Wider Cavity Walls: Becoming the Norm Today

Words: Building envelopes are continually changing and especially when it comes to masonry walls. Remember the days of four wythe thick solid brick walls, or the days when there was no continuous insulation out in the cavity? For most of commercial construction today, those days are long gone… kinda like the pager! What is driving the changes? The desire for a more energy efficient structure through energy codes that have become much stricter over the last 10 years, and don’t expect that to change anytime soon! Masonry veneers were typically used with masonry back-up walls which in many people’s opinion is still today the most sustainable and energy efficient system that you can construct based on its thermal mass. A number of years ago along comes wood and metal stud back-up walls with the idea of a quicker and easier wall to build thus decreasing the cost. However, recent studies have shown that masonry walls are quite cost competitive with their wood/metal stud counterparts, not only from an initial cost of construction but even more so when you factor in energy savings and even fire safety! With the ASHRAE90.1 Energy Code being the drive behind more energy efficient exterior envelopes for ALL structures, the use of continuous insulation is being required in most parts of the country today. And it seems like with each passing energy code upgrade the R-Value requirement for continuous insulation increases thereby requiring thicker insulations to meet the more demanding codes. This thicker continuous insulation requirement has essentially “pushed” the masonry veneer further away from the back-up structure. As our walls continue to get wider, the concerns about wire reinforcement and masonry veneer anchors have come into focus. TMS-402/602 code has stated that the maximum total cavity space (dimension from face of back-up wall to backside of veneer) had a maximum dimension of 4 ½”. Due to the increased use of larger cavity walls with thicker amounts of continuous insulation that code is transitioning to 6 5/8”, but PLEASE read the fine print because there are a number of stipulations in the new 6 5/8” code language that need to be considered and understood to fully meet the new code requirement. If the total cavity width is larger than the code requirement, then the reinforcement/anchoring system is to be engineered specifically for the project to include engineering calculations at an additional cost. This engineering cost responsibility needs to be clearly stated in specifications and/or drawings, and contractors need to pay special attention when they see this requirement. There are a number of factors that go into the engineering calculations including wind loads, locations of project and height of structure to name a few. One of the biggest positive factors for masonry veneers is their almost limitless design options, and an option that has once again become popular is corbelling of the veneer. In most cases this involves “stepping out” of the veneer but in some cases it may involve an in and out placing of the veneer material to create a look of depth and/or shadowing lines for aesthetic appeal. “Stepping in” can cause issues with the minimum amount of air space allowed by code, and can also create a spot where mortar dropping can accumulate potentially blocking the airspace and trapping moisture? However, the more common detail is for the brick to corbel out (as shown in drawing….) and in many cases these areas can exceed the cavity code maximum and often is overlooked. [caption id="attachment_19202" align="alignnone" width="1201"] Corbeling of brick creating extremely wide cavity[/caption] Even though, these areas can be smaller sections, they still must be reviewed and possibly involved engineered anchoring systems since many of these areas where corbelling occur tend to be at the top of wall sections, and are a very vulnerable area of a wall assembly. But corbelling can also commonly occur at the outside corners of a structure or around window/door openings, both can be vulnerable areas of the building and once again, extra consideration must be given to both design and construction of these areas. [caption id="attachment_19201" align="alignnone" width="2400"] Wide cavity with 5” of continuous insulation[/caption] Another factor that has exploded recently has been the issue of thermal transfer, especially with regard to metal stud back-up walls. Typically, metal stud walls included some type of insulation in between the studs with the primary product being batt insulation. As the insulating values of those walls were studied, it was determined that the “actual calculated R-Value” of the batt insulation was less than half of the printed R-Value for the product itself. This discrepancy came from the large thermal bridges that each metal stud created. Thus, today we are seeing a growing trend of not insulating in between the metal studs due to the insulating value losses, however this forces even more continuous insulation outboard to meet the energy codes making our cavities even larger. The other advantage to eliminating the batt insulation in between the studs is that your condensation or “dew” point now clearly is out in the cavity, which creates a much cleaner design with regards to air/vapor barriers. Wider cavities also mean that shelf/relieving angles are becoming larger thus making the structural steel thicker which can present numerous other issues. The most obvious issue is the potential thermal transfer of having a large piece of structural steel bolted or welded to your structure that can carry large amounts of hot and cold from the exterior portion of the building to the interior. Thermally efficient shelf angles have now become increasingly popular where the angle is offset from the structure using some type of attachment system so that exterior insulation can slide behind the actual shelf angle creating much less thermal bridging plus maintaining a reasonably sized angle and easily keep the typical 3/8” mortar joint size. Thermal shelf angles that have both vertical and horizontal adjustability can make the job of the mason contractor much easier in the field (see enclosure). Thermal modeling for these shelf angles is currently ongoing to determine exactly how much improvement they will provide. A simple calculation comparison of contact area up against your structure shows that the thermally efficient shelf angles create much less than 10% of contact area versus a standard shelf angle that is welded or bolted directly against the structure. The thermal transfer issue has even worked its way to the masonry veneer anchors themselves, and studies / testing (see enclosure) has shown that with potentially thousands of penetrations created by the anchors that enough thermal transfer can take place that they type and style of the anchor should be considered when selections are made attaching to metal stud wall assemblies. There can considerable difference between various types of anchors considering the type and number of penetrations, the size of the penetration, and the type of metal used to manufacture the anchor. For example, stainless steel anchors in general conduct much less thermal transfer than a similar anchor made of carbon steel. [caption id="attachment_19200" align="alignnone" width="1276"] Independent computer thermal modeling comparing standard anchors versus thermally efficient anchors.[/caption] The energy codes will continue to dictate that wider cavities are here to stay and architects, engineers, contractors, code officials and manufacturers must be aware of the impacts of wider wall sections with large amounts of insulation. This process will involve a learning curve for all parties involved including architects, engineers, contractor and product manufacturers. These issues are a part of today’s conversation regarding the building envelope, and they will continue to drive new product development as well as creating design and construction challenges as well. As our cavities get wider and more challenging, product selection, design/detailing and constructability will become even more critical.
Words: Chris Bupp Photos: Hohmann & Barnard
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