Flashing and air barrier seam tapes get buried deep in our walls where we rely on long-term performance without monitoring them. Are they doing their jobs?

Companies do their own more sophisticated testing, like this tensile test on ZIP Tape. The company has a whole video on its testing procedures, but it's more fun to try this at home!
Photo Credit: Screen shot from ZIP System video

NOTE: Read this whole series here.

Service life of tapes can determine the service life of an entire high-performance building assembly.

Performance testing of adhesives and sealants used in our weather barriers is improving due to new field-testing research, as we’ve written about before. However, the improvements in testing haven’t reached a critical product area: pressure-sensitive adhesive (PSA) tapes used for sealing seams in flashing, housewrap, and generally creating continuity in air and weather barriers. “I am unaware of any work being done on this issue, either laboratory or field tests,” says Christopher White, of the National Institute of Standards and Technology (NIST).

The most commonly cited adhesion tests for pressure-sensitive adhesive (PSA) tapes are as follows:

• ASTM D3330 – Standard Test Method for Peel Adhesion of Pressure-Sensitive Tapes
• ASTM D903-98-04 – Standard Test Method for Peel or Stripping Strength of Adhesive Bonds
• ASTM D1876-01 – Standard Test Method for Peel Resistance of Adhesives
• ASTM D3654 – Standard Test for Shear Adhesion of Pressure-Sensitive Tapes
• ASTM D3330 – Standard Test Method for Peel Adhesion of Pressure-Sensitive Tapes

But none of these tests is ideally suited for lab-testing high-stretch construction flashing tapes, and none go anywhere near testing under field conditions. And since just about all tapes are used in concealed weather and air barrier systems, we really need a field-service-life prediction test.

 “Workbench tests” of flashing tapes

So we took matters into our own hands—or rather, our own workbench. Lately I have been just sticking a slew of tapes on different building materials and gauging how hard it is to pull them apart.

Exterior adhesives and sealants are formulated for performance, but some contain chemicals that pose risks to unprotected workers or the environment

The silicone found in many window caulks is not much of a health risk to onsite workers, but the chemicals used to produce silicon are coming under greater scrutiny.
Photo Credit: ArmaCo Construction

 NOTE: Read this whole series here.

As discussed throughout this series, adhesives and sealants used outside the building envelope have to adhere to the substrate and seal gaps, and they often need to be as durable as the building itself. Performance is the primary concern, and the chemical constituents often take a back seat. Unlike products used on the interior—where VOCs and other potentially hazardous chemicals can concentrate to create indoor air quality problems for occupants—for exterior products, exposure risk is mostly limited to workers who manufacture and apply the products.

Compressible gaskets keep air and water barriers continuous without liquid sealants or adhesive tapes. But they don’t all last equally well.

This Holst Architecture-designed Passive House project, Karuna House, includes gasketing on the sill plate—common in Northern Europe for decades but fairly new to the U.S.
Photo Credit: Hammer & Hand

NOTE: Read this whole series here.

In the U.S., we tend to put a lot of faith in caulks, tapes, and wet-applied sealants. But in Europe it’s a different story.

Some Gaskets can be used in place of tapes or liquid sealants, mainly as part of residential air barrier systems. According to Lee Jaslow of Conservation Technologies, a leading U.S. distributor of high-performance gaskets and one of the high-performance gasket listings in GreenSpec, the market for gaskets in residential construction is small but growing, with increased interest due to high-performance rating systems such as Passive House.

Caulk joint sealants can be a major deciding factor in how long your building envelope lasts. Is there a better way to predict how long they last?

Mounted on the roof at NIST, this "weathering engine" tests sealant durability.
Photo Credit: National Institute of Standards and Technology

NOTE: Read this whole series here.

Durability, or service life, is critical to the overall performance of liquid caulk joint sealants in the water and air barriers in our buildings.

If we can figure out how long sealants actually last then we can come up with a prudent inspection schedule—and have a good idea of how they’ll fail and how to replace them. The good news about sealants is that they are generally exposed to view—unlike flashing tapes, which are generally buried and inaccessible. (More on tapes in a future post.)

Fairly assessing durability or service life

We are always hoping for that one magic test that fairly, accurately, and realistically portrays one or more performance attributes of our building materials.

The trouble is that, while field tests can be more realistic, they tend to introduce many uncontrolled or non-measurable conditions. And the trouble with laboratory tests is that they set, control, and measure many conditions, making them often far from what actually goes on in the field.

It turns out that we have plenty of useful standardized laboratory tests for both liquid sealants and tapes that we use in our weather and air barriers. That’s the good news. The bad news is that we haven’t had useful tests for the field service life prediction of those same sealants and tapes—until recently.

When you use tape to seal a seam or flash a sill, you need peel-and-stick performance—not “stick-and-peel.”

...or is it? Our confidence in tape might be a little misplaced.

NOTE: Read this whole series here.

While liquid sealants most often are used on the exposed surfaces of building enclosures, pressure-sensitive adhesive tapes (member link) and membranes are used one or even two layers deep in the building assembly to seal the margins of weather-resistive and air barriers and at penetrations such as window openings.

Their location means that they cannot be inspected, repaired, or replaced; we need to know that they will maintain their integrity and function for the full service life of the assembly.

What makes stuff “sticky?”

It’s pretty easy to take pressure-sensitive adhesion (PSA) for granted; from band-aids to masking tape to peel-and-stick membranes, we use them pretty much every day. But the science of PSA is complex and even a bit uncertain.

Any sealant can perform well in the right application, but knowing which to pick for your job is another thing. Our guide to sealants and how to use them.

NOTE: Read this whole series here.

Photo Credit: DAP

When selecting a sealant, these properties are typically the most important:

• movement tolerance (rated by Class per ASTM C920, with 25 meaning joint movement of 25% of the linear width dimension of the sealant bead)
• substrate compatibility
• workability, particularly based on temperature
• paintability and its converse—substrate staining
• relative cost
• service life
• material constituency and hazardous content

There are seven basic types of liquid sealants, largely based on their chemistries and subsequent strengths and limitations. Each sealant’s suitability to an application is based primarily on its performance properties, the properties of the substrates, and cost.

Latex

Latex sealants are water-based, easy to tool, easy to clean up, paintable, and relatively less expensive than other types of sealants. Some premium latex sealants may be appropriate for exterior use (appropriate service life) and are rated for movement in classes 12½ and 25. Latex sealants may be best suited to interior finish applications.

Acrylic

Acrylic sealants are also paintable but are solvent-based and more difficult to tool. They are used more in commercial and exterior applications than latex and have very limited movement capacity (Class 7½). Acrylic sealants tend to be used in commercial construction in low-movement joints. Their cost tends to be in the low to moderate range.

Butyl

Butyl sealants are solvent-based, synthetic rubber materials demonstrating strong adhesion to a wide variety of substrates. They have excellent weathering characteristics but tend to be stringy and difficult to apply. They generally have limited movement accommodation (Class 7½ ). Butyl sealants are sometimes used in curtainwall systems where adhesion to rubber materials is required. The cost of butyl sealants tends toward the moderate range.

The next group are sometimes called “high-performance” sealants and are most often used in commercial building assemblies.

Seals at window openings and other penetrations need to be done right the first time. Are your seals failing because of the most common application error—forgetting the bond break?

It's great they remembered the backer rod in this attempt to seal the joint, but with the depth of the sealant so skinny compared to the width, it's probably going to fail. Read on to learn why. 

NOTE: Read this whole series here.

Continuous air and water barriers are essential to healthy and high-performing buildings, but making these barriers truly continuous is more than just slapping on some building paper. It requires meticulous detail work. Sealants—properly applied—are a key part of that.

Sealants are liquid-applied substances tooled to a concave surface shape, with “edge bonding” to each substrate. In the case of air and water barriers, they connect one field of the wall to another or to the component in the penetration—the window, the pipe stack, the duct, etc.

Drawing a bead on proper joint sealing

Essential to any sealant application is a backer rod or bond breaker tape. These ensure that:

• adhesion is between the substrates only (no perpendicular stress from the back of the joint to weaken the focus on the connection between the substrates)

• the sealant is supported on the back side as tooling exerts pressure on the sealant

• the sealant bead is well-proportioned (ratio of width to depth of 2:1)

Backer rods come in various diameters so that they compress about 25% of their cross-section into the gap.

Open-cell backer rods have the advantage of “breathing,” allowing curing to the backside of the joint, and are not affected by any puncturing that might result during tooling. By contrast, closed-cell backer rods, if punctured during tooling, can offgas and create bubbling in the sealant. Closed-cell backer rods don’t absorb water, while open-cell ones do. A third type of backer rod is the “hybrid” bi-cellular backer rod; it does not outgas when punctured and only takes up moisture at cut ends.

Use the type of backer rod recommended or required by the sealant manufacturer.

Lots of building products offer some help in keeping air, water, and heat in our buildings, but without attention to the joints, you lose critical continuity in your barriers.

NOTE: Read this whole series here.

To keep out the weather, don't head for the stickum first. Take a page from the lobsterman's book and use weatherlapping, overhangs, and mechanical fasteners. Photo: KGBKitchen on Flickr.

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.