Glass Mat Sheathing

In the late-1980s, glass mat-faced sheathing was introduced by Georgia-Pacific. This product incorporated a very water-resistant, gypsum-based core. The product took off selling like hotcakes, in large part due to the extra water protection provided for exteriors, combined with the leaky condominium crisis. The perfect storm made this glass mat sheathing the “go-to” sheathing for most condominiums and commercial buildings across the U.S. and Canada.

Today, all gypsum manufacturers have a glass mat sheathing with some form of a treated core against water. These products come in a variety of colors and have slight variations in performance. The designer must select the product and should make that decision based on test data or compliance to ASTM C1177. In some cases, the design may get a substitution request to use a similar glass mat sheathing. Often, that substitution request is denied and contractors get mad at the architect.

Designing with Sheathing

A tip to contractors seeking a substitution request: make it clear and in writing that the product you want to use is at least equal to what was specified for performance, as well as why you want to use it. Working as project manager at an architect’s office, my job was to approve or deny substitution requests. I denied most requests and certainly was hated. The reason for the denial was that I had no time to research and compare products. While that seems like it should be easy, it can be, but most of the time it was not. I was not lazy, but I did have to account for my time. Researching and comparing products meant looking up data and verifying manufacturers’ claims. 

As a design professional, there is a liability in allowing a substitution if it is found not to be of equal value. The time for this research had to be paid for, and that meant the owner would get an invoice for my time. If there were no money savings components to them, they would understandably get mad at me. If the products seem very similar to each other and the substitution comes with a subcontractor’s warranty, it is more likely to be approved. However, there still should be a reason to switch. The reason could be as simple as product availability or your workers’ preferences. Lower cost to save you money is typically not a good reason, and it will likely raise red flags with the architect. The firm I worked at would honor the substitution suggested if it was done properly and did not burden our time.

Gypsum sheathing has made massive strides over the last few decades and is doing it again. Today, we have synthetic-faced, vapor-permeable face gypsum sheathing that is also an air barrier. To be approved as a system, they generally require the use of approved sealants, per that manufacturer. Pay attention to this if you submit a substitution request. The air barrier-rated sheathings can save time and cost versus adding a layer of another product. Installation of sheathing is per the Gypsum Association document GA 253 or per ASTM C1280. They are very similar in the language, and that is not by accident. Having a copy of these standards and occasionally reviewing them is a good idea. While changes in these standards are typically not dramatic, they can have an impact. Remember, you are responsible to know the rules on the application.

The construction industry is stagnant, stubborn and refuses to change. Really? Gypsum sheathing is just one example of progress and product innovation we routinely see. Innovation with construction materials may not be as sexy as the tech industry. But still we have done an incredible job of meeting demands and improving the efficiency of buildings for energy savings and water intrusion prevention.