December 1, 2011
By Chris Norris, P.Eng, PE, LEED AP, Bryce Brandon, MBA, LEED GA, and Medgar Marceau, PE, ASHRAE, CSI, CDT, LCA CP, LEED AP
In both Canada and the United States, there is increasing awareness of the building envelope’s role in conserving energy. New energy codes and standards include prescriptive requirements for continuous insulation to minimize heat loss associated with thermal bridging.
South of the border, the 2009 International Energy Conservation Code (IECC) contains requirements for ‘continuous insulation’ installed on the wall’s exterior in six of eight climate zones. The Canadian approach in the upcoming edition of the Model National Energy Code of Canada for Buildings (MNECB), to be published in 2012, will stipulate an overall heat transfer coefficient (i.e. U-factor) based on the heating degree-days for a specific project location. Although continuous insulation will not be explicitly required, the advantages of using this strategy to meet U-factor requirements are myriad.
Requirements for air barrier systems have evolved differently in Canada and the United States. Canada has been a leader in the development of, and requirement for, these building components. The country’s focus has been on durability and meeting the twin National Building Code of Canada (NBC) objectives of health and safety. Air leaks can cause condensation and bulk water penetration in buildings, resulting in deterioration of building elements and compromising health and safety. Energy conservation has been the American focus for the requirement of airtight construction. In 2001, Massachusetts became the first state to incorporate a quantitative air barrier code requirement, with six others following suit. The reasoning in both countries is valid, while the approaches reflect the differences in national code development.
This article explores the findings of a study that evaluated the provision for ‘continuous insulation’ in wall design and the important effect played by airtight construction. The U.S. prescriptive requirement for ‘continuous insulation’ requires the exterior cladding be cantilevered out from the wall structure by the thickness of the insulation. This cantilevering of loads can present structural challenges resulting in increased structural costs. It can also be particularly challenging for retrofit projects where the original walls were not designed to accommodate these additional loads and where the structural supports themselves constitute considerable thermal bridges.
For the study, the relative benefits of exterior insulation and finish systems (EIFS) were evaluated—comparing performance based on American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, compared to earlier versions as a baseline for evaluating the advantages of retrofit and new construction.
In CAN/ULC S716.1-11, Standard for Exterior Insulation and Finish Systems (EIFS)–Materials and Systems, Underwriters Laboratories of Canada defines the assembly as a:
non-load-bearing wall cladding system comprised of rigid thermal insulation board, an adhesive for attachment of the thermal insulation board to the substrate or water resistive barrier system, a glass fibre reinforcing mesh embedded in a base coat on the face of the thermal insulation board, and a finish coat.
The new standard applies only to EIFS “used in combination with a drained air space and liquid-applied water-resistive barrier (LA-WRB), as an exterior wall cladding system.” EIFS has been used effectively as an insulated cladding in Europe and North America for more than 30 years.
Energy modelling objectives
The objective of a recent energy modelling project was to evaluate the relative benefits of two continuous insulation EIFS systems—one with an air barrier, one without—for new construction and energy retrofit applications.
A prototype medium three-storey office building was modelled for three climates: Dallas, Texas, Seattle, Wash., and Toronto. Various scenarios for retrofitting existing buildings and for design upgrades for new construction were considered. The baseline case buildings were modelled as meeting the minimum requirements of ASHRAE 90.1-2004 for existing buildings and ASHRAE 90.1-2007 for new buildings. (Some of the prescriptive requirements in Canada’s MNECB are based on ASHRAE 90.1.) A baseline air leakage rate of 7.9 L/s•m2 (1.55 cfm/sf) of above-grade envelope surface normalized to 1.57 psf (75 Pa) was used. (For more information, see the 2009 ASHRAE Handbook of Fundamentals).
Existing building baseline and retrofit
The baseline existing building was:
– Seattle–0.39; and
Two retrofit cases were considered for each climate zone. For the first, a 50-mm (2-in.) thick barrier EIFS assembly, without an air barrier, was added to the baseline model without changing the building air leakage rate. For the other case, a 50-mm EIFS assembly with an air barrier was included, with the air leakage rate reduced to 2.0 L/s•m2 (0.4 cfm/sf) at 75 Pa (1.57 psf) pressure difference normalized to 4 Pa (0.08 psf). See Figure 1.
New building baseline and energy design upgrade
The baseline existing building was:
-Dallas–3.41 W/m2 K (0.60 Btu/h•sf•deg F);
-Seattle–2.84 W/m2 K (0.5 Btu/h•sf•deg F); and
-Toronto–2.56 W/m2 K (0.45 Btu/h•sf•deg F); and
The energy design upgrade cases replace brick with the air barrier/EIFS combination (Figure 2). In upgraded cases, the air leakage rate was reduced to 2.0 L/s•m2 (0.4 cfm/sf) or 1.3 L/s•m2 (0.25 cfm/sf) depending on the scenario. The thickness of the exterior EIFS was varied from prescriptive minimum up to a maximum of 250 mm (10 in.).
The prototypes used for the modelling were U.S. Department of Energy (DOE) commercial reference buildings models (Version 1.3), which employ the ‘design flow rate’ model for air leakage rates. (See M. Deru et al’s U.S. Department of Energy Commercial Reference Building Models of the National Building Stock, published by the DOE’s Energy Efficiency and Renewable Energy’s (EERE’s) Office of Building Technologies). Air infiltration is assumed in perimeter zones only; it is reduced to 25 per cent of full value when the ventilation system is running.
This modelling does not account for stack effect—the flow of air resulting from warm air rising, which creates positive (i.e. outward) pressure at the top of a building and negative (i.e. inward) pressure at the bottom. Had stack effect been included, a slightly higher air leakage rate would be expected.
While air leakage is typically specified and measured based on a differential pressure of 75 Pa (1.57 psf) across the building enclosure, the actual pressure under normal operating conditions is substantially lower. For this reason, the air leakage rates used in the energy models are normalized to a pressure of 4 Pa (0.08 psf) following DOE guidelines. (The procedure is described in K. Gowri, D. Winiarski, and R. Jarnagin’s Infiltration Modeling Guidelines for Commercial Building Energy Analysis [DOE Pacific Northwest National Laboratory, 2009]).
Taking a base air leakage rate of 2 L/s•m2 (0.4 cfm/sf) of above-grade wall area at 75 Pa (1.57 psf) and normalizing this to an air leakage rate at 4 Pa results in an air leakage rate of 0.3 L/s•m2 (0.0595 cfm/sf) of exterior surface area. The air leakage rates for the 1.3 L/s•m2 (0.25 cfm/sf), and 7.9 L/s•m2 (1.55 cfm/sf) at 75 Pa scenarios were then scaled from this base case.
Heating, cooling, lighting, and interior equipment energy consumption were modelled for each load case at 10-minute intervals, with the results summarized on a monthly usage basis in units of kilowatt hours (kWh). This data was then used to calculate heating and cooling energy consumption on an annualized basis; energy costs, energy savings over the base case, and carbon equivalent were calculated based on the annual heating and cooling costs.
Energy model results
The energy modelling results for Toronto are presented graphically in Figures 3 to 5. It is clear substantial energy savings can be realized through a combined air barrier/EIFS assembly. Annualized heating and cooling energy savings ranged from approximately 20 to 45 per cent with greater savings achieved in colder climates (in this case, Toronto).
In all climate zones, reducing air leakage was found to have a much greater impact on energy savings than adding continuous insulation alone. Again, greater savings were achieved in the colder climates.
The energy savings achieved was upward of 450 to 950 per cent greater than for adding insulation alone without reducing air leakage.
The following annualized heating and cooling energy savings were realized by adding a 50-mm (2-in.) EIFS assembly with air leakage controlled to 1.3 L/s•m2 @ 75 Pa (0.25 cfm/sf @ 1.57 psf) air barrier relative to a baseline ASHRAE 90.1-2007 new building:
The following annualized heating and cooling energy savings were realized by retrofitting a baseline ASHRAE 90.1-2004 building with 51 mm of EIFS and a 2 L/s•m2 @ 75 Pa (0.4 cfm/sf @ 1.57 psf)
Ultimately, there is a diminishing return in energy cost savings as R-value is increased.
The modelling demonstrates EIFS and air barriers are effective in reducing energy consumption both for new construction and for retrofit applications. Although durability was not considered within this study, the drainability of such systems, along with the hygrothermal properties, also improve the long-term success of the wall assembly. The beneficial hygrothermal properties of EIFS systems have been demonstrated in prior studies by Oak Ridge National Laboratories (ORNL).
Chris Norris, P.Eng, PE, LEED AP, is a principal and building envelope specialist with Morrison Hershfield. He leads new construction and retrofit building enclosure projects across the country. Norris has 12 years of experience in the building envelope field. He can be contacted at email@example.com.
Bryce Brandon, MBA, LEED GA, is the senior market manager for the exterior insulation and finish systems (EIFS) and coatings product lines in North America at Sto Corp. He is responsible for identifying new business opportunities and leading project teams to achieve business objectives. Brandon can be reached at firstname.lastname@example.org.
Medgar Marceau, PE, ASHRAE, CSI, CDT, LCA CP, LEED AP, is a building science consultant with Morrison Hershfield. For 12 years, he has been consulting in the areas of building science, sustainability, and environmental lifecycle assessment (LCA). Marceau specializes in high-performance building enclosure design and construction, whole-building energy simulation, and LCA. He can be contacted via e-mail at email@example.com.
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