Sealant Selection for Installation

Jim Snyder
October 18, 2017
COLUMN : From the Field | Materials & Components
Applying sealant to window

Exterior sealants require special qualities. (Image courtesy of iStock,

There is no installation component more critical to a successful window/door installation than proper exterior sealant selection. Unlike interior sealants or caulks, exterior installation sealants must seal out water intrusion, without fail, for many years. If they don’t, the consequences can be costly. 

That’s a tall order considering it must maintain its integrity and adhesion to adjoining surfaces despite ongoing exposure to the sun’s relentless UV light and extreme seasonal temperature swings (which also results in substrate and joint movement). Given the demands, it is difficult for installers to select the right product amidst all the options. But there are some considerations that might help. 

Sealant or Caulk?

After extensive research, I found that the terms sealant and caulk are frequently used interchangeably in the construction industry and are not universally defined. I looked to experts from Dow Chemical, including Research Scientist Brian Lieburn and Andrea Wagner Watts, high performance building application engineer, to clarify. 

“Sealants are specifically formulated to create a durable seal against weather,” Lieburn explains. “Caulk, on the other hand, is typically used to fill (minor) joints as part of painting prep.” 

“Sealant,” then, is more descriptive for our purposes. But that’s just a small step toward selecting the best product. More factors need to be considered. For example, both Lieburn and Watts agree that sealants should be selected based on application temperature, expected exposure, movement capability and desired performance life, as well as one that adheres well to both the window and underlying substrate.

Also, while many sealant products are labeled for “window, door and siding” as well as “exterior,” these labels are still kind of generic. Look very specifically on the label for applicable substrate materials, workability temperatures and UV tolerance. Substrate-to-sealant adhesion recommendations are also found in Table A4.2 of ASTM E2112, the Standard Practice for Installation of Exterior Windows, Doors and Skylights.

Many sealant tubes also refer to “ASTM C920,” the Standard Specification for Elastomeric Joint Sealants. While the specification is very complex, it essentially qualifies a sealant on multiple levels. The takeaway for us is to look for ASTM C920 with “Class 25” or greater. This may also be listed as “movement capability” (because that’s what it is) and may be shown in a percentage, for example, “Movement capability: +/- 25%.”

Chemical Compatibility 

Chemical compatibility is perhaps the most under-respected and hidden danger of incorrect sealant selection. It has broad meaning, part of which is doing no harm to wall membranes and flexible flashings. The deceiving part is that incompatibility is not usually immediately obvious, yet the consequences can be severe. 

According to Dr. James Katsaros, research fellow at DuPont, “Incompatibility is typically not evident right away and is something that will develop over time, more likely within months. The degradation could impact the membrane/self adhered flashing, or sealant, or both.” Membrane drainage failure leads to internal water intrusion, which is hidden, at least for a while, causing costly damage. 

For information on chemical compatibility of sealants with a given product, look to the product manufacturer of the membrane and flashing.

Ease of Application?

Ease of application should not necessarily be a factor in selection. It’s tempting to reach for a water cleanup product because it’s easier to use. Most of these products will fall short of more important requirements. 

Some time ago, I had my house re-sided and then hired painters to paint the new siding. I chose to provide the sealant myself after learning of the product the painters intended to use, which was clearly based on ease-of-use and cost. 

As professionals, you’ve learned the skill to apply the right product for the job, even if it costs more. In fact, in general, expect to pay more. Sealants that meet all of the above qualifiers cost more on the front end, but cost less in the long run by reducing callbacks and providing the homeowner with a problem-free installation lasting many years.

Warning Labels

After finding the right sealant for the job, don’t ignore the label warnings, as I stubbornly did at times. After a couple of hard lessons, from skin irritation to face numbness, I always made time to put on gloves and eye protection and heed the other warnings.  

Recognize the importance of proper installation sealant and take the time to make the right choice. It’s to everybody’s benefit. It’s even our obligation. 


Jim Snyder is an AAMA-certified FenestrationMaster and InstallationMaster who shares his years of installation field experience as an industry writer, speaker, trainer and project/product consultant for dealers and manufacturers. A member of various industry organizations, Snyder also is involved in instructional document creation and revision. Contact him at