July 12, 2018
By Andrea Yee
It is often said, the devil is in the details. Increasingly stringent energy requirements are resulting in a push towards higher performing roof, window, and wall systems; however, the interface detailing between these systems remains a weak link in actually achieving high, or sometimes even adequate, performance of envelope systems. When water leakage, air leakage, and thermal loss issues arise, they typically signal discontinuities in the control layers and, more specifically, at interfaces and tie-ins between systems. These are not new challenges, so why do transition details remain such a challenge, and how can this situation be improved?
A casual observer will note the design of buildings is becoming more innovative, moving away from traditional rectangular designs and favouring, instead, unique, curvier, and more complex geometric shapes. More sophisticated design, as a result, requires greater attention be paid during detailed design and construction. It can also, occasionally, involve the use of new materials to achieve durability and effectively control moisture, air, thermal, and structural loads.
In addition to increasingly complex geometry, there are always changes during construction and, despite best efforts, things rarely go as planned. Wall systems change due to cost or availability of systems or materials, geometry changes to accommodate new layouts, floor slabs and structure do not always get constructed exactly where they were expected. Many changes can affect envelope systems and create a butterfly effect without designers or contractors realizing it. Even when such changes are acknowledged, projects are typically pressed for prompt delivery and teams are forced to redesign quickly, on the fly. Occasionally, the effect of a change on the tie-in details is not factored into the redesign and contractors are left to develop their own solutions, which may or may not be consistent with the original design intent.
Regardless of the reasons, there are actions designers can take to focus attention on specific details and give the project the best chance for success.
Understand and identify the control layers
The goal of designers is to achieve continuous control layers (e.g. water, air, or thermal) throughout the building envelope. Understanding and identifying the location and function of various control layers are fundamental first steps toward achieving this goal.
Without understanding and identifying the layers, it is very difficult to ask builders to do the same.
Show how control layers connect for each system
Once the locations and functions of the control layers in each system are understood, designers must show how these layers connect. While it is impractical to show every tie-in detail and interface on a building, it is important to show sufficient detail so builders are not left bridging large design gaps. At a minimum, the head, sill, and jamb transition details for each system should be provided. Very often we find problems occur where insufficient or no detail was provided, either by the design or by the shop drawings.
Designers should put themselves in the builder’s shoes and try to find gaps in their own work. A good practice is to make a careful scan of the drawings, looking for atypical interfaces and identifying whether continuity of the control layers will need to be achieved in a different way from the rest of the building.
One effective strategy can be to develop an “air barrier plan”: a set of simplified drawings clearly showing the line of airtightness across the entire enclosure. Including this plan in the tendered documents can also assist the builder to understand the design intent.
Lastly, interfaces between multiple systems in multiple planes are very difficult to convey through two-dimensional details. Although not common practice, consider whether 3D details or even 3D printed models might be a more effective means of communication. Many designers are already accustomed to working with 3D software so it may be worth incorporating these workflows into the contract documents.
Make the design constructible and practical
A common challenge with many designs is constructability. Almost anything is achievable with the right amount of money; however, most building designs are bound by financial limitations requiring one to consider the cost implications of designs. Increasing awareness of common construction practices will allow more creativity in developing cost-effective and constructible solutions. During design, ask:
Simplifying details or making them easier to construct will increase the likelihood of success.
Selecting the right materials for transitions and tie-ins is also important for achieving the desired performance and durability. Self-adhered membranes, while commonly specified for transitions, are not always the best choice. Self-adhered membranes are often used because of their flexibility; however, this flexibility also makes them susceptible to wrinkles, which can result in breaches at laps and terminations. Transition materials also must accommodate differential movement and provide thermal continuity.
Further, self-adhered membrane materials typically are not designed to bridge large gaps; continuous, rigid support is required. Thus, in some instances, a different flexible material, sheet metal, or sealant may be a more appropriate selection. Although the initial material costs for some alternate solutions may be higher, the costs to repair leaks can often far exceed these material cost premiums. Designers should be willing to consider different solutions and be open to considering new product solutions as technology advances. At the same time, designers should ensure materials are compatible and be vigorous in vetting new products to ensure they can meet performance requirements and stand the test of time.
Once the design is complete, designers turn it over to the contractor’s hands to bring the vision to life, but this does not mean the designer’s job is done. Turning the design into a functional system typically occurs during the shop drawing submittal process. It is critical for the designer to review shop drawings, material samples, and other submittals with the same principles followed through design. Do the submittals demonstrate a clear understanding of all control layers? Do they correctly show control layers within their systems and their connections with the adjacent systems? Do they show the adjacent systems accurately and are they consistent with drawings/construction by other trades? Is the system constructible? Is the material selection consistent with the original design? Often the submittal review process can be an iterative one, but it is an important step to complete thoughtfully prior to starting construction.
Mockups can be a critical “first test” for any project and are an important tool for achieving success. Mockup requirements should be clearly communicated in the project specifications, including the location of the test(s), the frequency, and whether they are to be constructed in a laboratory, onsite, and/or in situ. Mockups can save lots of headaches down the road by confirming the design is being followed, providing an established benchmark for construction, and, if needed, validating performance through testing before advancing with construction production.
Ongoing and regular field reviews by a building enclosure specialist can significantly enhance the delivered product. Construction review includes checks that the design, approved shop drawings, and/or approved mockups are being followed. They also allow the project team to provide input to changes that may happen during installation. Contractors may also make changes to materials, believing them to be harmless to the design; however, these changes may have a significant effect on performance if not reviewed against the project requirements and design.
Full building testing
While many projects rely on dated laboratory test reports for building enclosure systems, field testing can be the final step in confirming delivered performance meets the design intent. Typical testing can include in-situ air and water infiltration, roof leak detection, and infrared scanning. Recent changes to regulations such as the Toronto Green Standard (TGS) have raised awareness of whole building testing techniques such as quantitative air leakage testing.
Project specifications often include target air leakage for individual enclosure systems and, occasionally, for the entire building; however, projects have only started to occasionally implement testing to confirm whether these targets are being met. Including testing requirements in the specifications, especially for building as a whole, signals to the construction team targets will be checked and diligent attention to the enclosure will be required through the entire project life cycle, from design to operation, to achieve air leakage and other performance requirements.
Of course, testing is not a substitute for good design or construction reviews since it is much more difficult to fix transition defects once the building is complete. At a minimum, full building testing can be a good final check before the building is turned over and before any errors in construction translate to future warranty or performance issues.
The trend towards higher performing buildings and envelope systems requires a re-doubling of effort towards achieving durable, constructible, and maintainable transition details. There are many ways for these details to fail. Fortunately, there are also many opportunities throughout the design and construction process to focus on getting them right.
Andrea Yee is senior project manager and technical lead at WSP. She has more than 10 years of experience in the building science industry. Her diverse background includes new construction, existing building restoration, building condition assessments, and capital planning. She also specializes in building envelope consulting and façade renewal at WSP. She can be reached via e-mail at firstname.lastname@example.org.
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