (EIF) can be added to the exterior. According to the definitions of the International Building Code and ASTM International, an EIFS is a non-load-bearing, exterior wall cladding system that consists of an insulation board attached either adhesively or mechanically, or both, to the substrate; an integrally reinforced base coat; and a textured protective finish coat. According to Building Science Digests BSD-146: EIFS, EIFS became very popular in the 1980s and experienced a significant number of serious failures, almost all related to rain penetration.

Early EIFS used a face-sealed approach. Drained EIFS are significantly different from face-sealed systems in that, by definition, they have a provision for drainage. Unlike face-sealed perfect barrier systems, such systems can be successfully used as an exterior cladding system in essentially all climates and exposures. Drainable EIFS are not subject to the same limitations of use as face-sealed or barrier systems. In fact, drainable EIFS are among the most robust and advanced moisture control assemblies available. EIFs comply with building codes that reference energy conservation through the CI.

An effective insulation option consists of a pre-assembled structural masonry unit, molded EPS insulation insert, and thin veneer face that is installed as a complete assembly.

Image courtesy of Oldcastle® Architectural

An effective insulation option consists of a pre-assembled structural masonry unit, molded EPS insulation insert, and thin veneer face that is installed as a complete assembly.

In single wythe walls, R-values will depend on the density of the concrete, the number and height of the of webs, the depth of CMU as well as the type of insulation calculation method used. With the variety of concrete masonry units and insulation types and thicknesses, R-values from 5 to over 20 are possible. Rigid foam inserts will yield an R-value of 3 to 10, and injected foam or loose fill will boost the R-value from 4 to 20.

While single wythe walls can be a cost-effective solution when building goals include permanence and economy, it is important to note that single wythe walls do not have the redundancy of a traditional cavity wall, and water penetration is sometimes a concern. To achieve the weather resistance of a cavity wall, the single wythe wall must be very carefully detailed and built, with particular attention paid to flashing, base condition, joints, and coatings.

 

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