Mullion Trim Between Windows — Mull Covers for Twin Units
Two windows set side by side with a strip of foam or bare framing showing between them. We cover the mull and trim the pair as one window.
Mullion trim between windows is the cover over the joint where two window frames meet: a flat mull cover, a piece of casing, or a wood jamb extension that spans the gap. JMB Builders installs mull covers and cases twin and triple windows as one unit across the Twin Cities metro, with a single continuous stool and matching casing. Priced per window, quoted from your photos within one business day.
- Priced per window in the group
- Mull cover, shared casing and one continuous stool
- Quote from photos in 1 business day
- Licensed MN contractor BC809005
When two windows go in side by side they are either mulled at the factory, joined on site with a mull kit, or set separately with a stud between them. On the inside all three leave the same thing: a vertical strip between the frames that is narrower than a normal jamb and has nothing to nail casing to. The installer foams it and moves on. Trimming that strip so the pair reads as one window is the job.
The width of the strip decides the trim. A factory mull is thin, and a flat mull cover ripped to width and set between the two frames closes it. Two windows with a stud between them leave a wider gap that takes a jamb extension, and then a piece of casing on the face that matches the outside casing. Get the width or the reveal wrong and the pair looks like two windows that happen to be near each other.
JMB Builders trims twin, triple and ganged windows across the Twin Cities metro. We rip the mull cover to the exact gap with an even reveal on both frames, run one stool across the whole group with horns at the outside ends, and case the outside with the same profile as the rest of the room. Painting or staining is quoted as a separate line, or leave it primed and caulked and paint it yourself.
What you supply
- A straight-on photo of the pair and a close-up of the gap between the frames, with a tape across it
- A photo of another window in the house that has the casing you want matched
- Whether the outside is already trimmed or the whole group still needs jambs and casing
- Your paint or stain color if we are finishing it; painting is quoted as a separate line, or paint it yourself
What JMB supplies
- A mull cover ripped to the gap in paint-grade or stain-grade stock, or a jamb extension when the gap is wide
- A center casing or flat stock on the face of the mull in the same profile as the outside casing
- One continuous stool across the group with horns at the ends, and an apron under it
- Jamb extensions and casing at the outside of the group when the pair was left bare
- Foam trimmed flush, backer rod and sealant in a wide mull gap before the cover goes on
- Photos of the finished group in your job folder
- Painting or staining when you want it done, quoted as its own line
How it is priced: per window
Quoted in writing from your photos within one business day. What moves the number:
- Two windows vs. three or more in the group
- A thin factory mull vs. a framed gap that takes a jamb extension and a center casing
- Whether only the mull needs a cover or the whole group still needs jambs, stool and casing
- Paint-grade vs. a stain-grade species matched to the rest of the trim
- Frames that were set out of plane with each other, which takes shimming behind the cover
- Matching an older casing profile at the center and the outside
- Number of mulled groups in the house
A tape measure across the gap between the frames in the close-up tells us whether it is a mull cover or a jamb extension, and that sets the price.
How long it takes
A mull cover on a pair that is already cased: about an hour. A twin or triple unit trimmed from bare — jambs, mull cover, stool, apron and casing — two to three hours. A house full of ganged windows in a day or two.
How your project works
- 1
Send photos, get a written quote
Send three photos of the space and a sentence about what you want. You get a clear, itemized quote within one business day. We only come out to measure first when the job needs a tape measure — no pressure, no games.
- 2
Pick materials, or bring your own
We help you choose profiles and materials that fit the house and the budget, or match your existing millwork exactly — or you buy the door or trim yourself and we install it. Everything is confirmed before the crew is scheduled.
- 3
Clean, precise installation
The crew shows up on time, protects floors and furniture, and installs with tight joints and consistent reveals. Photos of the finished work go in your job folder.
- 4
Walkthrough and invoice
We walk the finished work with you. If anything is not right, we fix it before we call the job done. Then one invoice, paid by check or direct deposit — no card fees, no payment links.
Situations and options
Flat mull cover on a factory mull
The frames are joined tight and the gap is narrow. A flat piece ripped to the width, with an even reveal on both frames, glued and pinned into the mull. The casing on the face, if any, is a single narrow piece. This is the common version on new builds.
Jamb extension and center casing on a framed gap
The windows were set with a stud between them. We fit a jamb extension to the gap so it reads like the side jambs, then run a center casing on the face in the same profile as the outside casing. The stool runs under both windows.
Case each window separately
When the gap is wide enough for two full casings with wall between them, each window gets its own casing and stool. It is the right call when the windows are more than a casing width apart, and the wrong call when they are close.
Drywall return between windows
On a drywall-wrapped group the mull is finished with a strip of drywall and corner bead, or a thin wood mull cover set flush, so the pair matches the drywall returns on the outside.
Mullion Trim Between Windows — Mull Covers for Twin Units: questions, answered
What is a mull cover on a window?
It is the trim piece that covers the mullion, the joint where two window frames are fastened together. Factory-mulled windows come with a plastic or aluminum mull cover on the outside; the inside is left for the trim carpenter. An interior mull cover is a flat piece of wood or PVC ripped to the width of the gap, set between the frames with the same reveal on both sides, and cased on the face if the outside casing calls for it.
How do you trim two windows that are right next to each other?
As one unit. The mull between them gets a cover or a jamb extension depending on the width, one stool runs across both with horns only at the outside ends, and the casing wraps the outside of the pair with a center piece over the mull. Casing each window on its own only works when there is enough wall between them for two full casings, and it usually looks busier.
The gap between my twin windows is bigger on one side than the other. Can that be trimmed?
Yes, but it is fitted rather than ripped straight. One frame was set slightly off the other, so the mull cover is scribed to follow each frame and the reveals are adjusted so they read even from the room. If the frames are out of plane, the cover is shimmed so it does not rock. The photos usually show it, and we allow for it in the quote.
Should the stool run across both windows or stop at each one?
Across both. One continuous stool with horns at the outside ends is what makes two windows look like one unit, and it is stiffer than two short stools meeting at the mull. The apron under it runs the full width as well. The only time each window gets its own stool is when they are far enough apart to be cased separately.
Can you match the mullion trim to the casing on my other windows?
Yes. Send a photo of a cased window in the house. The center casing over the mull is cut from the same profile as the sides, or from flat stock that matches a flat casing. On a stain-grade house the mull cover and center casing are the same species, sanded and ready for stain. Painting or staining is its own line on the quote.
Does the outside of the mull need anything?
Factory-mulled units come with an exterior mull cover and it should already be on. Windows joined on site need the exterior gap flashed and covered before the siding or trim closes over it. If your exterior mull is open or the cover is missing, send a photo; it is a separate small job and we can do it in the same visit.
Get a written quote for this job
Three photos and a sentence is enough. You hear back within one business day. Or text the photos to 763-204-2099.
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Read morePage reviewed September 24, 2026 · JMB Builders LLC, MN license BC809005 · All window trim pages
Where we work
Window trim near you
Based in Zimmerman, scheduling across the Twin Cities metro. Closest first: