by Katie Daniel | March 5, 2015 11:21 am
By Michael DeLaura, LEED AP, and John Edgar
The road to energy independence is paved with conservation. In spite of new methods of producing ‘clean’ energy, nothing beats conservation as the most cost-effective solution. This is why recent changes to building codes—such as the new National Energy Code of Canada for Buildings (NECB)—have emphasized the requirements for airtight building envelopes and continuous insulation (ci).
In an effort to build a more robust building envelope, wall assemblies are being improved to meet the specific requirements of the local climate. Air barriers are becoming an integral part of the design in wall assemblies and have been re-engineered to meet practical installation considerations. Fluid-applied air barriers can be used over any substrate and behind any cladding over a range of surface geometries without the requirements for origami overlaps.
Some of the more advanced fluid-applied air and moisture barriers were initially designed to function as substrate protection of drainage exterior insulation and finishing systems (EIFS).
EIFS have always been rigorously tested for evaluation services—both the Canadian Construction Materials Centre (CCMC) and the International Code Council Evaluation Service (ICC-ES) have required performance testing from drainage efficiency to fire resistance. Recently, questions regarding fire performance of some air barrier materials in conjunction with continuous insulation have arose. EIFS already has years of tested experience and this performance advantage has allowed the fluid-applied membranes to find new markets behind other claddings.
As the demand has increased for a more energy-efficient building envelope, air and moisture barriers became the products for substrate protection. The use of one continuous air and moisture barrier simplified the specification and design process. Air and moisture barriers provide protection against unrestricted water vapour movement, water leakage, and the resultant mould and mildew. One major difference between building wraps, self-adhesive membranes, and a fluid-applied air barrier involves the performance. Some fluid-applied air and moisture barriers, depending on the type and manufacturer, can be exposed to the elements for up to six months without a significant change in water penetration resistance. In what may appear to be backward thinking, the more robust fluid-applied waterproof air barriers become the functional cladding of the building and exposed decorative cladding becomes the screen to deflect rain from the inner barrier, hence ‘rainscreen’ cladding.
Other benefits of fluid-applied air and moisture barriers include:
Air barriers also lower heating and cooling cost and increase occupant comfort, and research has been conducted to prove their energy efficiency. The U.S. Department of Energy (DOE) finds that up to 40 per cent of the energy used to heat or cool a building is lost through air leakage.
The National Institute of Standards and Technology’s (NIST’s) study, “Investigation of the Impact of Commercial Building Envelope Airtightness on HVAC Energy” confirmed air barriers promote energy savings ranging from 30 to 40 per cent for heating climates and 10 to 15 per cent for cooling climates. The use of an air barrier in the wall assembly can contribute to points on a Leadership in Energy and Environmental Design (LEED)-certified project in the Energy and Atmosphere (EA) category. Points can be awarded in this category for conducting air barrier and/or building envelope testing, including Optimize Energy Performance and Enhanced Commissioning.
Suggestions for specifying air barrier and sheathing membranes
The first step is the selection of the appropriate air barrier and sheathing membrane based on the wall design parameters and the environmental differences between the interior design loads and the local climatic conditions. Most designers are familiar with their local climate, but caution is given to those who design away from their local environment. Even local micro-climatic conditions can have a significant impact on the cladding assembly’s performance.
A determining factor in the selection process may involve conducting a performance analysis of the wall assembly under various climatic conditions. Simple steady-state conditions can be easily determined for climate extremes such as worst-case summer and winter conditions. More complex situations may require two-dimensional modelling which will define how well a wall dries given dynamic environmental conditions. Once the appropriate design conditions are determined, the product best meeting the requirements may be specified.
The next step following product selection is the installation’s specification and detailing. Performance is everything and research is a critical element for product selection. When the assembly’s performance requirements are unknown, how can a durable design be specified? If dynamic modelling of a wall assembly is conducted, the product manufacturer should provide the water vapour transmission characteristics including the range of vapour transmission characteristics. A simple vapour transmission number from ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials, is not enough. Wet-cup and dry-cup measurements can be different for the same product, and a range of vapour permeabilities under different humidity conditions are possible.
Products should be ‘characterized’ and the design professional should contact the manufacturer to review any special details prior to the project being sent out for bid. This can be accomplished by scheduling a meeting with the manufacturer or sending a copy of the plans and specifications to the manufacturer to allow for a plan review.
Continuous foam plastic insulation is not a new concept. The EIFS industry has pioneered it for the last 50 years, but the growing demand for improved building performance has transferred the requirement to all claddings. There are many durable insulation products suitable for use in an exterior wall assembly. As with air and moisture barriers, the characteristics of the selected assembly must be compatible with the wall design.
Questions to consider include:
These considerations are elements of a good wall design—casual mixing and matching of elements that often comes from the realities of bidding and ‘value-engineering’ can destroy the designer’s best intentions.
Insulation products are available in a wide variety of materials. Closed or open-cell configurations are available as expanded polystyrene (EPS) or extruded polystyrene (XPS) products with differing vapour permeability. The R-value of a 25-mm (1-in.) thick insulation ranges from 3.85 to 6.5, depending on the product’s chemical make-up and configuration. Recent studies have shown the value of continuous insulation installed outbound of the sheathing is 99 per cent effective as compared to 60 per cent or less for batt insulation installed in the stud cavity [source?]. In lay terms, a 50-mm (2-in.) insulation with an R-value of 8, has an effective R-value of 7.92 as compared to a R-11 batt insulation with an effective R-value of 6.6.
A whole building energy modelling study conducted by Medgar Marceau of Morrison Hershfield shows how much energy can be saved by upgrading standard building code requirements with added insulation and an air barrier assembly in the wall. The study compared the value of airtightness and increased insulation thermal value in a variety of North American locations. The modelling proves insulation with a barrier is more effective that just insulation alone, even with substantial increases in R-value.
Based on an assembly in Toronto, a building with an air barrier and 50-mm of continuous insulation will pay for itself in approximately three years. The baseline building for this energy modelling analysis is a three-storey brick office building with 33 per cent glazing. The building has 5016 m2 (54,000 sf) of floor space, and is built to conform to ASHRAE 90.1-2013, Energy Standard for Buildings Except Low-rise Residential Buildings. The study also shows the payback in the number of years based on the insulation thickness of 101 mm (4 in.), which can be as little as two years, based on the market. At some point, depending on the location, there is a maximum thickness used to produce optimal results.
The Morrison-Hershfield study, “Building Envelope Thermal Bridging Guide–Analysis, Applications, and Insights,” was published by BC Hydro and shows the real effect of thermal bridging in a wall envelope. Additionally, the BC Hydro Guide was developed by Morrison-Hershfield in collaboration with BC Hydro Power, Canadian Wood Council (CWC), Fortis BC, FP innovations, and Homeowner Protection Branch. The guide covers an evaluation of the cost-effectiveness in building types and the underappreciated impact of thermal bridging. While the study showed EIFS as one of the best performers as an insulated cladding, it still depends on proper interface detailing to eliminate thermal bridging.
The next step in the design process is the cladding selection to complete the wall assembly. The design professional has a wide range of cladding options such as brick, EIFS, stucco, precast, limestone, granite, vinyl siding, wood siding, or metal panels.
EIFS can be a logical choice for the cladding since the air/moisture barrier and continuous insulation are part of the same assembly. The assemblies also provide a wide range of options in surface geometry, colour, texture, and exotic finishes.
EIFS with 50 to 75 mm (2 to 3 in.) of EPS will meet all existing building code requirements for continuous insulation and air barriers—however, it can be installed with up to 150 mm (6 in.) of EPS in order to meet more stringent expectations for energy efficiency.
EIFS is one of the few claddings that has air/moisture barrier, insulation, and esthetics in one engineered system, installed by a single contractor, with single-source warranties available. Assemblies are tested for durability and energy efficiency; the material has more than 40 years of proven performance in North America as a continuous insulation wall cladding.
As indicated, EIFS offer specialty finishes, which can replicate brick, granite, limestone, metal panels, and precast. These finishes are easier to install and require fewer specialty trades than the traditional cladding material. Specialty finishes offer a cost-effective esthetic option, increase energy efficiency, and moisture resistance with less mass on the building exterior structure.
The combination of a fluid-applied air and moisture barrier and 50 mm (2 in.) of continuous insulation provide an energy-efficient method for meeting most current code requirements in North America for air barriers and continuous insulation. Independent studies have shown how this portion of the assembly reduces heating and cooling costs, increases occupant comfort, and may offer the option to resize the HVAC for additional savings. EIFS can be a logical cladding option since adding basecoat, mesh, and finish to the fluid-applied air barrier and continuous insulation is a cost-effective option.
Michael DeLaura, LEED AP, has been with Sto Corp., as an exterior cladding specialist since 1996. A 28-year veteran of the EIFS and coatings industry, he is an active member of the U.S. Green Building Council (USGBC), specifically the Middle Tennessee and Hampton Roads Chapters. DeLaura can be reached firstname.lastname@example.org.
John Edgar is president of John R. S. Edgar Consulting Inc., and chair of the EIFS Council of Canada. He has held positions including technical director at Sto Canada, member of the Standing Committee on Environmental Separation (part 5) of the National Building Code of Canada (NBC), and chair of Underwriters Laboratories of Canada (ULC) S716 Task Group for EIFS. Edgar can be reached at email@example.com.
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