DIY replacement muntins or mullions for windows

Question

Recently, I started a window replacement project at my house. All of the original windows were single-pane with storm windows, and performed poorly in hot or cold weather.

Of all the replacement windows needed at the house, two units had to be custom ordered since they were much larger than what was available as ready made items. The rest of the windows were conveniently available as stock items, priced less than half of the custom ordered units. However, the stock windows lack mullions, which are required by my HOA.

Since all of the replacement windows are double pane insulated glass, the mullions would have been installed between the panes if I custom ordered them. However, I am now limited to placing the mullions on the indoor surface of the glass. These mullions have several requirements:

• maximum 6 mm material thickness
• opaque
• robust enough to resist cleaning
• convincing when viewed from indoors or outdoors

I have tried several approaches without a clear success.

Electrical tape has the proper dimensions, but is translucent and difficult to apply in straight lines. It also leaves a stubborn residue on the glass if removed.

Next, I cut strips of leftover laminate floor and then spray painted them. The laminate strips are sturdy and opaque, but don’t hold paint very well. It dries in blotches or drips, and also chips off from the plastic coated surface.

I next tried cutting strips of foam core board, the type used to construct trifold posters or presentations. These strips are lightweight and opaque, but will probably not be easy to clean. They comprise porous paper layers and a delicate foam middle, which will likely not endure scrubbing.

This approach https://thechelseaproject.com/how-to-easily-diy-custom-muntins-for-windows/ relies on custom cut moldings, which I also want to avoid if possible.

Likewise, https://www.newpanes.com/ doesn't appeal to my budget.

Has anyone else constructed their own mullions? I have found them described variously as mullions, muntins, lattices, grills, grilles and grids.

Answer 1

Since you seem happy to use "strips" of other materials, just use strips of wood, without getting into "custom-cut moldings" (since you are obviously not profiling electrical tape, foamcore, etc.) so the surface profile evidently does not matter. Plain rectangular strips of wood, notched where they cross (easily done with a handsaw and chisel, though allowing a few "practice on scrap" attempts is advisable.)

An alternate possibility is to use copper foil tape, which can be had specifically for use on glass (primarily for stained glass work) and is completely opaque. Mark both sides of the frame where it will go to aid in getting it straight (hold one end on the mark, stretch to the other mark without touching the glass, then lower one end onto the glass and roll or slide along the tape to attach it as you work towards the opposite end being held in place. Or, attach a wide strip and razor-cut with a straight-edge to get the final width, straight.