Shadow Box Design

To Vent or Not to Vent



Photo of Mark Walsh

Mark Walsh

Principal, Technical Director




Shadow boxes are commonly used in curtain wall construction, but can be problematic if not designed and detailed correctly and appropriately for the climate. Much has been written about shadow box design, both successes and failures, but there is no consensus about how, or if, the cavity should be vented. There are 4 approaches to ventilating the shadow box cavity: venting directly to the exterior environment, venting indirectly to the exterior through the mullion cavities; venting directly to the interior building environment and sealing the cavity. Venting the cavity directly to either the exterior or the interior building environment introduces moisture- and particulate-laden air that can condense under certain climatic conditions and will likely deposit dust and debris on the interior surfaces of the cavity, creating both permanent and temporary aesthetic concerns. When venting to the exterior, exterior air can produce extreme hot or cold temperatures inside the cavity that is transferred to the surfaces of the surrounding mullions that are exposed to the building interior to detrimental effect. Sealing the cavity typically eliminates condensation and debris buildup, but the cavity can become overheated as in venting to the exterior. Additionally, there is some evidence that heat and pressure may build up to a point where it could damage the glass and degrade the sealants and coatings inside the cavity. After considering all of the options, indirect venting to the exterior appears to address most of the issues, but with the caveat that it is only feasible with certain unitized curtain wall systems. This approach vents the cavity into the vertical mullions, which ultimately connect to the exterior environment, but do so indirectly, relieving the heat and pressure but also tempering the exterior air that is allowed to enter.


A curtain wall shadow box is a spandrel assembly consisting of vision glass at the building exterior and an opaque infill at the interior side of the curtain wall system

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Modes of Shadow Box Failure

A review was completed of literature related to the design of shadow boxes or their failure and subsequent remediation. A summary of the literature review and salient points appears in

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Ventilation Strategies

Ventilation Directly to the Exterior

Shadow box cavity ventilation directly to the exterior is commonly done by leaving gaps in the glazing gaskets of the vision glass and putting porous

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Conclusion and Recommendation

Having considered the benefits and drawbacks of the ventilation strategies identified above, it is this author’s primary recommendation that shadow box cavities be ventilated indirectly to the exterior through the

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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

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