Mulled Windows Explained: Mulling Kits, Structural Mullions and Impact Ratings

Mulled windows are two or more separate window units joined side by side or stacked into a single assembly. The joint between them is called a mullion, and the process of joining them is called mulling. Common examples are a picture window flanked by two single-hung windows, or a row of casements. In Florida, the mullion is a structural part: it has to carry the same wind pressure as the windows, so mulled impact windows require a mullion that is tested or engineered for your design pressure.

If you are still choosing between window styles for each unit in the group, see our guide to single-hung vs double-hung windows.

Mullion vs Muntin: Clearing Up the Terms

These two words get mixed up constantly:

• Mullion: a structural member between two separate window frames. It joins units and transfers wind load.
• Muntin (grids): decorative or dividing bars within a single sash that split the glass into smaller panes. Today they are usually grids between the glass or applied to its surface.
• Meeting rail: where the two sashes of a single or double-hung window overlap. It is part of one window, not a mullion.

Common Mulled Window Configurations

• Twin or triple single-hung: two or three hung units side by side, common in bedrooms and living rooms.
• Picture window with operable flankers: a fixed center unit for the view, with single-hung or casement windows on each side for airflow.
• Stacked units: a fixed transom or arched unit mulled on top of a window or door.
• Window walls: multiple fixed and operable units combined to fill a large opening.

Window Mulling Kits vs Structural Mullions

What a window mulling kit is

A window mulling kit is a manufacturer-supplied set of parts, typically connectors, clips, a mull cover and sealant, that joins two units in the field or at the factory. For non-impact windows in low-wind areas, a basic kit mostly aligns the frames and keeps water out.

What a structural mullion is

In high-wind regions, the mullion becomes a load-bearing member. Wind pushes on the glass, the glass pushes on the frames, and the frames push on the mullion, which must carry that load to the anchors at the top and bottom of the opening without bending too far. A structural mullion is usually a heavier aluminum or reinforced section with its own anchoring clips at each end.

Integral vs separate mullions

• Integral (factory) mulled units: the manufacturer joins the units and the assembly is tested or rated as a combination.
• Field-mulled units: separate windows are joined on site with a tested mullion system listed in its own product approval.

Either approach can be correct. What matters is that the specific mullion, size and anchoring are covered by an approval or engineering for your design pressure.

Mulled Impact Windows and Design Pressure

Under the Florida Building Code, 8th Edition (2023), mullions between window and door units must be tested or engineered to carry the design wind load for the opening. In practice for impact work in Southwest Florida:

• The assembly is only as strong as its weakest part. If two windows are rated for a high design pressure but the mullion is rated lower, the whole assembly is limited to the mullion's rating.
• Longer spans mean more load. A tall mullion between two full-height units sees much more load than a short one, so tall mulls often require heavier sections.
• Anchors matter. The mullion clips must be fastened into the buck, block or framing exactly as the approval specifies.
• Impact rating applies too. In the wind-borne debris region the mullion and joined units must form an impact-resistant assembly, as covered by the product approvals.
• Paperwork: your permit package should include the approval for the windows and the approval or engineering for the mullion. Verify the current Florida Product Approval for the exact products.

Pros and Cons of Mulled Windows

Pros

• Large glass areas without one oversized, expensive unit.
• Mix fixed glass for the view with operable windows for airflow.
• Individual units are easier to handle, ship and install.
• A single damaged unit can sometimes be replaced without replacing the whole group.

Cons

• More joints to seal against wind-driven rain.
• Structural mullions add cost and can widen the sightline between units.
• Size and DP limits of the mullion can restrict the design.
• Poorly sealed mull joints are a common leak source.

What Mulled Windows Cost

Each unit in a mulled group is priced like any impact window: typically $1,200–$3,800 per unit installed in Southwest Florida in 2026. The mullion adds cost on top, and structural mullions for tall or high-pressure assemblies cost more than simple short mulls. Ask your contractor to list the mullion as a separate line so you can compare quotes fairly. For a single-hung group, our single-hung window replacement cost guide shows what moves each unit's price.

How to Spec Mulled Windows on a Quote

When you review a quote for a mulled group, look for these details:

• The product and approval number for each individual unit.
• The mullion type, its approval or engineering, and the maximum span and design pressure it is rated for.
• The overall assembly size and how it is anchored at the head and sill.
• How the mull joint is sealed and covered, inside and out.
• Whether the units are factory-mulled or joined on site.

Signs of Mullion Problems

• Water stains or soft drywall directly below a mull joint.
• A visible gap or cracked sealant between joined frames.
• Bowing or flexing of the joint on windy days.
• Rust stains from mullion fasteners in older aluminum assemblies.

If you see these, have the opening inspected. Re-sealing helps with minor gaps; structural movement usually means the assembly needs to be rebuilt to current code with an impact window installation.

Frequently Asked Questions

What does mulled window mean?

A mulled window is a combination of two or more window units joined together with a mullion into one assembly, so they install and look like a single larger window.

Is a window mulling kit enough for impact windows?

Only if that specific mulling system is approved or engineered for your required design pressure and impact rating. In coastal Florida, a basic alignment kit is not a substitute for a rated structural mullion.

Can you mull a picture window to a single-hung window?

Yes, it is one of the most common configurations. The mullion between them must be rated for the opening's design pressure.

Are mulled windows more likely to leak?

Each joint is a potential leak point, so sealing and flashing matter. A properly installed mulled assembly should perform as well as a single unit.

Can I add a mullion to existing windows?

Generally not in a code-compliant way. Mulling is done when windows are installed, using approved components for those specific products.

Bottom Line

Mulled windows let you build large, flexible window groups from standard units. In hurricane country, the mullion is a structural part of that assembly: make sure it is rated for your design pressure, anchored per its approval, and listed clearly on your quote.

Planning a window wall or combination unit? Call Prestigious Home Improvements at (941) 223-4875 for a free in-home estimate.