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To Mull, Or Not To Mull

For many homes, more glass means more appeal to homebuyers. Whether two Double Hung windows joined together at the factory or a wall of windows overlooking a backyard, mulled window units are a popular way to allow light into homes. Less visual obstructions with mulled units means wider views and less framework. 

To help builders and remodelers gain a stronger understanding of mulled units and options, Simonton Windows offers the following “Q & A Briefing” with responses from experts in the company’s coastal, code and sales areas.

Q: What is a mullion?
A: A mullion is a vertical or horizontal connecting unit between two or more windows. A mullion may use aluminum or steel reinforcement to give it strength. 

Q: What are the most critical aspects of working with mulled units?
A: Making sure the design pressure ratings match up with the correct mullion system to assure the strength of the overall window unit is very important. It is also recommended that units are resealed once they are installed in the opening.

Q: What are the various ways two or more window units can be mulled?
A: There are quite a number of different ways new window units can be mulled (or “joined together”) including factory mulled, field mulled, semi mulled (partially mulled in the factory and then completed in the field), T-mulled (generally referred to as an integral mullion --- instead of being two totally separate mainframes installed into a single opening, these units have a continuous head and sill) and H-mulled (generally the mull looks like an H from an aerial viewpoint). Mulled units can be used for both new construction and replacement applications. In new construction, the H-mull requires the removal of the fin and J-channel from the sides of two units and the two windows are inserted into the sleeve of the mull.

Q: What type of reinforcement is used in mulled units?
A: The use of the mullion reinforcing material is typically determined by performance desires of the finished window unit (including how much wind pressure it must stand up to), the unit size and the quantity of units to be included in a particular mulled assembly. At Simonton, aluminum mull reinforcement is used in most product lines with the exception of ProFinish Brickmould 300. This series use steel reinforcements in their mulled units.

Q: How many Double Hung new construction windows can be factory-mulled?
A: At Simonton, mulling capabilities differ by product line. The H-mull can join up to five Double Hung units for new construction windows for a total width of 150 inches. As an example, the T-mull can go up to three Double Hung units in the Brickmould 600 Series (up to 109 inches in width) and three Reflections Casements (up to 106 inches wide).

Q: What if I want to mull the units in the field?
A: Components can be ordered and assembled successfully in the field by an experienced contractor. A common rule of thumb is that anytime there is an intersecting mullion and the mullion length exceeds 60 inches (or when the assembly is to include more than two units) it is recommended to include reinforcement in the mullion. It is advisable to have the configuration mulled in the factory to ensure quality construction.

Q: Which is better, a T-mull or an H-mull?
A: T-mulls perform better with regards to water and air infiltration due to their continuous head and sill construction. They are also more structurally sound, have a cleaner appearance and require no post-installation caulking and clean-up. 

Q: What industry code(s) regulate mulled units?
A. The American Architectural Manufacturers Association (AAMA) 450 standard has been adopted in the International Residence Code (IRC) 2006 version of the building code. Some states and jurisdictions are recognizing this standard in the code. More than 90 percent of Simonton customers commonly request mulled assemblies. Most of these meet AAMA 450.

Q: What’s the difference between AAMA 450 and AAMA 101?
A: AAMA 101 is specific to individual windows and is a nationwide requirement. It does not specify anything for mulled products. The AAMA 450 standard was created specifically for mulled products. It only regulates non-integral mullions, or H-mulls and has only been adopted by eleven states to date. T-mulled units are certified to AAMA 101 and do not require AAMA 450 compliance due to their continuous head and sill construction. 

Q: Are there differences in the ways different manufacturers mull units?
A: Yes. At Simonton, factory mulling uses a multitude of products such as: silicone, reinforcements, steel strapping and screws. This is to assure the strongest possible mulled unit that can meet AAMA codes.

Q: What (if any) are the advantages of getting a factory mulled unit versus field mulling a unit?
A: Experience says that factory mulling is better because of the amount of reinforcing materials used and the experience of the people performing the work. In field mulling, you sometimes risk that the person doing the mulling is not as experienced as individuals in the factory, or may not have the proper tools and products. As a result, they could leave water and air infiltration points. Another advantage of factory mulling is reduced labor at each job site. When Simonton mulls the units in the factory, they can be installed faster at the job site.

Q: What’s the biggest risk regarding the use of mulled units?
A: The risk comes when (and if) the units are not properly mulled with enough reinforcements on large openings or in large configurations. There’s also a risk if the window is not properly installed in the opening. Results of an improper installation or mulling could be windows that are hard to operate, excessive air or water infiltration or the “shaking” of mulled windows in their openings when doors are opened or closed in a home. 

Q: What type of windows can be mulled?
A: Virtually any combination of windows can be mulled together to create a wall of windows. Often time Half Rounds are mulled to the top of Single or Double Hungs. Another popular choice is to mull a variety of Double Hungs together or several operable Casements to a Picture window to create a massive window showpiece. There are limitations as to what can be mulled together. Contractors should check with manufacturers for their recommendations on field mulls and with their local building codes. In addition, if doing field mull work, contractors should remember that weeps should never be obstructed by mulling.

Q: How are mulled units successfully transported and installed?
A: Because of the weight of mulled units, it’s important that shipping, moving and storing the units receive top priority. They need to be stored in a safe place in an upright position on and against a flat, even surface. All mulled units should be installed plumb, level and square and shimmed as needed. Make certain the proper fastener type, size and spacing are used to properly anchor the mulled unit in the opening. Installers should make certain the unit is sealed with the appropriate sealant around the entire perimeter to prevent water and air infiltration. When the unit requires a reinforced mull, make certain the base plate is installed and properly anchored to the opening.

Simonton Windows produces ENERGY STAR® qualified replacement and new construction windows and doors, including a line of impact-resistant products. The company was ranked #1 in quality in the 2007 Brand Use Study sponsored by Builder magazine and has won two consecutive Best In Class Awards for being ranked “Overall Top Rated Vinyl Window Brand” in unaided industry studies.

Founded in 1946, Simonton’s hallmark has been to deliver its made-to-order products in seven days or less. Part of consumer products company Fortune Brands, Inc. (NYSE: FO), Simonton has manufacturing facilities in Pennsboro, Harrisville and Ellenboro, W.Va.; Paris, Ill.; McAlester, Okla.; and Vacaville, Calif. Simonton is a founding sponsor of The Weather Museum and a corporate partner of Homes for Our Troops. For information, call (800) SIMONTON or visit www.simonton.com


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