Let’s Talk Vapor Barriers

In my first architecture job after graduation, I was relegated to drawing interior elevations, plans, and ceiling plans for 2 years. There was no way architectural interns like me with minimal experience would be allowed to do any work related to the exterior of the building.

Make a mistake on the interior and you can talk to the contractor to adjust things during construction. Make a mistake on the façade and you risk serious lawsuits from clients – “the roof is leaking, the walls are cracked, mold is growing in the walls, the floor is flooded, etc”… I used to marvel at the older architects who could draft details and crank them out as if they were pulling a continuous magic roll of toilet paper.

Over time, however, I managed to escape interior prison work to tango with facades. Gradually, after endless hours of drafting, and seeing construction processes unfold, but mostly through dumb mistakes, I learned how to detail such disparate exterior systems as window wall, metal panel, curtain wall, cast in place concrete, mineral fiberboard, ultra high performance concrete, terracotta, brick, etc… While every material has its own tolerances, advantages, and limitations, there are two fundamental keys for any well detailed building enclosure: (1) control the flow of air, and (2) control the flow of moisture.

In terms of detailing process, I have 2 main life experiences that have shaped the way I work. First, when a stressed out colleague of mine had a heart attack at 45 years old, I was appointed as his replacement supervising exterior detailing on a Renzo Piano building in construction in NYC. Luckily I survived my tour of duty without cardiac failure to recount my tale. Piano’s office is run like an army.  They taught me to never compromise - never give up. You’re given an impossible goal like design a thin plate metal parapet coping that negotiates silicone sealant and urethane roofing material incompatibility to provide a warrantable aesthetically pleasing solution. Months of I-kid-you-not 20 person conference calls between roofing and sealant manufacturers, subcontractors, façade consultants and owner’s representative, yielded endless sketches of different component configurations until the final goal was attained through pure dogged heuristic iteration.

Second, when I was renovating my own first apartment, I was too cheap to hire actual plumbers and contractors. I learned to never be afraid to ask for help. I asked my unqualified superintendent of the building, Chris, to help me hang cabinets and do minor plumbing and would pay him minimally to get the job done. For gas line work, Chris disconnected our old stove and connected a new stove with crude wrench work. It wasn’t pretty but he charged $20 instead of $300 a real plumber would’ve charged for the same work. Since he wasn’t a licensed plumber Chris  ‘asked for help’ by calling ConEd gas company to report an ‘emergency gas leak’. When the ConEd utility service man hastily came to the apartment looking to prevent the building from exploding, Chris calmly concocted a story how he smelled gas and that the ‘real’ plumber had left for the day. The unsuspecting utility man then checked Chris’s gasline work for leaks confirmed that while the connection should’ve been made using plumber’s putty rather than Teflon tape, there was no detectable gas leak according to his special gas monitoring equipment.

In the design of the current project I’m working on, I’m not afraid to ask material manufacturers for help or advice. In fact, I’ve been quite shameless and aggressive in asking the commissioning agent hired by the owner’s representative for advice on how to best detail the building. Although you admit vulnerability when you ask for help, you also open yourself to learning from other people who have many years of experience with certain materials or detailing issues. Vendors and commissioning agents are like free façade consultants.

Air Management 101

I grew up in a suburban stucco house built a century ago. Every time the winter wind blew, you could feel the draft. It was uncomfortable and annoying, but you learned to cope. Nowadays, there are a multitude of ways to mitigate air infiltration with modern materials. After you build the primary structure of the house and sheathe it to form an enclosure, you can wrap or paint it with a continuous rubber like coating like a Henry Air and Vapor Barrier. This prevents the air from coming in and energy loss from occurring.  Some air and vapor barriers are trowelled, others are applied in sticky sheets, some are installed at low temperatures for wintertime construction, some can be left exposed to the sun’s damaging UV rays… depending on the project, you must select the right air and vapor barrier for your job.  

Water management 101 - Liquid

On top of the roof insulation you place an impermeable membrane so no water can leak into the building. Consideration should be given to the materials used on the roof. For example, if you use a metal roof, you place a high temperature membrane like Grace Ultra. Composed of two waterproofing materials—an aggressive 100% butyl rubber adhesive backed by a layer of high density cross laminated polyethylene film, the Grace membrane is specially formulated to withstand temperatures up to 300°F. If you have a concrete roof with exterior rigid insulation you can apply a Siplast paradiene modified bitumen roofing.  Whenever possible, at roof surfaces, it’s best practice to try to divert the flow of water by sloping the roof away from walls and parapets a minimum 1/8”:1’-0” slope. Many building edges fail when ice freezes and thaws at the junction between wall and roof thereby spalling off critical exterior layers of the envelope and allowing water to damage the building. Don’t invite the water barbarians to the gate. Once they break down the city walls, they can wreak havoc! Another pro tip you’ll thank me later for is if there are curbs or walls adjacent to roofs, they should be flashed 8” above the drainage plane of the roof to avoid water infiltration from snow buildup.   

Water management 201 - Vapor

In liquid form, water is easy to control. Specify an impermeable umbrella like membrane and no liquid water should enter the building. In gaseous vapor form, water management is more nuanced. In a northern climate, you want to position the exterior wall air and vapor barrier on the inside face of the insulation. By placing the barrier on the warm side of the insulation you ensure vapor from the interior of the building never gets cold enough to condense in the wall to start an unsightly black mold situation. If you have insulation on the exterior and interior of the sheathing (i.e., rigid insulation and batt within studs), you should specify and air impermeable and vapor permeable membrane. Any excess moisture can thus escape through the exterior wall without condensing within the wall to cause a mold situation.

The ability of a material to transmit vapor through a material is measured in Perms (permeance). A low permeance <1.0 means the material is highly impermeable and doesn’t allow any vapor passage. A high permeance means the material is permeable to vapor. In specifying materials for wall construction, you never want to have vapor condense within the wall. Following this logic, putting a vapor impermeable wall material like vinyl covering on the interior face of an exterior wall will potentially trap condensation within the exterior wall and lead to disastrous moldy situations. I’ve found, if you talk nerdy about perms and vapor drive control with contractors or clients, they’re impressed with your use of arcane jargon and assume you’re quite knowledgeable about building construction.

Windows

At windows, the head, jamb and sill should have 2 lines of sealant and backer rod to control air and water from infiltrating the building. The insulated plane of glass should be designed in line with the plane of insulation in the wall construction so the thermal insulating envelope is continuous throughout the building. Additionally, the exterior line of sealant should be inset and positioned in line with the thermal break of the window for optimum performance. Should both sets of sealant lines fail, detailing the window such that the stainless steel flashing can let any inboard water weep out by gravity is the most robust failsafe strategy.  We call this the belts and suspenders and underwear approach to window detailing.   

The Final Analysis

On any given project, you may have multiple air and vapor barrier, insulation, and water management materials to suit specific conditions for the project. Sometimes you’ll need an air vapor barrier that heals itself when drilled with fasteners to support rails for a rain screen, other times you may need a sprayfoam insulation that provides a low permeance, or you need an expansion joint product that ties into roof membranes without curbs, etc... To sort out all these conditions I suggest creating a table of roof membranes, insulations, expansion joints to keep track of their material properties (permeance, R value, material compatibility, etc..) and correlate this with simple envelope diagrams for the building enclosure.

Envelope Matrix

Vapor Barrier Diagram

Thermal Insulation Diagram

Every time one product overlaps another, you must call the manufacturers to confirm the layers are compatible, that the overlapping layer can adhere to the layer below, and that their material contact will not cause mutual deterioration. Over a series of refinements, one can then link all these envelope components together to form a continuous line of air and moisture management armor for the building. If the design is clear and resolved, one should be able to verbalize these layers of defense by writing beautiful English sentences like this to describe their final chemically compatible, spatially acceptable configurations: at the seismic joint between new and existing building the Emseal horizontal colorseal backs up the Emseal roof joint material which is tied curbless into liquid applied reinforced parapro flashing which is applied to the siplast paradiene modified bitumen membrane which transitions up an adjoining roof wall with liquid applied reinforced parapro flashing which is lapped over with a termination bar and henry Blueskin self adhered air and vapor impermeable membrane which in turn laps over henry trowel applied air and vapor permeable barrier which transition to a high temperature Grace Ultra underlayment under metal roofing as it slopes up to a skylight where henry vapor impermeable Blueskin wraps up the curb providing additional protection to the skylight stainless steel flashing and sealant and backer rod lines of defense….