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Pictorial Guide to Reinforcement Mulls

by Dec 9, 2019

What is mulled window?

Mulled windows are two or more separate windows that have been joined together to fit into a single opening in a wall.

Mulled sets require windows to be permanently attached together during installation. Windows are mulled together (joined) with mull strips and reinforcements through the use of our proprietary interior and exterior accessory pockets that are standard on all Ringer Windows. Very large openings with multiple windows connected together may require special heavy duty reinforcement parts.  Our heavy duty reinforcement mulls provide enhanced stability to fully support these heavier windows.  This pictorial blog provides step-by-step guidelines on how to mull windows.


Ordering Windows with Reinforcement Mulls

When placing an order, IT’S IMPERATIVE that you inform your Ringer Windows representative about any windows which are designated to be mulled into single openings. This will ensure that window mulling costs and appropriate parts are included in your original bid.  Your Ringer Windows representative will also help you to determine whether you need regular or heavy-duty reinforcement parts.  Reinforcement mulls may be ordered with any of our window products including stock windows.


Please Read Our Window Mulling Instructions For More Detailed Information

Please refer to the section entitled “INSTALLING MULLS USING REINFORCEMENT WITH A 1/2″ STRIP” in the mulling instructions.

Once the proper types of mulling parts are determined, your window order will include all required components including reinforcement metal (if required), 1/2″ or 0” mull strips, and L-Brackets used to reinforce mull window installations.

Replacement window installations may need special Vin-L-Hold fasteners as well as other unique accessory trims that are essential for replacement applications.  These additional trims and parts may be purchased from us.  This is worth repeating…..  IT’S IMPERATIVE that you communicate as many details as possible about your window installation to your Ringer Windows representative so we can accurately quote the necessary parts for your particular application!

A three window replacement installation in brick was used for this description and pictorial guide. The three window set was placed in a 12′ x 7′ opening.


To Begin – Prepare Opening

STEP 1: (For New Construction) Frame Opening and Prepare for Window Set

Use Ringer Windows new construction install guidelines for installing new windows. Prepare the new framed opening for installation of mulled windows.

STEP 1: (For Window Replacement) Remove Old Windows

Use Ringer Windows replacement window installation guidelines for replacement windows. Remove old windows and caulk. Prepare window opening for new window installation.

Removing the glass of the old window

Removal of old glass and careful handling

Glass removal for large windows takes two people

Next – Install Window Number One

STEP 2: Plan, Measure, Mark Locations and Install 1st Window

Once you have a prepared opening, the next step in window mulling is to measure, mark, and remeasure the windows and the opening. It is critical to make sure the new Ringer Windows will properly fit with either regular or reinforced window mull bars and 1/2″ mull into the opening. You must know each window’s location and orientation in its respective opening. Measure the windows, mark their locations in the window opening and don’t forget to accommodate the 1/2″ spacer for the reinforcement bar between each window.

Throughout this multi-window installation, continue to measure and remeasure to ensure all windows will fit within the opening with the reinforcement mull hardware. 

Finally, with precision measuring and marking completed, you are ready to install the first window and secure it to the framing.

Plan, Measure, Mark, Measure, Mark, Plan

Measure and Mark

Mark 1/2″ intervals for reinforcement mulls

Setting Window 1 into opening

STEP 3: Align, Set, Plumb, Secure, Measure

Window Number One is aligned with the markings made earlier. Align with the edge of the marking. Check for alignments and make sure any operable window will open properly. Secure Window One to the building frame. Measure again.

Line up Window to marking

Measure to make sure Window 2 will fit properly.

Secure window to framing.

Measure and place window properly

STEP 4: Install 1/2″ Reinforcement Window Mull Bar

The metal reinforcement bar(s) have been pre-cut to be used with the windows that were ordered. Remove the blue protective backing on the adhesive strip on one side. Adhere the metal reinforcement to the open side of the already installed Window One.


Remove Blue Protective backing from Metal Bar

Adhere Reinforcement Bar to Window 1

Secure Reinforcement Bar

Reinforcement Metal Bar

STEP 5: Install L-Brackets

Fasten the L-Bracket to the window through the reinforcement metal bar to the window and to the building frame. Two L-Brackets should be secured at the bottom and the top of the window.

Two L-Brackets for top and bottom of window

Attach Lower L-Bracker to window and house frame

Secure L-Bracket

Secure Upper L-Bracket to Window 1

Next – Install Window Number Two

STEP 6: Installing Window Two

Once the L-Brackets are installed and measurements are assured, Window Two is ready for placement and installation.

Remove blue protective strip on refo bar 

Install Window Two

Place Window Two

Measure for Window 3

STEP 7: Install 1/2″ Reinforcement Mull Strips

After Window Two is placed, Window One and Window Two are connected with the special trim, the 1/2″ mull strip. This 1/2″ window mull bar is one of the proprietary installation trims available for purchase as part of the trim kit. Starting at the top, the mull strip connects the two windows together. Trim the bottom of the mull strip to fully connect the 1/2″ mull strip to the windows. Once the windows are connected, secure window two to the building frame.

Attached 1/2″ mull strip to accessory pocket near top on both sides of window

Snap in half inch mull

Secure within the accessory pocket

Securing 1/2″ reinforcement mull into the accessory pocket

STEP 8: Trim and Secure 1/2″ Reinforcement Mull Strip, Secure Window

Outside bottom of Mull strip

Trim mull strip to be able to secure top to bottom within the accessory pocket between Window 1 and 2

Completed reinforcement mulling of Window 1 and Window 2

Secure Window 2 after completing mulling to Window 1

Last – Install Additional Windows As Needed

STEP 9: (For Second Window Installation)

If your window mulling is for a two window installation in one opening, your mulling is complete . Window trim-out (if required) and seal for replacement window installation are the final steps. You now have two successfully mulled windows.

STEP 9: (For Three Window Installation)

For more than two window windows in an opening, measure and check to be sure the remaining window (s) will be able to fit within the opening. Repeat Step 3 – 8 above.

Summary Of Window Mulling

Below is a brief pictorial summary of each step above (to complete out the 3 window installation in this case). After all of the windows are installed with the reinforcement mulls, complete the window installation. Replacement window installations may require additional insulation, installation accessory replacement windows trims and sealant.

Ringer Windows offers full-service window installations for replacement windows Austin area homes and businesses. NOTE:  Window mulling is not complicated, but it requires good technical knowledge of how windows are installed.  Allow our in-house professional installation crews to provide knowledge and superior service for your replacement windows. Contact us online or call today at 512-989-7000 to discuss Ringer Windows installation services for replacement windows.

STEP 3 – Plan/measure for next window

Step 5/6 If needed, Secure reinforcement bar, L-Brackets to open edge

Step 8: Secure Window to outer building frame

STEP 4 – Place and set next Window

Step 7/8 – Secure 1/2″ Mull Strip inside and out. Trim and complete



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205 Mustang Cove
Taylor, TX 76574



(512) 989-7000




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