A comprehensive scrutiny of building enclosure design and construction

by sadia_badhon | January 21, 2021 6:17 pm

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By Andrea Mucciarone, BSS

The building enclosure is the ‘skin’ of a structure and protects occupants from the exterior environment. The driving factors of enclosure design have evolved through the decades. Today, they include reducing carbon footprint, meeting greenhouse gas (GHG) emission targets, climate resiliency, and minimizing building energy use (aiming for net zero). Thus, there is a pressing need to design and construct thermally efficient, air- and water-tight, and durable building enclosures.

Although there is eagerness to address the factors listed above with new and innovative materials and construction techniques, building professionals may be putting the cart before the horse. Instead, it is better to focus on improving and implementing fundamental building envelope principles in the design, preconstruction, and build stages.

Design phase

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Implementation of fundamental building science principles in the design phase is critical to the successful performance of the facility (Figure 1). It is highly recommended to engage a building envelope specialist at the schematic design stage to achieve the highest opportunity for influence on the project with the least impact on overall cost.

At this phase of the project, the design team/building envelope consultant is able to discuss and validate high-level ideas to establish buy-in with the client and help set realistic expectations. These meetings should determine building envelope objectives including the general plan for the control layers (i.e. air, water, thermal, and vapour barriers), R/U-value targets for the assemblies, the planned maintenance life cycles, and the building’s designed lifespan in accordance with the Canadian Standards Association (CSA) S478, Durability in Buildings, be relevant in these meetings.

Shop drawings

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An enhanced review of building enclosure shop drawings is highly recommended, as they are critical documents. They can impact the overall performance of a building if insufficient and incorrect detailing is unaddressed at the review stage. For building envelope elements, such as light/medium-weight cladding on thermally broken framing or windows/curtain wall, it is recommended shop drawings be designed, reviewed, and stamped by a professional engineer for air/water performance along with structural. It is recommended shop drawings contain a comprehensive list of all materials being used for the project, and all components on the details are labelled and dimensioned.

Shop drawings should include and be checked for close co-ordination of adjacent trades so critical connections/interrelationships are thoroughly detailed, as these create most of the discontinuity failures on a building. For example, a roof shop drawing detail at a curtain wall curb may only show the roof membrane wrapping the curb and the curtain wall installed on top. However, the curtain wall subcontractor may be installing a membrane transition between the curtain wall and the granulated roof membrane. As such, in most cases, the roof membrane will need to be degranulated (i.e. heated and granules embedded in the bitumen) for the transition membrane to properly adhere to the roof membrane (Figure 2). If this transition is included in both trades’ shop drawings, it is less likely to be missed at the site where it will lead to discontinuity in the air and moisture barrier.

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Reviewing details

The drawings should be reviewed (in conjunction with the specifications) in the schematic design, design development, and construction documentation phases. The evaluation would focus on determining if a sufficient number of details have been provided for most/all building envelope transitions, as missing or poor detailing can spiral down to misleading pricing, incorrect shop drawings, and improper execution at the project site (Figure 3). This culminates in poorly performing buildings with an increased risk of leaks and maintenance costs.

Building envelope transitions between assemblies should be assessed for continuity of fundamental control layers to ensure durable detailing of the building enclosure is achieved. These control layers, in order of importance, should something fail, are:

• water;
• air;
• thermal (insulation); and
• vapour.

Generally, discontinuity in the water control layers have an immediate impact or cause the most deterioration, followed by air, and then thermal and vapour issues, which may be tolerable to a degree, with potentially a lesser expected level of deterioration.

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Achieving continuous control layers is a challenge in both the design and construction stages, as water and air can enter buildings in multiple ways and concerns surrounding thermal bridging are still prevalent, all of which lead to failures and occupant discomfort.

Durable detailing

Durable detailing in the contract documents is the first step in constructing a low-maintenance, durable, and energy-efficient building. Figure 4 illustrates potential constructability and durability concerns, which can arise during the design phase. While the intent in this design is to provide a well-insulated precast assembly, with membrane transitions to adjacent windows, it would be infeasible to install spray foam to provide continuity of the thermal barrier due to the location of the transition (behind a column) and shape of the concrete. Additionally, direct membrane transitions to typical window-wall framing are not durable due to the profile of the frames. An alternate approach such as a combination of membrane, metal flashings, and backer/rod and sealant would maintain air barrier continuity from the back of the window frame to the precast concrete.

Window location

Something as simple as window location can significantly impact a building envelope’s thermal performance and reduce the overall performance of an insulating window or wall assembly by 25 to 50 per cent. If the thermal plane of the insulating glass unit [IGU] does not line up with the thermal control layer in the surrounding wall, thermal flanking around the perimeter of the transition reduces the effective thermal performance of both assemblies. The greater the offset, the greater the impact. Figure 5 illustrates the thermal flanking phenomena by the sharp changes in direction of the temperature isotherms at the transition. In this example, the energy model assumed windows would have an effective U-value of 0.357 (~R-2.8), and spray foam insulated precast concrete walls U-value of 0.038 (~R-26). However, due to the offset in thermal planes, the modelled U/R-values were U-value 0.562 and 0.071 (R-1.78 and R-14) respectively, a roughly 35 and 46 per cent reduction. If such discrepancies between assumed and calculated U/R-values are not identified in the design stage, the as-built performance of the constructed facility will not meet the planned energy use targets.

Isometric details

As building materials and transitions become more complex, the importance of thorough detailing is more critical. Contract document and shop drawing details should provide as much information as possible with respect to materials and continuity of control layers—comprehensive detailing equals fewer unknowns during construction for contractors, leading to lesser impromptu transition details. Where two-dimensional detailing may be insufficient to fully illustrate continuity in contract document, isometric/three-dimensional detailing is recommended (Figure 6). Areas/details where isometric drawings are recommended are:

• intersection points of three different planes, such as a terrace parapet crossing with a curtain wall;
• continuity of expansion joints between different assemblies and planes like roof-to-wall-to-foundation; and
• where complex drainage/venting paths are required, such as skylight transitions to curtain walls.

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Reviewing specifications

During the design phase, the specifications are recommended to be reviewed by the building envelope specialist in conjunction with the drawings. The evaluation would include:

• durability of specified materials and if they have a demonstrated history of effectiveness;
• potential compatibility/adhesion concerns between different materials (e.g. pressure preservative treated wood reaction to metals, bituminous materials reaction to some plastics);
• requirements for project-specific thermal analysis (effective U/R-value calculations) of window and wall assemblies and roofs;
• applicable performance standards (e.g. for window performance);
• shop drawing requirements and specific information needed for each work result; and
• inclusion of quality assurance and control (QA/C) measures such as site air and water testing.

The author recommends including a separate Division 1 building envelope QA/C section to highlight to the contractors the required level of site reviews and testing.

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Test reports

Manufacturer and contractor test reports, such as window performance testing or thermal calculations (i.e. U/R-value calculations and condensation resistance), are recommended to be evaluated in detail by the design team/building envelope specialist to verify the testing/modelling is in conformance with the performance requirements of the specifications and associated specified standards, as this could impact the in-situ performance of the building enclosure and overall energy efficiency of the facility.

Preconstruction phase

The preconstruction phase of a project is where the design team/building envelope consultant reviews the contractor’s understanding of the design intent and fundamental building science principles. This is completed through shop drawing and other submittal reviews and pre-installation meetings.

Submittal reviews

Building enclosure submittals typically include product data sheets, manufacturer installation instructions, and applicable test reports. Data sheets often include product limitations that should be reviewed along with the planned time of installation and potential issues with transitions to other materials or assemblies.

Similar to the design phase, isometric details in shop drawings can be very useful to illustrate complex relationships before they are installed. They can help identify potential concerns, critical sequencing, and provide a clearer picture for the contractor and all trades on how to execute the work. In Figure 7, the detail illustrates the sequencing of installation of a curtain wall stack (movement) joint at a custom vertical fin, and shows the continuity of the control layers. Similarly, Figure 8 illustrates a curtain wall with an extended guardrail supported by internal aluminum posts penetrating the top of the curtain wall. Continuity of both and air and water barriers is critical at these penetrations, and these isometric details help both the contractor and specialist better understand how this detail should be installed, reducing the potential for error, such as an in-situ discontinuity, and future leaks.

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Pre-installation meetings

Pre-installation meetings are often ignored or overlooked. These meeting are recommended to include the general contractor (GC), architect, building envelope specialist, site foreperson, and trades. These meetings allow the contractor to explain how he or she intends to execute the work and discuss critical installation procedures, sequences, or compatibility concerns with the design team. It provides the opportunity for feedback and collaboration between the design team and contractor, and to get a thorough understanding from the trades so everyone is working toward the same goal—a durable, well-performing enclosure. It can also be an opportunity for the contractor to get acceptance from consultants on how they plan to execute the work and any unique installation procedures they may be employing.

Construction phase

The construction phase of the project is where the all the above is executed. To achieve the contract documents’ design intent, the following QA/C activities should be implemented:

• building envelope assembly mockups;
• enhanced building envelope site review;
• in-situ performance testing; and
• onsite quality control (QC) including co-ordination between trades, checklists, and a dedicated building envelope superintendent.

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Mockups

Mockups are set out in the technical specifications, but they get missed or executed after full-scale installation has already commenced, which is a lost opportunity. Mockups help establish the quality of work and review installation procedures before full-scale installation begins. In this way, one can reduce the potential for systemic deficiencies that could lead to poor performing assemblies or rework. Often the simple mistakes are the costliest.

Figure 9 is an example of an insulated metal panel (IMP) system that was installed without a mockup. Consultants were also not notified when work commenced. Upon review, it was noted the interior air seal sealant had not been installed or installed incorrectly, and the contractor had to remove and reinstall 30 per cent of the overall panels to rectify the issue. A one-hour mockup could have saved both the consultant and contractor time and headaches.

Mockups also provide the opportunity to work-out complex details, which may otherwise be concealed and potentially lead to significant discontinuities in the building enclosure. The goal of mockups is to reduce the potential for control layer discontinuity, thereby producing leak-free, airtight, and thermally efficient buildings.

Enhanced building envelope review

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Although the contractor is responsible for undertaking the project, a key element to ensuring success and the long-term performance of the building is QA. It is recommended a building envelope specialist perform site review twice a week, as dictated by the contractor’s activities. This allows the project team to identify issues/deficiencies before being covered up or progressing to a point where it becomes costlier to rectify during construction or when the building is occupied and operational (Figure 10). It also assures the owner the specified performance will be achieved due to direct reviews.

Site testing

In-situ testing of the building enclosure during construction provides another level of QA/C that the envelope is performing in conformance with the specified targets. Tests for air and water penetration resistance are recommended to be performed on windows and glazed assemblies. The author recommends tests not only in a mockup, but also periodically during construction. In a mockup, testing can help identify potential installation issues that could lead to systemic failure. Periodic testing provides a level of assurance the quality of work and performance of the system is consistent throughout the construction process.

In recent code changes, whole-building airtightness testing during and post construction became mandatory. This test aims to produce more airtight buildings and improve durability, occupant comfort, and effectiveness of mechanical ventilation systems, lower utility costs, and enhance resiliency. Although in some jurisdictions airtightness targets are not being enforced at this time, it is critical fundamental building science principles are given the attention they need at all stages of the project to increase the probability these targets can be met once they become enforceable. Failing an airtightness test near the end of a project’s construction is a risk all parties should plan to avoid.

Onsite quality control

QC can have a significant impact on the overall performance of the enclosure/building. Co-ordination between trades is where a number of discontinuities can occur through incorrect overlaps, compatibility issues between materials, timing/sequencing of trades, etc. Figure 11 is an example of how the timing and sequencing of trades can lead to discontinuity in the building enclosure. In this example, the brick shelf angle and offset plates were installed prior to the wall’s air and vapour barrier membrane transition to the foundation wall’s waterproofing, creating an air and moisture barrier discontinuity at the top of the foundation wall.

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As building enclosures become more complex and performance targets more stringent, building envelope checklists are recommended to be implemented by the GC and subcontractors to increase site QC and reduce the potential for non-conforming work. Such checklists are already common place in curtain wall fabrication facilities, and therefore, there is no reason why they cannot be implemented for site installation work. Checklists will start to become common place when building envelope commissioning requirements are adopted more widely. Checklists should be enforced by GC and periodically checked by the building envelope consultant and commissioning team.

Historically, site QC was enforced by GC’s site superintendent. However, building superintendents do not have the time or technical knowledge to supervise the trades to the degree required to achieve high-performance buildings. It is recommended GC employ a full-time building envelope superintendent to ensure continuity of control layers are maintained and enforced. This level of QC is currently being implemented on Passive House projects with a dedicated air barrier superintendent, typically called the ‘air boss,’ to achieve stringent airtightness and energy use targets, and has been found to be quite successful.

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Conclusion

Climate resiliency, reducing energy use, and striving for net-zero buildings will continue to drive building envelope design and development. Getting the building science fundamentals right is the first step to meeting these new goals. At the very least, one will be able to achieve more durable buildings.

Engaging an objective building envelope specialist from the project onset, who is not part of the same firm as the prime designer, provides several benefits to overall execution of the building envelope from design to construction, and through the life cycle of the building. Clients and designers should also be made aware this higher level of review and QA can impact costs and increase time due to a higher level of QC required to satisfy project objectives (i.e. site checklists, testing, mockups, etc.).

As such, it is advisable to engage a building envelope consultant as soon as possible during the initial schematic design meeting in order to fully understand the benefits and impacts to the overall project.

[13]Andrea Mucciarone, BSS, is project building science specialist with the building science and restoration division of RJC Engineers. Mucciarone is responsible for evaluations, investigations, and remediation of building envelope systems (walls, windows, roofing), as well as the implementation of rehabilitation and preventative maintenance programs. He has a broad range of experience, including design, contract administration, construction review, and thermal and hygrothermal analysis of building envelope assemblies. Mucciarone can be reached at amucciarone@rjc.ca[14].

Source URL: https://www.constructioncanada.net/a-comprehensive-scrutiny-of-building-enclosure-design-and-construction/

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