The design team should also review shop drawings and identify areas for further development such as corner conditions, drainage strategies, and other internal seals, such as heel beads the manufacturer or installer may offer as a means of improving a system’s integration with its sub-framing. This, coupled with stringent requirements for mockup testing, can provide better assurance that the system will perform as intended. However, accommodating a change to use a mulled system after the design phase typically requires significantly more work from the designer, especially when attempting to develop mulling systems for the already lower-performing R and light commercial (LC) windows.
Case study: Water penetration
The risks associated with mulled windows can be most easily seen in practice. For example, one of the author’s projects involved an educational institution targeting an AW-40 rating for its punched windows in existing 3 x 3 m (10 x 10 ft) concrete openings (Figure 5). On this project, the author provided building envelope commissioning services to
the owner.
The contract documents specified a curtain wall system for the large, punched openings on the project. However, the contractor opted for an unrated receptor frame and vertical mull to combine two narrower rated windows and accommodate variations in the existing concrete openings. This mulled assembly had no performance rating and during field quality control (QC) testing, it exhibited severely compromised performance compared to a window tested in isolation. This consistently resulted in leakage at significantly less than the specified performance rating (Figure 6).
In particular, the receptor frame presented a major weak point in the system. However, the vertical mulls also played a role by further complicating the assembly as installers attempted to integrate all components without published installation instructions or support from the manufacturer tailored to this mulled configuration.
Repairs were eventually implemented to bring the windows back into compliance with the contract documents. However, this issue brought with it significant costs, as well as additional seals that would need to be maintained at the owner’s expense, and aesthetic changes had to be made to the windows as the drainage path needed to be modified by adding field-drilled weeps.
