Tape It? Seal It? Glue It? Sealing Weather Barrier Seams

This is part of an ongoing series. Read all the Sticky Business posts here.

To keep out the weather, don't head for the stickum first. Take a page from the fisherman's book and use weatherlapping, overhangs, and mechanical fasteners. Photo: Clinton Steeds. License: CC BY 2.0.

In just about every climate in which we live and build, the number one job of any building envelope or enclosure is environmental separation. Keeping water, air and heat in or out of buildings can make them more resource-efficient, durable, and safer for occupants.

The number-one difficulty or challenge in environmental separation is continuity of our air barriers, drainage planes, and insulation layers, particularly at penetrations, transitions and margins of building assemblies.

Many of us jump straight to adhesives, sealants, tapes and membranes to achieve barrier continuity, but there are strong reasons to employ three strategies: weatherlapping, mechanical fastening, and then selective, task-specific use of sealants and the like.

Basic Storm Gear: The Weatherlap

We need look no further than the storm gear of any fisherman to understand this strategy. From brimmed hat to boots, many overhangs, drip edges, and healthy, gravity-honoring overlaps shed bulk water down and away from the fisherman.

In buildings, weatherlaps are the foundation of bulk water barriers and should not be ignored. Many buildings suffer from lack of overhangs, poor flashing details, and similar issues. However, these fundamental protections may not be sufficient if the wind blows hard enough or the structure stands tall enough for even small wind pressure to push or suck water over weatherlaps against gravity. And managing both air and heat requires more than the weatherlap.

Reinforcement: Mechanical Fastening

When the weather gets nasty enough, the fisherman's weatherlaps may be reinforced with cinches, belts, and hook-and-loop fasteners. Secure all of these tightly enough, and physical sealing can achieve continuous or nearly continuous contact of one barrier element to the next, keeping water out even as the wind pulls and pushes at each overlap.

In buildings, we can pin down the exposed edges of our barriers with fasteners, gaskets, and rigid bars or cleats, rendering the overlaps physically sealed, or nearly so. On tall commercial buildings or very exposed low-rise residential buildings, mechanically fastening or "trapping" the exposed edges of our barriers should be routine.

The writer airseals sash pockets in his home after sash replacements.

Sealing the Deal with Chemistry

Sealants and adhesives (along with materials that make use of them, like tapes and membranes), rely upon surface chemistry to "stick" one material to another to make a barrier. Unlike weatherlapping and mechanical fastening, the quality of the bond or "stick" is dependent on the nature of the underlying materials, as well as the conditions of application: usually the ideal is clean, dry, and moderate in temperature.

We use an incredibly wide array of these materials to make our barriers continuous, but how do we know which ones to use and in what combinations with the various sheet goods that make up the fields of our barriers?

Defining the Options

Adhesives

Adhesives are chemical compounds whose sole purpose is to join two substrates--to be "sticky." Their performance is mostly related to the strength of the bond achieved between two substrates. Adhesives are strictly two-dimensional in function; they are not exposed.

Adhesives are classified according to how they cure--physical hardening, chemical curing, pressure-sensitive--and their chemistry--water-based, solvent-based, and two-part reactive. The ability of an adhesive to manage stresses--temperature range, application temperature, ultraviolet-light exposure, expansion/contraction, moisture (bulk water and vapor), and time--can be related to both curing and chemistry. The environmental profile of each adhesive can also relate to curing and chemistry.

Sealants

These materials are meant to span gaps between two building components; they are formulated to adhere to the substrates on each side of the gap. They have lower strength than adhesives but greater elongation, or elasticity. Their primary function is to seal, to keep air and water out (or in). Their function is a three-dimensional one; they are very often exposed.

Sealants are classified as one-component, two-component, or sealant tape. They are also categorized by chemistry: acrylic, butyl, latex, polysulfide, polyurethane, and silicone. As with adhesives, the performance properties and environmental profile of a sealant is often tied to its chemistry.

Mastics

Mastics are adhesive materials that do not "dry out" but instead remain pliable during their service life. They are generally a pretty heavy consistency, airtight, and waterproof. Mastics are typically troweled or smeared, making them a "flat," thin, three-dimensional application. Typical applications include HVAC ducting, roofing, and foundations.

Putties and caulks

These materials are strictly fillers, meant to span gaps but not provide air or water sealing. Many of us in the industry use the terms caulk and sealant interchangeably, but chemists and manufacturers like to make a distinction between filler materials and true sealants.

These definitions may seem basic and self-evident, but how many times have you seen a sealant being used as an adhesive, or the reverse? I can't tell you the number of flanged windows I have installed applying a bead of sealant to the backside of the flange, compressing the sealant to a flattened smear during installation--per the manufacturer's directions. Yet, this treatment eliminates any three-dimensional bead configuration of the sealant, treating it like an adhesive!

Stay tuned for Part 2 in this series: Matching Performance Properties to Application.

And for a head start on an enclosure system with appropriate--and continuous--weather-resistive materials, our product guide on weather & air barriers is a great place to start.