A Guide To Installing Mullions For Your Indow Window Inserts

A mullion divides a window into smaller units. An Indow mullion comes in an ‘H’ profile to divide our window insert into more manageable units when needed. The acrylic inserts slide into the white space of the ‘H’.

Sometimes a specialty ‘F’ profile mullion is needed for windows that meet at an inside corner, like a fish tank. If this is the case for your windows, let us know so we can provide different instructions.

Consider the Indow Mullion when:

  • Your window, or the opening you’d like to seal, is too large for a standard Indow insert.

  • You will be installing & removing your insert often—for cleaning or opening your window.

  • You want to open part of your window while keeping part of the insert installed.

  • Your existing window has frame dividers & you want your insert to match.

The Indow Mullion comes in the same colors as our insert frames - white, brown, & black - but can be painted to match your window frame. The ‘F’ bar is only available in a “mill” finished aluminum.

The Indow Mullion includes everything you need for installation:

  • ‘H’ Bar

  • Mounting ‘L’ Brackets

  • Screws

Please let us know if you did not receive the mullion sample piece with the mountings brackets and screws.

H Bar

F Bar

Color Options

NOTE: This guide includes instructions for installing brackets, measuring with only the brackets in place, and installation once inserts and mullion have been delivered. For alternate installation techniques or when using ‘F’ bars, you will need to completely install the bar and hardware before measuring.

Horizontal Split Installation

What you will need:

  • 2 ‘L’ brackets

  • 2 Screws per bracket

  • Drill with 1/16" Drill Bit

  • #1 Phillips Bit Screwdriver

  • Pencil

  • Tape Measure

  • Laser Measuring Device

Bracket Installation

  1. Determine placement of the window split. We recommend using an existing visual line, like a window frame, sash or divide, if present.

  2. Measure from the bottom of the frame up to the point at which the mullion is desired and mark the frame with a pencil.**

    Measuring for bracket position
  3. Repeat on the other side.

  4. Position an ‘L’ bracket so that your pencil mark is at the TOP of the bracket and mark a center point for each counter sunk hole.

    Marking drill holes for L bracket
  5. Repeat for the other side.

  6. Pre-drill each marked center point.

  7. Once holes are drilled on both sides, secure the ‘L’ brackets with the screws provided.

    Drilling holes for L bracket

NOTE: MAKE SURE you position the bracket at the correct depth (& frame step) within the frame. If you aren’t sure what a frame step is, walk your fingers from the surface of your window out toward your wall. Every time you have to take a step with your finger, that’s a frame step.

Watch the Video:

“How to Measure Your Window for Mullions and Indow Inserts”

Horizontal Split Measurement

  1. Follow the instructions included with your Laser Measure Kit to capture the six measurements for each opening.

    1. When measuring the “A” value for the bottom panel, place the laser tool just below the ‘L’ bracket and shoot straight across the window frame.

    Horizontal split measurement diagram A Horizontal split measurement diagram B Horizontal split measurement diagram C

    The blue ‘L’ bar represents the Mounting ‘L’ Brackets

  2. Double check your measurements. The ‘B’ value for the upper insert should be nearly the same as the ‘A’ value for the lower insert.

  3. Enter all data into the Measure by Indow portal, check, and save.

    1. When entering your measure data into the Measure by Indow portal, designate UPPER” or “LOWER” in the Location field. This will be labeled on each insert so you know the correct location to install each panel.

Insert Installation

  1. Receive and un-package insert packaging.

  2. If the mullion bar was not ordered pre-cut, you will need to measure the opening and cut it now.

    1. Measure the distance across the frame at the specific location the mullion will be installed.

    2. Measure and cut the aluminum bar using any saw with a metal-cutting blade.

  3. Lay the mullion in place on top of the previously installed brackets.

  4. Install the Indow inserts.

    1. Install the TOP insert FIRST to compress the mullion down on top of the ‘L’ brackets.

    2. When installing the bottom/lower insert, install the TOP of the panel first, so it nests in the channel of the mullion.

Vertical Split Installation

A vertical split is when the ‘H’ bar is on its side. Imagine the ‘H’ tipped over 90 degrees and both the inserts sliding in from the side.

What you will need:

  • 4 ‘L’ Brackets

  • 2 Screws per Bracket

  • Drill with 1/16” Drill Bit

  • #1 Phillips Bit Screwdriver

  • Pencil

  • Tape Measure

  • Laser Measuring Device

  • Mullion Sample (provided by Indow)

Bracket Installation

For a vertical installation, you will need to install two sets of ‘L’ brackets on either side of the mullion to hold it firmly in place.

  1. Determine placement of the window split. We recommend using an existing visual line, like a window frame, sash or other divide, if available.

  2. On the window frame header, measure from the left of the frame to the point at which the mullion divider is desired and mark the frame with a pencil.**

  3. Repeat on the bottom of the window sill.

  4. Position an ‘L’ bracket in the top of the frame so that your pencil mark is at the RIGHT of the bracket and mark a center point for each counter sunk hole.

  5. Repeat on the bottom of the frame.

  6. Pre-drill each marked center point.

  7. Once holes are drilled on top and bottom, secure the ‘L’ brackets with the screws provided.

  8. On the bottom of the window frame, place the mullion sample against the installed ‘L’ bracket.

    Placing mullion sample against installed bracket
  9. Place the other loose bracket inside the mullion channel and mark the location of the hole in the bracket.

  10. Remove the mullion divider and pre-drill marked center point and secure ‘L’ bracket in place with the screws provided.

    Marking and drilling for top bracket
  11. Repeat steps 8-10 for the top of the window frame.

**MAKE SURE you position the bracket at the correct depth or frame step within the frame.

Vertical Split Measurement

  1. Follow the instructions included with your Laser Measure Kit to capture the six measurements for each opening.

    1. When measuring the ‘D’ value of the left panel and the ‘C’ value of the right panel, ensure your laser tool is flush against the window frame, not resting on the ‘L’ bracket, and shooting straight up to the top of the window frame. These values should nearly be the same.

    Vertical split measurement diagram A Vertical split measurement diagram B Vertical split measurement diagram C
  2. Enter all data into the Measure by Indow (MBI) portal.

    1. When entering your measure data into the MBI portal, designate “LEFT” or “RIGHT” in the Location field. This will be labeled on each insert so you know the correct location to install each panel.

    Measure by Indow portal - product and frame details Measure by Indow portal - product and frame details

3 Panel Splits

If you have a large window opening requiring 2+ mullion bars, you will need to place additional brackets in the window, in the places you wish to install the mullions. Where you measure from will be important as it can impact the outcome of your inserts. Here is a diagram illustrating our recommended measurements in a 3 panel split:

3 panel split diagram
  • BLUE

    Heights measured on the frame, next to the brackets.

  • GREEN

    Diagonals from the bracket to the frame.

  • BROWN

    Widths from the bracket to the frame.

  • RED

    Downward diagonals, from the bracket to the frame.

  • ORANGE

    Heights measured on the frame, next to the brackets.

  • PURPLE

    Widths from bracket to bracket.

  • YELLOW

    Diagonals measured from ‘L’ brackets into ‘L’ brackets.

Insert Installation

  1. Once you’ve received your inserts and mullion bar, unpackage the panels to identify which should be installed to the left of the mullion and which should be installed to the right.

  2. If the mullion bar was not ordered pre-cut, measure and cut it now.

    1. Measure the distance across the frame at the specific location the mullion will be installed.

    2. Apply that measurement to the aluminum bar. Cut it with any saw with a metal-cutting blade.

  3. Place the ‘H’ bar against the previously installed brackets so one set of brackets is nested in the ‘H’ channel.

  4. Position the second top bracket so the pre-drilled hole is properly aligned, and secure into place.

  5. Repeat on the bottom.

  6. Install the Indow inserts, taking care to install the side that will be nested in the mullion first.

For alternate installation techniques or different mullion profiles, you will need to completely install the bar and hardware before measuring.

Insert installation into mullion

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Customer Reviews

Excellent

4.49 Average 1505 Reviews

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Anonymous

WE just took delivery of the two window inserts we ordered. they both fit perfectly. The most difficult part of the process was me trying to hold the laser measuring device still while taking the diaginal measurment! The Indow team was helpful and the ordering process was catch free. the finished product is just as they described. once installed, you can not even tell that they are there. I will warn you, if you have larger sizes they will come in a pretty large packing crate/box to protect them so be prepared. The only reason for 4 stars insted of 5 is they have just been istalled and we shall see how effective they are at reducing cold air, but they fit so snug that I anticipate they will be very effective. Just in time too, because our area is in a low temp/low wind chill pattern at the moment.

Posted 1 week ago

Craig F

The Indow I ordered was a perfect fit. It looks great and is an extremely well made product. The ordering process was seamless.

Posted 1 month ago

Catherine J

Noise reduction by at least 50%! I cannot hear most car that drives by now. My bedroom is so quiet! Thank you!

Posted 1 month ago

Walter M

Easy to order and install.

Posted 2 months ago

Dianne F

Window inserts help with sound since I live facing the street and the highway

Posted 2 months ago

Dianne

This is a 5 star company from beginning to end. I purchased the I inserts in 2017 because the bedroom on the second story faced the street. These are very effective for street noise. Noticed a problem on one of the windows recently and they quickly took care of it due to their warranty. Customer service personnel are very helpful. The price of the windows was acceptable for top quality of sleep and less noise during the day

Posted 3 months ago

Charles C

Recently purchased the acoustic window inserts for a couple of old windows facing a busy street and they've made a huge difference in terms of noise reduction! The measuring process was relatively easy with the laser measuring tool that Indow sent and the windows were delivered faster than expected. Martin was easy to work with and we would highly recommend Indow to those looking for window inserts.

Posted 5 months ago

Dean C

Installed noise canceling acrylic Indow window inserts to deal with the road noise at my condo, 4 lane state road is 40 feet from the condo. Expensive custom fit, but oh my gosh, they cut the noise by 75%! Decibel meter app I trust shows a 6 Decibel noise reduction, average Db over 30 minutes. Turns out 6 Db is letting only 25% of noise through! I am one happy camper…

Posted 5 months ago

Harry T

The Indow team was great from start to finish! They were very clear about each step. After installing the inserts my home has been significantly while still preserving the fabulous city views. For anyone who lives in a noisy street or city like NYC I would highly recommend it. Leave the chaos outside and come home to piece and quiet.

Posted 5 months ago

Paula S

From start to finish, we were very pleased with our rep, Reed, and the other staff at the PDX will call! The inserts themselves are stupendous! We purchased six total for the noisiest, road side(s) of our home. The outcome was beyond our expectations! Not only did the three rooms that bordered the road become at least 70% quieter, the other end of our home did as well! I cannot recommend Indow inserts more! The technology makes sense to add the inserts rather than purchasing new windows. Great results and thank you! Paul and Paula Spilsbury

Posted 6 months ago

JD

Brian was very helpful & informative. He helped me order the correct windows for my needs. They were shipped with the utmost of care & arrived ahead of schedule. Ultimately, the Indow Windows have been very helpful in muffling unwanted noises & they have helped save on our energy costs. Thank you, Indow Window!

Posted 7 months ago

Cynthia M

Very happy with the reduction in noise. Easy to install and use. Great customer service.

Posted 8 months ago

Juli E

We are thrilled with our Indow window inserts! I'd been on the lookout for a more permanent solution to leaky windows in our third floor living space. I discovered Indow on social media but since then have learned of friends who'd had Indow recommended to them by their rennovation contractor. The entire process went smoothly with friendly reps at hand to talk through questions during measuring, etc. They fit perfectly and help us to maintain the beautiful design of the windows on our 100 year old home! Highly recommend!!

Posted 8 months ago

Jen F

Worth the money!

Posted 9 months ago

Elizabeth H

For townhouse living, this solved my problem with noise from my neighbor's with my kitchen window. A considerable decrease of noise indeed! Measurements were a bit frustrating so I recommend using a laser measurement device and following directions to use a string for the diagonal. They look great and were easy to push into place. I highly recommend as an alternative to buying a new window.

Posted 9 months ago