WHY PROBLEMS OCCUR
The air tightness, continuity, structural integrity and durability of the air barrier system are dependent upon three factors; materials, design and installation practice. Flaws in any of these elements can have negative ramifications on the ability of the completed system to perform to specification in the short and/or long run.
Materials
When specifying air barrier materials, the designer must confirm that the material or materials chosen have an air permeance rating equal to or less than 0.02 L/(s·m2) measured at an air pressure difference of 75 Pa. Many materials may meet this requirement, but care must be taken to ensure that the material will maintain its air permeance rating (and not have any adverse effect upon the system’s ability to meet the other three requirements of continuity, structural integrity and durability) once it has been installed in the wall. For instance, two-part materials that are fabricated on site, such as some spray-applied materials, may be rendered ineffective if not mixed correctly. All relevant information regarding the material, including air permeance, fabrication instructions and material characteristics, can be found in the technical literature as supplied by the manufacturer.
Most commonly specified air barrier membrane materials demonstrate similar air and vapour permeance characteristics (in reference to their scope of use on a building). However, other performance characteristics, such as adhesion, elongation, puncture resistance and tensile strength may vary considerably and must be taken into consideration when specifying materials, especially when used around roof/wall junctions, wall/window junctions and control joints where movement is expected. The variance may be enough to compromise the ability of the system to function correctly. As an example, the elongation of regularly specified self-adhered air barrier membranes can range from 4% to 200%. Where movement between system components is expected, materials with greater elongation properties should be selected.
The installed materials must not react adversely to either other materials that comprise the air barrier system, or adjoining components within the building envelope. While it is beyond the scope of this paper to document every potential incompatibility, the designer must be aware that incompatibilities can occur, and should carefully consider the physical and chemical properties of the materials being specified. Physical incompatibilities occur when the physical characteristics of different materials make them incompatible. A common example is where a hot-applied material is installed over heat-sensitive material. For instance, if torch-grade membrane is installed over self-adhered or spray-applied membrane, the excessive heat may cause the self-adhered or spray-applied membrane to melt (this may also occur if hot mopped asphalt is used around the roof/wall junction). However, specifications often allow for different trades to select between a range of acceptable materials, and a situation may occur where one trade has selected self-adhered membrane and a second trade chosen torch-grade.
The general contractor should monitor the work of the sub-trades and identify any concerns regarding material compatibility or sequencing to the designer, who should be aware of the materials being used on the project. Chemical incompatibilities occur when the chemical properties of different materials make them incompatible. Consider substrate preparation. If walls are not primed properly and in keeping with manufacturers’ recommendations, or the incorrect primer is used, not only may the membrane not bond adequately to the substrate, but the chemical composition of the primer may damage the membrane itself. In fact, the chemical compositions of certain membranes may make it impractical to use them concurrently on a wall section. The chemical composition of asphalt membranes is such that it will cause certain rubber membrane to decompose. Similar results may be attained when a membrane of a particular makeup comes in contact with high solvent-based sealants or uncured solvent-based primers.
The Canadian Construction Materials Center (CCMC) has published technical guides that detail specific structural, durability and air leakage test criteria for air barrier materials and systems. Air barrier materials can be tested both as stand-alone materials (tested for air permeance) and as part of a system (tested for air permeance, structural integrity and durability). For optimum results, all system materials should be evaluated under this protocol. However, while the results of evaluations like this can be used as a reference to provide assurance of the material’s ability to perform as part of a system, the evaluations do not pre-approve the system. It is the responsibility of the designer and installer to bring the individual materials together as an effective system.
Design
Meeting specifications does not necessarily guarantee that the air barrier system will perform well. An incorrectly designed system will not function effectively regardless of how well it has been installed. It is not uncommon for an air barrier system failure to be attributed to a flaw in design. Common examples are improperly locating the air barrier within the wall; discontinuity within the system (for instance, gaps in the system at major joints, such as roof/wall, wall/foundation, and window and door frames to wall junctions); sequencing of structural, mechanical and electrical systems which may make air barrier continuity impossible to achieve, and; failure to differentiate between air barriers, vapor barriers and/or materials that act as both.
In cold or severely cold climates2, where a material is to act both as an air barrier and a vapour barrier, it should be placed on the warm side (or high-vapour pressure side) of the wall3. It should be placed at a sufficient depth within the building envelope so dew point temperature occurs to its exterior side. Where air barrier and vapour barrier functions are to be performed by different materials, the vapour barrier should be placed on the warm side of the wall. Again, it should be placed so dew point temperature occurs to its exterior side. In this instance, the air barrier may be placed anywhere within the wall provided it restricts the flow or movement of conditioned air, preventing this air from coming in contact with cool surfaces where temperature is below dew point.
If the air barrier is placed outside the insulation plane, the air barrier material must have a vapour permeance characteristic, or the system be designed, such that water vapour will diffuse to the exterior of the building envelope, or a vapour barrier of lesser permeance is used on the inside. In comparing warm and cold climates, the ‘science’ behind where the vapour barrier is placed within the wall does not change ?? it is always placed on the warm side of the wall. However, in warm climates, because the warm side of the wall will be closer to the exterior than in areas of cold climates, the vapour barrier will be placed closer to the exterior as well (and may even form part of the exterior wall).
In most instances, to best meet the requirement of durability, the air barrier should be placed within the exterior cladding and outward of the structural frame. This not only protects the air barrier from exterior environmental conditions, but by keeping the structural frame of the building within the air barrier, the system design is more straightforward in terms of maintaining continuity at penetrations associated with structural elements. Reviewed by Moishe Alexander the CEO of Canadian Funding Corporation