![](/_app/immutable/assets/LP-Mullion_Guide-01.6k-Em7T1.jpg)
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
![](/_app/immutable/assets/LP-Mullion_Guide-04.DIWkPB_W.jpg)
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
Determine placement of the window split. We recommend using an existing visual line, like a window frame, sash or divide, if present.
Measure from the bottom of the frame up to the point at which the mullion is desired and mark the frame with a pencil.**
Repeat on the other side.
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.
Repeat for the other side.
Pre-drill each marked center point.
Once holes are drilled on both sides, secure the ‘L’ brackets with the screws provided.
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”
![](/_app/immutable/assets/EnSqxtCIZ6o.DeB4mA13.jpg)
Horizontal Split Measurement
Follow the instructions included with your Laser Measure Kit to capture the six measurements for each opening.
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.
The blue ‘L’ bar represents the Mounting ‘L’ Brackets
Double check your measurements. The ‘B’ value for the upper insert should be nearly the same as the ‘A’ value for the lower insert.
Enter all data into the Measure by Indow portal, check, and save.
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
Receive and un-package insert packaging.
If the mullion bar was not ordered pre-cut, you will need to measure the opening and cut it now.
Measure the distance across the frame at the specific location the mullion will be installed.
Measure and cut the aluminum bar using any saw with a metal-cutting blade.
Lay the mullion in place on top of the previously installed brackets.
Install the Indow inserts.
Install the TOP insert FIRST to compress the mullion down on top of the ‘L’ brackets.
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)
![](/_app/immutable/assets/Hm9csxMvffU.D6CEddF3.jpg)
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.
Determine placement of the window split. We recommend using an existing visual line, like a window frame, sash or other divide, if available.
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.**
Repeat on the bottom of the window sill.
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.
Repeat on the bottom of the frame.
Pre-drill each marked center point.
Once holes are drilled on top and bottom, secure the ‘L’ brackets with the screws provided.
On the bottom of the window frame, place the mullion sample against the installed ‘L’ bracket.
Place the other loose bracket inside the mullion channel and mark the location of the hole in the bracket.
Remove the mullion divider and pre-drill marked center point and secure ‘L’ bracket in place with the screws provided.
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
Follow the instructions included with your Laser Measure Kit to capture the six measurements for each opening.
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.
Enter all data into the Measure by Indow (MBI) portal.
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.
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:
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
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.
If the mullion bar was not ordered pre-cut, measure and cut it now.
Measure the distance across the frame at the specific location the mullion will be installed.
Apply that measurement to the aluminum bar. Cut it with any saw with a metal-cutting blade.
Place the ‘H’ bar against the previously installed brackets so one set of brackets is nested in the ‘H’ channel.
Position the second top bracket so the pre-drilled hole is properly aligned, and secure into place.
Repeat on the bottom.
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](/_app/immutable/assets/LP-Mullion_Guide-14.D_En3gB9.jpg)
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Customer Reviews
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
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!
Cynthia M
Very happy with the reduction in noise. Easy to install and use. Great customer service.
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!!
Jen F
Worth the money!
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.
Anonymous
Easy to install, and instantly stopped drafts and dampened outside noises very effectively!
Robert C
The process could've been smoother with the sending of the measuring kit. There was a delay with weather in Oregon but then it felt like my kit was still delayed due to other reasons. Most new homes (like mine) have textured paint and it can also be inside your window frames. I measured everything out but I still have little tiny gaps due to the texturing. Not indows problem but just be aware you made need to sand or fill in those gaps yourself to get a perfect seal. I did the acoustic grade inserts and they have reduced some noise but they will not completely block or significantly reduce road noise. This is most apparent with deep or lower frequencies like truck exhausts or modified street racing cars that are quite popular these days. Taking the inserts out to open your windows isn't an easy task. The little metal hook they provide needs to be either bigger or you can attach something to it as a way to leverage yourself. Overall the process was average. I think they're definitely a good option for weatherproofing and some sound reduction depending on the type of windows you have. I would purchase again.
Shelley C
The process for ordering and installing the Indow Widnow inserts was so easy. They sent the measuring information and it had great instructions. The inserts were delivered by UPS was crushed on one corner, but the company had packed them well, so there was no damage. We installed them the same day. It works best if you have two people completing the installation. We felt the results immediately and are glad we spent the money to make our home just a little more efficient.
Barry B
The insert fits like a glove and insulates the window! No more frost and condensation on the interior of the window! Thank you for an easy, effective and attractive fix!
Susan A
The support during the entire process was wonderful, thank you so much for the extra quiet space!
Volodymyr R
Very good good quality. Overall very satisfied with this product and service. Thanks
Jane H
Our home was built in 1913 and has beautiful old wavy glass single-paned windows. I’m so glad we were told about Indows and didn’t have to replace the original windows. We live in Canada but had no difficulty placing the order and were very impressed by how accurately the company met its deadlines, including cross border delivery. The Indows inserts blend in perfectly with our home and the existing window frames. You don’t see them unless you’re actually looking for them. But what we did notice was how much warmer the rooms feel the minute the inserts were installed! I can’t speak to any heating bill savings because they just went in this week, but our home feels much, much cozier. Our contractor who took all the measurements and installed the Indows says: “Excellent instructional videos: clear and informative. Very impressed with the design and the product. Overall, the ease of installation is quite impressive. Working with all the staff at the company was great – everyone was very helpful.” I’m already recommending this company to any of my friends who live in drafty older homes and want to reduce their heating/cooling bills and do some good for the environment.
Anonymous
Don't be afraid of buying an older home with single pane windows. This company has you covered. Their inserts are super high quality and fit well thanks to the laser measuring kit they send. Highly recommended!
Katie M
We purchased three Indows to reduce drafts and noise in our bedroom, which faces the street. The improvement was immediate. Our house was built in 1958 and although we have exterior storm windows we didn't want to replace the windows outright due to cost and architectural consistency. These were an excellent solution.