Technical Series 96-235
Air leakage through a building envelope can seriously affect its service life. Yet many air barriers do not perform as effectively as they should or could. Unfortunately, the technology of design for an air barrier system and its details is not developed well enough to allow designers to predict their performance with respect to air permeability and air pressure loads. Better information is needed by all involved in building delivery, from the owner through to the eventual user. A methodology must be developed to encourage designers and builders to advance air barrier system design and construction from an art to a science.
It has been suggested that "commissioning" the building envelope could improve its performance. Commissioning is normally associated with mechanical and electrical systems in a building. It is the process of verifying the performance of a completed system to determine if it complies with the design documents and the specified performance ratings. To apply commissioning concepts to an air barrier system, the performance requirements of the installed system need to be established and methods need to be developed for evaluating the design to ensure that, if constructed as designed, the intended performance will be achieved. CMHC through various research and development projects, has made significant advances towards this objective. The results of that research have been used in this project in formulating the basis of a procedure to commission the air barrier system.
The steps required to design, construct and commission the air barrier system of the building envelope are outlined in Figure 1.
Figure 1. Steps in the Design, Construction and Commisioning
of an Air Barrier System
The project brief is a document developed by the owner's team that contains the character, the attributes and the constraints governing the design and construction of a new building project. On most projects, the brief should contain information on the type of project (e.g., office, residential) and the interior and exterior design conditions (e.g., temperature, humidity, wind loads). The project brief must also identify the control functions to be certified and commissioned (e.g., air leakage, water leakage, sound attenuation) and must set realistic performance objectives for key components, assemblies and systems, and for the building as a whole. Durability and maintenance expectations should also be stated.
The design process for the building envelope proceeds in the normal manner. A conceptual structural system and method of construction are resolved for the building, followed by the development of systems for the roof, walls and windows. It is during this stage that consideration is given to the air barrier system. In order to improve the air barrier design process to allow for certification of air barrier performance during construction and final commissioning, the air barrier design must be validated for basic characteristics of air impermeability, continuity, structural capacity and durability. If a contractor is to be held responsible for constructing an assembly of materials in a manner that provides a specified level of airtightness, it is necessary for the designer to provide proof that the design, if constructed as per the tender documents, can achieve the required performance. An audit process must be conducted at the conclusion of the design stage which determines that the air leakage rates of all the elements proposed will, in combination, fall within the overall building specifications for total air leakage.
For most construction projects, the air barrier system is described in drawings and specifications. The sections and details show the materials and their respective position within each part of the envelope. The type of materials, the preparatory work and installation procedures are presented in the specifications (Section 07195). Quality compliance test options will be provided in the specifications. Depending on the scale of the project, the contractor may be required to follow progressively more stringent options to demonstrate compliance with the performance requirements, from an air leakage test of the completed air barrier system, to tests of individual areas as they are constructed, to testing of on-site mock-ups prior to construction; if the air leakage rates and structural attributes comply with the performance requirements, the construction of the envelope may proceed so long as construction of the air barrier is identical to the mock-ups. The specifications may even go so far as to require a series of site briefings during construction to explain the design objectives and testing requirements. Such field compliance tests are generally conducted by an independent company and paid for by the building owner. Repeat testing resulting from failure to meet performance requirements are paid for by the contractor.
As construction proceeds on major projects, it is generally cost effective to certify performance of the air barrier in stages to ensure the as-built performance meets the design requirements. As the building nears completion, the overall performance of the air barrier system may be tested. If the building envelope does not pass the final test, a specialist is retained, at the contractor's expense, to investigate and identify the locations of extraneous leakage for repair. Following successful testing, the builder will be provided a certificate of compliance for the air barrier system. The builder should also receive a commissioning certificate that certifies the design, construction and performance of the air barrier system in its entirety, including materials and workmanship, were tested and found to perform within the envelope performance limits prescribed by the owner's project brief.
To ensure that the air barrier performs its functions adequately over time, a program of monitoring and testing may ensure durability. Monitoring can be done indirectly, by visual observations of symptoms related to air leakage (e.g., efflorescence or staining). The monitoring can also be direct, by instrumenting the wall to measure air pressure differences, and temperature and humidity in cavities with respect to outdoor and indoor conditions. If an area of wall is suspected of increased air leakage, an air leakage test can be conducted.
A building operating manual is important in ensuring the correct maintenance and repair procedures are followed. It should identify the air barrier materials, their location within each assembly, and the means by which they are linked together to provide a continuous building envelope plane. The building operating manual should also describe the intended operating limits for the air barrier. A change in the operating conditions, such as increased operating pressure or increased humidity, may cause symptoms that indicate excessive air leakage. In such a case, the design should be considered before embarking upon repairs.
A process for "commissioning" the building envelope has been outlined. The commissioning involves extra steps not normally required in the design and construction process and would therefore involve extra costs, for specialist consulting fees, additional documentation, and commissioning testing. However, the extra steps (and associated costs) advocated in the commissioning process are justified by more durable building envelopes that contribute to increased energy efficiency and comfort, and reduced life cycle costs.
Project Manager: Jacques Rousseau
Research Report: Commissioning and Monitoring the Building Envelope for Air Leakage
Research Consultant: Morrison Hershfield Limited
A
full report on this research project is available
from the Canadian Housing Information Centre.
The information in this publication represents the latest knowledge available to CMHC at the time of publication, and has been thoroughly reviewed by experts in the housing field. CMHC, however, assumes no liability for any damage, injury, expense or loss that may result from use of this information.
©1999 CMHC-SCHL. All rights reserved.