Replace a Triple Window Unit — Mulled Twins and Triples as One
You have two or three windows joined into one wide unit — triple casements, twin double-hungs or a picture window with casements — and the whole thing is failing.
Reply within one business day. Or call / text 763-204-2099.
To replace a triple window unit or a mulled twin, the whole assembly comes out in one piece and a new one goes in: two or three windows joined at the factory or with a mull kit, reinforced where needed, set level in the same opening, flashed and trimmed. Twin Cities metro to St. Cloud. Priced per window in the unit; photo reply within one business day, or book a free in-home estimate.
- Priced per window in the assembly
- Replaced as one joined unit
- Configuration can change in the same opening
A mulled unit is two or more windows joined into one frame at the factory. From outside it looks like one wide window; behind the trim there is one rough opening and one header. Replacing one window in the middle of it rarely works: the new frame does not match the old sightlines, and the mull joint is where leaks start.
So the assembly is replaced as one. We measure the full opening, order the new windows joined the same way, and add mull reinforcement where the width or height calls for it. The old unit comes out in one piece with enough hands to carry it, the opening is checked for rot and level, and the new assembly is set on blocks, shimmed, fastened and flashed across its full width.
It is also the time to change the layout. Three casements can become a picture window between two casements, or the reverse, inside the same opening. Bedroom units are checked so at least one window still meets egress.
Get a written quote for this job
Three photos and a sentence is enough: a reply within one business day, or ask for a free in-home estimate. Or text the photos to 763-204-2099.
What you supply
- The layout you want: same as now or a new combination
- Frame color, glass and grille pattern
- Furniture and window coverings cleared from the wall
What JMB supplies
- Measure of the full opening and each window position
- New windows ordered joined, with mull reinforcement where needed
- Removal of the old assembly in one piece
- Sill pan, flashing and sealant across the full width
- Assembly set level, shimmed and fastened
- Interior and exterior trim, with one continuous stool if you want it
- Haul-away and photos in your job folder
How it is priced: per window
Quoted in writing from your photos, with a reply within one business day. What moves the number:
- Number of windows in the assembly and their styles
- Overall width and height, and mull reinforcement
- Changing the layout vs. matching it
- Insert vs. full frame
- Rot at the sill or header found at the measure
- Second-story or hard-to-reach location
Send a photo of the whole unit from inside and outside, and tell us which windows open. A measurement across the inside trim helps.
How long it takes
Most mulled units are replaced in one day once they arrive. Lead time depends on the brand and is written on the quote.
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 reply within one business day. Prefer a look in person? A free in-home estimate is always an option.
- 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
Triple casements
Three cranking windows in one frame. All three can open, or the middle one can become fixed glass for a wider view.
Picture window with casements
Fixed glass in the middle, an opening casement on each side. The flankers give air and egress; the center gives the view.
Twin double-hungs
Two double-hungs mulled side by side, often in a bedroom or dining room. Can stay double-hung or change to casements in the same opening.
Replace a Triple Window Unit — Mulled Twins and Triples as One: questions, answered
Can you replace just one window in a mulled triple unit?
It is possible on some units but rarely worth it. The new frame will not match the old sightlines or color, and the mull joint next to it is usually what has failed. Replacing the assembly as one gets a single flashed opening and matching frames.
What is a mulled window?
It is two or more windows joined side by side or stacked into one unit, usually at the factory. The joint between them is the mull. A mulled unit sits in one rough opening under one header.
Can I change my triple window to a different combination?
Yes, as long as the new assembly fits the same opening. Common changes are three casements to a picture window with casements, or twin double-hungs to twin casements. In a bedroom, at least one window must still meet egress.
Is a wide mulled window too heavy to replace?
It is heavy, which is why it is planned. The old unit is cut free and carried out with enough people, and the new one is set on blocks and shimmed before it is fastened. Very wide units get mull reinforcement so the joints do not flex.
Does a triple window need a new header when replaced?
Not if the new unit fits the same opening and the existing header is sound. We check for sag at the measure. If the opening is being widened, the new header is sized for it, and a header that size is sized by an engineer when needed.
How is a mulled window unit priced?
Per window in the assembly, with the mulling and any reinforcement listed. A triple unit is three windows on the quote plus the joining, so you can compare it line by line.
Recent JMB work, from our job log
59 installs recorded across 30 Twin Cities communities since Dec 2025. Window and door installs for homeowners and the builders we work with. Counts come straight from our job log; no customer names.
Related jobs we do
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.
Read morePicture Window Replacement — Large Fixed Glass Set and Trimmed
A fogged living room window, a picture window that frosts at the bottom, or an old unit you want to open up with casements. We replace it and finish the trim.
Read moreCasement Window Replacement — Crank-Out Windows Measured and Installed
The crank strips, the sash will not pull tight, frost builds on the glass edge. When the casement itself is done, we replace it and finish the trim.
Read moreBay Window Replacement — Bay and Bow Units, Seat, Roof and Trim
The seat is cold, the roof over it leaks, the glass is fogged or the whole unit has dropped. We replace the bay or bow and finish it inside, outside and on top.
Read moreEgress Window Installation — Basement Opening Cut, Window and Well
A basement bedroom needs a way out. We cut the wall, set the window and the well, and finish the inside so the room is legal and usable.
Read more1.5 Story Dormer Window Replacement — Upstairs Knee Wall Bedrooms
The upstairs windows in the dormers are drafty and hard to reach, and the bedrooms up there are the coldest rooms in the house.
Read morePage reviewed October 9, 2026 · JMB Builders LLC, MN license BC809005 · All windows pages
Where we work
Window Replacement & Installation near you
Based in Zimmerman, scheduling across the Twin Cities metro. Closest first: