Appendix: Shadow box: To vent or not to vent AIA course

appendix

The following is a review of relevant literature that has been published in the recent past and represents the most rigorous and scholarly resources found by the author. These resources have been used to inform the recommendations in this paper. The bulleted lists that accompany each resource are the salient points found in each paper.

(1) Michno, Michel (Enclos). “Analysis and Design of Spandrel and Shadowbox Panels in Unitized Curtain Walls.” Glass Performance Days 2009

• In a sealed cavity, the effect of temperature rise on cavity pressure is negligible, amounting to 4.5 psf for 280°F temperature rise.
• In a sealed cavity, there is load transfer from the outer panel to the inner panel, especially if the outer panel is flexible (e.g., 1/4" monolithic glass.)
• Study evaluated three shadow box configurations: sealed, vented, and pressure equalized.
• Sealed cavity systems provide a marginally higher benefit than vented systems, but rely heavily on high manufacturing standards.
• Pressure-equalized systems provide good performance in harsh environments, but need to be protected prior to installation to prevent contamination of the cavity.

(2) Boswell, C. Keith (SOM) and John Walker (Enclos). ““Shadow Boxes.” – An Architect and Cladding Designers’ Search for Solutions.” Glass Processing Days 2005

• In a sealed cavity installation in Beijing, the low-E coating was installed on the #3 surface to reflect heat back into the cavity and minimize condensation on the #4 surface.
• Suggested solution to prevent condensation in the cavity and “scum” buildup is to provide either a sealed cavity or a cavity that is vented internally, from the cavity to the insulation layer, but not through the vapor barrier.

(3) McCowan, Derek, Mark Brown, and Michael Louis (Simpson Gumpertz & Heger). “Curtain-wall Designs.”Glass Magazine (April 2015).

• Identifies two potential problems with shadow boxes: imperfect seals between the shadow box back pan and the mullion framing that allow condensation into the cavity, and offgassing of sealants inside the cavity due to high temperatures.

 (4) Vigener, Nik, and Mark Brown (Simpson Gumpertz & Heger). “Building Envelope Design Guide – Curtain Walls.” Whole Building Design Guide (a program of the National Institute of Building Sciences) (25 June 2012).

• Suggests an interior back pan behind the insulation to help prevent condensation in the cavity.

(5) Kragh, Mikkel, Stanley Yee and Larry Carbary (Dow Corning Corporation), and Neil McClellan (HOK). “Performance of Shadow Boxes in Curtain Wall Assemblies.” CTBUH 2014 Shanghai Conference Proceedings.

• The use of laminated glass (for structural performance or impact resistance) could limit the acceptable temperature inside the cavity. Manufacturer information on the temperature limits of laminated glass should be consulted before using it in a shadow box assembly.
• Nonvented cavities can cause deformation of the glass or back pan, resulting in a compromised appearance.
• In vented cavities, baffles should be provided to reduce the possibility of dust and debris entering the cavity.
• Back pans should be sealed to the surrounding framing, rather than rigidly attached, to prevent deformation from differential movement and pressure cycling (for nonvented cavities).
• Suggests sealed, desiccated systems as a new standard.

 (6) Kaskel, Bruce S., and Carole M. Ceja. “Case Study Repair of Shadow Box Spandrel Condensation.” ASTM Selected Technical Paper 1549 (2014).

• Condensation and ice formation was observed both in the shadow box and in the corners of lights adjacent to the shadow box.
• Back pan insulation was observed to be incomplete in filling some shadow box cells, and the sealing tape was incomplete or poorly adhered to the mullions.
• The building was designed to maintain interior humidity at a minimum of 30% throughout the winter.
• Testing indicated that the insulation at the back of the shadow box, in conjunction with venting to the outside (by cutting gaps in the interior glazing seal) reduced the temperature inside the shadow box enough to cause condensation on mullions at vision glass panels adjacent to the shadow box. The high interior humidity levels contributed to this effect.

 (7) Centre for Window and Cladding Technology. “Shadow Boxes.” Technical Note No 94 (2014).

• Venting the shadow box cavity to the exterior in cold climates can bypass the curtain wall thermal break and create cold mullions surrounding the shadow box.
• Annealed glass is not suitable for the inner light of the IGU as it may break due to thermal stress caused by high temperatures inside the shadow box cavity.  Suggests heat-strengthened or tempered glass. Alternative suggestion for laminated glass, but the interlayer must be suitable to withstand the expected high temperatures.
• Suggests that shadow boxes be vented to the exterior in UK-like climates. To maximize the effectiveness in rejecting heat and humidity, it is suggested that the ventilation openings be located at the top of the shadow box.
• Suggests that there is no current guidance on calculating the area of ventilation openings.
• Ventilated shadow box cavities should not be considered pressure equalized because the openings will be too small to allow the rapid pressure changes that an equalized system requires.
• A fine mesh or open-cell foam in the ventilation openings will reduce the amount of dust entering the cavity, but will not eliminate all dust and could become clogged and close the ventilation openings.

 (8) Apogee Advanced Glazing Group. “A.A.G.G. ‘Shadow Box’ Design Guidelines.” Technical Bulletin 505 (May 23, 2005).

• Shadow box glass must be heat strengthened to avoid thermal stress failure.
• Difference in temperature between the interior of the shadow box and the adjacent vision glass areas may be transferred through the shadow box’s perimeter mullions. In cold climates this can result in condensation on mullions at vision areas. In hot climates mullions in vision areas can become hot to the touch due to high temperatures inside the shadow box.
• Shadow box venting is required by code in Massachusetts.
• At the perimeter of shadow boxes, horizontal-to-vertical-mullion seals must be full depth to prevent interior air from entering the shadow box cavity around the installed vapor barrier.
• The shadow box cavity cannot be vented to the exterior in face-sealed (four-sided-SSG) systems.
• If the cavity is generously vented to the exterior, the back pan may be required to resist the same wind pressures as the building exterior. Smaller vents will reduce the wind pressure on the back pan.
• Residual solvent release from aluminum finishes due to high cavity temperatures isn’t an issue due to the high application temperatures of these finishes.
• Other materials in the cavity, including sealants and foam plastic insulation, must be suitable for the expected high temperatures.
• If laminated glass is required, venting may be required to keep the cavity temperature below 170°F to prevent damage to the laminate.
• Any vents to the exterior should be baffled to minimize transmission of dust and debris into the cavity.
• Avoid fixed connections at the back pan perimeter to minimize oil canning due to thermal expansion.
• Care must be taken with vented, unitized curtain wall sections to ensure that water and debris is not introduced into the cavity when the units are stored on site.
• There is no industry consensus on venting of the cavity.
• Use of solvent-release sealants, such as butyl, acrylic, or acetoxy-cure silicone, is not recommended in the cavity.
• Silicone, SCR, or EDPM glazing gaskets are recommended.
• Always maintain a seal between the shadow box cavity and the building interior.