How to Build and Install Custom Window Mullions (Grids)

May 22, 2025 - By 4idiotz

Article Summary

The article is about how the author added window grids to their home, and the process they went through to choose the right materials and install them. They were inspired by the traditional mullion grids but wanted to create a DIY version for a more affordable option. The author chose polystyrene vinyl screen mouldings from Home Depot and provides a step-by-step guide on how they installed the grids on their windows. The process is simple, easy to follow, and results in a polished, aesthetically pleasing look for the windows.

What This Means for You

You can achieve the look of traditional mullion grids without the high cost of replacement windows.
The process of installing the grids is simple and easy to follow, making it accessible to many homeowners.
You have the flexibility to choose the material that best fits your needs, whether it’s wood, vinyl, or another material.
Dividing the windows into smaller panes does slightly reduce the amount of natural light, but it doesn’t impact the structural integrity of the window. It’s a personal preference and should be considered carefully.

Original Post

With this in mind, I decided to add my own window grids. I will be honest with you – I didn’t know where to start. Naturally, I took to the internet and was so disappointed by what I found. I found tutorials that either were too complicated or looked really, really cheap. So I did what I normally do with projects – drank coffee and just thought on it. I walked around the house and looked at the windows with grids. Then I realized the grids were simply made up on two vertical pieces and 6 horizontal pieces. With that bit of knowledge, I headed to Home Depot to find the perfect trim.

Quick note: traditional mullion grids as made by manufacturers are thin strips of aluminum between the panes of glass on a double-pane window to make for easy cleaning and energy efficiency. These DIY windowpane dividers are easily removed for cleaning the glass panes and saved me a bunch of money by making my own grilles and giving me the aesthetic appeal I wanted. Homeowners who want the true architectural style of the window muntins grids can talk to a window manufacturer to retrofit or replace their windows.

Dividing the windows into smaller panes of glass does slightly reduce the amount of natural light in the room, but this DIY doesn’t impact the structural integrity of the window, so consider your personal taste and make an informed decision about what you like for your windows.

Step 1: Choosing the Mullion Grid Material

While at HD, I looked at a ton of trim. I thought about wood. The only problem is that the pieces on the front are not wood, so they wouldn’t match. After a bit of searching, I came up with these polystyrene vinyl screen mouldings: