Phius Building Scientist & Certification Manager Al Mitchell outlines best practices for parapets.
Parapets. Who needs them?
Well, any building with a low slope roof. Parapets keep the roof membrane protected from uplift from wind forces, help collect snow and rain to prevent runoff, and hide unsightly (or exciting if you ask me) rooftop mounted mechanical equipment. While my stand-up career is still a ways off, I would like to take the time to clarify some of the concerns Phius has had regarding parapet walls and some potential solutions to remedy them.
Just like the realtors motto of “location, location, location,” parapets function similarly with “location of air control, location of vapor control, and location of vapor diffusion.” (Maybe this analogy is a stretch, but don’t let that make your interest taper (to a roof drain)).
We want to ensure the insulation in the parapet is protected from exfiltration from the interior conditioned spaces in the building, vapor diffusion from the interior, and that there is a vapor permeable air barrier to relieve any moisture build up in the assembly. This is fairly straightforward in a typical parapet detail where the wall is either balloon-framed or platform-framed on top of the roof decking. However, there has been a growing trend to include large cants in the parapet. This canted area creates a large enough air space that most fire code inspectors would flag it as a risk for fire, so then it needs to be filled.
The common method is to fill this with a fluffy insulation, such as mineral wool, cellulose or fiberglass. I am generally concerned about cellulose due to its hydrophilic nature. But even with a loading of hydrophobic mineral wool, we at Phius were concerned about the inner face of the sheathing becoming a condensation plane. This is similar to the concerns about the sheathing rot of double-stud walls in cold climates. Unlike the double-stud walls, this is even more risky as the sheathing is covered with a very vapor-closed roofing membrane, shown below in blue.
In similar fashion to the mid-wall air barrier that solves the hygrothermal concerns in a double-stud wall, this can be solved by carefulling minding the control layers and their respective locations. By platform framing the parapet wall, a peel-and-stick air control membrane can be applied to the transition. This can continue to an air control layer under the insulation of the roof, but on top of the deck. This detail is easy to quality control on the site, and protects the air barrier from damage.
By limiting the air flow into the parapet cavity, which was uncontrolled in previous details, we limit the flow of moist air up into the insulation around the cant, and protect the sheathing. It is best practice to also allow some diffusion of moisture out of the top of the parapet wall, so a vapor permeability air barrier and exterior insulation on the parapet wall would be advantageous.
One other solution that might work is to use closed cell spray foam as the control here, but this is more sensitive to construction sequencing, as this often involves a contractor on a ladder spraying blindly into the cavity. This will also better control the air and vapor flow into the parapet cavity.
In summary, Phius will continue to carefully check details for hygrothermal risks, and I hope this clears up what the best practices for parapets with large cants are. This is an open discussion, so please follow up with other ideas you may have.
