I think a Section here on building details is a great idea and the parapet, a very common problem area, is a perfect place to start. The details presented compare two routes for the thermal barrier to follow at the parapet. The discussion suggests that short-cutting the thermal barrier across the base of the parapet will have the advantage of reducing the total surface area of heat-transmitting wall exposed to the exterior. The proposed thermal shortcut uses spray foam inserted between the metal studs. Some things to consider:
1. The vertical studs penetrate the spray foam so the R-value of the foam has to be de-rated accordingly.
2. To place the spray foam as shown would require a horizontal sheet "backer" closing off the space between studs. The detail does not show what the backer material might be or how it would be held in place. Often, vertical studs are doubled around openings and close-packed at corners multiplying the labor and practical problems of placing the backer and the foam.
3. Placing the spray foam: Is it placed from below via long nozzle inserted up behind the beam? Installers have told us this is not practical. Placed from above before the parapet is sheathed? This can create construction sequence issues.
4. But the most unpredictable risk is the possibility of condensation forming in the un-vented, un-tempered, space above the foam.
The discussion assumes the necessity of a parapet. If optimum thermal performance is a goal, then early design discussions should question this necessity.
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Alan Swift AIA
Director, Design Integration
dbHMS Design Build Engineering
Chicago IL
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Original Message:
Sent: 05-12-2019 17:26
From: Joseph Deringer
Subject: "Details Forum"
The "Details Forum" is a major project in 2019 of the Building Performance Knowledge Community (BP-KC).
The objective is to provide online information that shares good details among architects via a web location within the BP-KC site. Another focus is to provide a vehicle for publicizing the activities of technically-oriented architects.
A draft detail of a Parapet has been prepared as a test case for implementing a first draft of the "Details Forum."
See the attachment.
Stay tuned for this first draft to be published soon.
Project participants are:Brad Gellert (lead)
Stephen Katz
Drake Wauters
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Joseph Deringer AIA
Institute for Sustainable Performance of Buildings
Berkeley CA
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