Mulling Over AAMA 450-06

Paul R. Gary
June 15, 2007
COLUMN : Legal | Codes & Standards

Mulled windows are the darling of designers—“the introduction of light and the feeling of space”—you know what I mean. A proliferation of mulls done poorly, however, can represent an “Achilles’ heel” that can cause an otherwise vigorous company to falter.

We have struggled with mulls through joinery and sealant concerns, education in the trades regarding flashing practices and communication issues with respect to instructions. Unfortunately, even if you made it through those, there is no time to rest because yet another source of potential issues arises from the different nature of a mulled window. We are going to review how the mulling of windows has a “ripple effect” on the use of AAMA ratings in the market.

By definition, a “mull” involves the joining of individual window products without a continuous frame to fill a single window opening. Assembly (or creation of the actual mull) can occur at the factory, a distributor facility or on-site in the field. Mull configurations require greater than normal discipline with respect to both engineering and workmanship. If you are like me, you have learned about mulls in no small part from mistakes. For purposes of this article, we are moving on from this history.

About a year ago, AAMA adopted 450-06, a Voluntary Performance Rating for Mulled Window Assemblies. The 450 document is a necessary tool for standardization within the industry and the issues surrounding it need to be understood. It provides a methodology for rating the mulled assembly as a whole. The standard is appropriately conservative in that the overall assembly cannot bear a rating beyond that of any of the individual component parts, i.e. the total may not equal the sum of the parts. 

The operative phrase for application of 450-06 is “mulled fenestration assemblies.” Despite an aggressive Section 5.0 of definitions, the term “knocked-down field mulled fenestration assemblies” is not defined. As a result, we have to anticipate that knocked down assemblies will be interpreted to include any field mull that is, may be or should be contemplated by the manufacturer. I propose that you should seriously consider obtaining a rating for the mulls that you know, or have a factual reason to anticipate, are being done in the field. A category of approved mulls and disapproved mulls should be developed and communicated to customers.

Why? Manufacturers that certify their products to AAMA standards are justifiably proud of the fact. It only makes sense to publicize certification. “AAMA certification” has been an appropriate general reference for use in marketing and product specification materials. With that, buyers often understand that the entire product line of a company is AAMA-certified, allowing for a uniform technical basis for product selection.

As a result, the case may be made that the “reasonable consumer expectation” is that all products sold by the manufacturer to be installed in a given structure are AAMA-certified. That same consumer may not realize that mulling of windows (either in factory or field) can dramatically change the overall test performance of the mulled unit. To them, even a field mull is still a product manufactured by the named company (individual windows mulled together may still have labels indicating individual product certification). 

The issues of foreseeability and reasonable expectations are real. Given this, manufacturers and resellers would be advised to either take steps to assure that certification requirements are met or that the consumer is advised (in a way that can be proved) of the fact that the windows as installed in their structure (mulled) have not been tested to meet all AAMA certification requirements. Looking at it this way, certification of mulls may not quite be “voluntary” after all.

Let’s go back to the two principal locations for mulling of windows—factory and field. Assume the situation in which a manufacturer markets an AAMA-certified product line, and that mulls are fabricated in the factory. Here, I strongly recommend that the company either certify the mulls to AAMA 450-06 or clearly advise the buyer that the mulled products have not been certified. The best practice is for all factory mull configurations to be rated. To facilitate the number of options this may entail, 450-06 allows for rating by “Groupings” of configurations that would meet or exceed the overall rating.

With respect to mulls fabricated in the field, the case for either rating the mull or disclosing the lack of testing may seem less compelling, but the same principles apply. The question is one of foreseeability. If a manufacturer incorporates AAMA certification into its overall marketing, then it may have a tough time explaining why it never disclosed that certain “foreseeable uses” of its product do not fall within the parameters of the AAMA rating. Even if the evidence shows that the manufacturer/reseller never actually knew of the specific field mull configuration, it will be left open to the argument that it had reason to know and could have had processes in place to gain actual knowledge. The same rationale may be applied to communicating size and configuration limitations of field-assembled mulls. Someone is going to say, “You’re the expert and you never told me.” If the judge buys it, then you will have to let the jury decide.

In summary, for both manufacturers and resellers the appropriate promotion of products as “AAMA-certified” brings with it the need to develop and maintain a strategy with respect to both factory and field-assembled mulled windows. Companies should consider either rating the mulled product or communicating the fact that testing has not occurred.

Paul R. Gary is the prinicipal of The Gary Law Group, a law firm based in Portland, Ore., emphasizing legal issues facing manufacturers of windows and doors. He welcomes feedback about articles published in Window & Door and can be reached at 503/227-8424 or