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The layers of material that make up a wall assembly have different air permeability.
The chart below provides a comparison of typical materials used in wall assemblies and their air permeability values.
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Energy codes in the United States have begun to require air tightness of the building envelope, but they are not specific about levels of air permeability for air barrier materials. The generally accepted level based on National Building Code of Canada requirements is 0.02 L/(s·m2) at 75 Pa pressure [0.004 cfm/ft2) at 1.57 psf)]. While many common building materials like plywood and gypsum wallboard meet this standard, a sheathed wall assembly will not perform well as an air barrier unless the joints are treated with an air barrier material. The sheathed wall assembly with treated joints then becomes an air barrier sub-system of the total building envelope air barrier system. The total building envelope air barrier system consists of all the interconnected air barrier materials—for example, wall sheathing with joint treatment, roof membrane, foundation waterproofing, windows, and doors, and the air barrier connection materials between them.
Air Barrier Continuity
The overall design concept of air barriers in building construction is the creation of a continuous airtight membrane around the building envelope. Therefore air barrier materials in wall assemblies, to be effective, must be continuous.
Breaks in air barrier continuity cause air leaks. In cold climates the breaks can allow significant amounts of warm moisture laden air to escape from the interior environment and condense on a cold surface in the wall assembly. Conversely, in hot humid climates, breaks in the air barrier permit moisture laden air from the exterior environment to infiltrate the building envelope and potentially condense on a cold surface in the wall assembly. Any penetration through the wall assembly or termination of the wall assembly must therefore be detailed to maintain the continuity of the building envelope air barrier materials to create an air barrier system. Air barrier system performance is compromised by any discontinuity of the air barrier materials that make up the air barrier system. The design/construction professional must take material compatibility and construction sequencing into account when designing an airtight assembly to ensure continuity. A number of connecting air barrier materials exist that are compatible with Sto Guard® fluid applied air/moisture barrier to make transitions from one material to the next. For example, rubberized asphalt membrane tapes can be used to connect from wall sheathing to foundation, or low expanding urethane foam sprays can be used between windows and rough openings.
Air Barrier Structural Integrity
Structural integrity of air barriers is important because wind loads are transferred to the most airtight components in a wall assembly, the air barrier materials, and in turn, are transferred to the structure. Negative and positive wind loads stress air barrier materials. If an air barrier material tears or displaces with loading its effectiveness as an air barrier is diminished. Some building wraps have low air permeability, but they do not perform well as commonly installed, not only because they have many seams that reduce their effectiveness against air leakage, but they are non-structural. If the seams in building wraps are not taped they do not perform well as air barrier materials. Because building wraps are non–structural they are susceptible to displacement and tearing with negative wind gusts in cavity wall construction. This compromises their performance in service.
Sto Guard® is a fully adhered air/moisture barrier. Adhesion to sheathing exceeds the strength of the sheathing. Tensile adhesion tests show that the paper or glass mat in gypsum based sheathings fails, while unfaced sheathings like plywood show adhesive failure at loads in excess of 344 kPa (50 psi, could equate to more than a 2560 km/hr [1600 mph] wind speed). The structural strength of Sto Guard® fluid applied air/moisture barrier in effect equates to that of the sheathing. Deformation while in service is limited to the deformation of the sheathing. This means no tears and no compromise in performance caused by structural loading, provided the sheathing and supporting frame are adequate to resist loads.
Air Barrier Durability
While capable of resisting wind loads without compromise in performance, air barrier materials must also demonstrate durability in a number of other ways, particularly if the air barrier is concealed and inaccessible for maintenance. Durability criteria include:
Resistance to puncture
Resistance to pests—rodents, termites, carpenter ants, and other insects
Resistance to low but sustained negative pressures from building stack effect and HVAC fan effect
Ability to withstand stress from thermal and moisture movement of building materials, and stress from building creep
Resistance to UV degradation (during the construction period)
Resistance to mold growth
Resistance to abrasion
Sto Guard® does not provide a food source for insects or other pests. By virtue of its excellent adhesion to sheathing and to prepared concrete or masonry substrates, it is resistant to puncture and it resists loads imposed by stack effect and fan effect, as well as wind loads. Its resistance to stresses imposed by thermal and moisture movement, and building creep, is achieved by the ability of Sto Gold Fill® joint treatment to span gaps in sheathing. Similarly, the UV resistance, resistance to mold growth, and abrasion resistance is a function of the physical and material properties of Sto Gold Fill® and Sto Gold Coat®. |
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