The Misconception Series #5: Parapet Design

Contributed by Roy Schauffele

​I just reviewed another project from a different architect with the same recurring design misunderstanding. This is isolated to the parapet area of a building. I see this detail dozens of times a year and yes, I’m being Texan polite by calling it a misunderstanding.

The building is an administrative office for a Fortune 100 company. It is of steel stud construction with R-13 batt insulation between the studs, 5/8” exterior gypsum sheathing, an ABAA (Air Barrier Association of America) approved liquid applied air barrier (from Company XXX), 1” of an ABAA evaluated foil faced polyisocyanurate insulation (from Company XXX) and a metal façade. This assembly has an effective R-value of >13, which exceeds code. It is also an ABAA evaluated assembly and completely compliant with the Code Mandated Fire Requirement to NFPA-285, and since the air barrier and insulation were from the same corporation, chemical compatibility issues are avoided.

All is well on the front of the building.

Unfortunately, all is not well on the backside of the parapet.

The detailing of the parapet backside showed steel studs, R-13 batts and 5/8” exterior gypsum sheathing for an effective R-value of about 6.6, at best. Undoubtedly, this area is going to be highly energy inefficient and it will have a different dew point (the temperature to which air must be cooled to become saturated with water vapor) than the front of the building. When the dew point occurs, the condensate will drop straight in to the conditioned space, possibly causing mold and definitely leading to reports of a “roof leak”. 

The fix is easy.

Just put 1” of your specified continuous insulation over the top and backside of the parapet and now your entire building has the same thermal envelope, with the same dew point profile on both sides of the building. 

The advent of code required effective R-value and the use of continuous insulation has led to certain misunderstandings, but as I said above, there is an easy fix and it is in the paragraph above.