French Doors: Interior vs Exterior, Sills, Seals and Security

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Few upgrades change a room as quickly as a pair of glass-paneled doors. French doors bring in light, open a dining room to a patio and make a small den feel connected to the rest of the house. They also introduce more moving parts than a single swinging door: two leaves that must meet cleanly, a center seal, a long sill that has to shed water, and a lot of glass that has to meet safety rules. This guide covers the differences between interior and exterior units, why the swing direction matters for water, how sills and flashing keep rot out of the floor, how multipoint locks and astragals affect security, what the federal safety glazing standard covers, why wider openings may need a bigger header, and what installed costs commonly look like in 2026. Prices throughout are broad ranges that vary with size, material, region and installer.

Part of our Structure & Exterior guide — start there for the full picture →

What Are French Doors and How Do Interior and Exterior Units Differ?

French doors are hinged doors with glass panes, called lites, running most of their height. The classic version is a pair of doors that swing from the outside jambs and meet in the middle with no fixed post between them, but the term is also used for a single glass-paneled door or for a pair where one leaf is fixed and the other operates.

Interior French Doors

Interior units separate rooms such as an office, dining room or sunroom from a hallway or living area. They usually use single-pane tempered glass, a lighter frame and simple hinges with a ball catch, roller catch or passage latch. There is no sill, no weatherstripping and no flashing to worry about. Their main challenges are fit, alignment and sound.

Exterior French Doors

Exterior French doors, often sold as patio doors or garden doors, are a weather barrier. They have insulated glass units, a sill with a threshold, compression or bulb weatherstripping, a center astragal and a lockset designed for security. Frames come in wood, fiberglass, steel, vinyl and aluminum-clad wood. Because they sit in an exterior wall, they need proper flashing and a sill pan, a properly sized header and careful air sealing. Swapping an interior unit into an exterior opening is a common mistake that leads to drafts, warped leaves and water damage at the floor.

In-Swing vs Out-Swing and How Water Gets Managed

Most exterior doors in American homes swing inward. In-swing French doors are familiar, keep the hinge pins inside where they cannot be removed from outdoors, and do not collide with storm doors, screens or furniture on a patio. Their weakness is water. When wind drives rain against an in-swing door, the weatherstripping is pushed away from the frame, and the sill relies on a raised threshold, a sweep and drainage paths to keep water out.

Out-swing doors are pushed against their stops by wind pressure, which tends to tighten the seal, and they free up floor space inside. They are common in coastal and high-wind regions. On the other hand, they need security hinges or non-removable pins because the hinges are exposed outside, they can be blocked by snow drifts or furniture, and a screen has to be mounted inside or use a retractable design.

Along the Colorado Front Range, wind-driven snow and spring storms are more common than hurricane rain, so in-swing units dominate. Either style can work well when the threshold is the right height, the sill slopes to the exterior and the opening is protected by an overhang or porch roof where possible.

Checking the Threshold and Sweep

Signs of trouble at the bottom of an exterior French door include daylight under the leaves, a sweep that no longer touches the threshold, staining on the subfloor or finished floor inside, and cupped hardwood or soft spots near the sill. Many thresholds are adjustable by turning screws along their length. If the threshold itself is cracked or worn, our guide to door threshold replacement covers the repair and when a full sill replacement is the better call.

Sill Pans, Flashing and Rot at the Sill

Water rarely enters through a door leaf. It usually gets in around the frame, at the corners of the sill and through the gap between the sill and the rough opening. A properly installed exterior French door sits on a sill pan, which is a formed or site-built tray with an upturned back leg and end dams that collects any water that gets past the threshold and directs it back outside. Flashing tape or membrane is lapped shingle-style at the sides and head so that water moving down the wall stays on the outside of the weather-resistive barrier.

Older installations, and some newer ones done in a hurry, skip the pan and rely on caulk. Caulk eventually cracks, and water that gets under the sill soaks the subfloor and rim joist below. Because the damage is hidden under the flooring, it can progress for years.

Warning signs include:

• Soft or crumbling wood at the bottom of the jamb legs or brick mold
• Paint that peels repeatedly at the lower corners of the frame
• Dark staining or a musty smell in the basement or crawl space below the door
• Loose or springy flooring right inside the threshold
• Gaps where siding, trim or stucco meets the door frame

Rot that stays in the trim can often be cut out and patched. Rot that has reached the sill, subfloor or framing is a bigger job. Our wood rot repair guide explains how contractors scope that damage and when epoxy consolidation, dutchman patches or full replacement makes sense.

Multipoint Locks, Astragals and Security

A pair of doors with no center post presents a security question: what does the active leaf latch into? The answer is the passive leaf, which is held closed by flush bolts or surface bolts at the top and bottom and covered by a vertical strip called an astragal. The astragal seals the gap between the leaves and gives the active door something to latch and lock against.

That arrangement is only as strong as the bolts holding the passive leaf. If a top bolt does not fully engage the head jamb, or a bottom bolt barely reaches the sill, a firm push can open both doors at once.

Multipoint Locking Hardware

Many modern exterior French doors use multipoint locks. Lifting the handle engages hooks or bolts at several points along the edge of the door, often at the top, middle and bottom, before the key or thumb-turn locks the system. The benefits are better security and a tighter weather seal, because the door is pulled evenly against its weatherstripping along its full height. The trade-offs are cost and repair complexity. Multipoint mechanisms are often proprietary, and replacement parts may need to come from the original manufacturer.

A few other security checks are worth making:

• Strike plates and hinges anchored with long screws that reach the framing, not just the jamb
• Non-removable hinge pins on out-swing doors
• Laminated or impact-resistant glass if break-in through the glass is a concern
• A deadbolt or multipoint system that cannot be opened by reaching through a broken lite, where local fire and egress rules allow

Weatherstripping and Air Sealing

Two leaves meeting in the middle create more seal length than a single door, and every inch is a possible leak. Exterior French doors depend on compression or bulb weatherstripping along the jambs and head, a seal on the astragal, and sweeps or door-bottom seals against the threshold. When those seals flatten, tear or shrink, drafts and dust follow.

ENERGY STAR’s guidance on sealing behind window and door trim lists drafts around a closed door, visible gaps around the trim, pests and dust entering through gaps, outdoor odors and peeling paint around the frame as common signs of air leaks. The same resource describes a paper test, closing the door on a slip of paper and checking whether it slides out easily, as a quick way to judge whether the seal is tight. It also notes that the space between the wall framing and the door jamb, hidden behind the trim, is often poorly sealed and can be filled with appropriate sealant or low-expansion foam.

Replacing worn weatherstripping is usually inexpensive, but many exterior French doors use kerf-mounted seals specific to the manufacturer. Our guide to weatherstripping doors covers the common profiles and how to match them. Lead paint is a consideration on older doors and trim; ENERGY STAR points homeowners in pre-1978 homes to EPA’s Renovation, Repair and Painting resources before disturbing painted surfaces.

Safety Glazing in French Doors

Because French doors are mostly glass, the glass itself is a safety feature. The Consumer Product Safety Commission’s Safety Standard for Architectural Glazing Materials, 16 CFR Part 1201, was established in 1977 and sets testing requirements for glazing used in certain architectural products. According to CPSC’s final rule briefing package amending that standard, the covered products include storm doors or combination doors, doors (both exterior and interior), bathtub and shower doors and enclosures, and patio-type sliding glass doors. Those materials must pass impact testing, and the standard covers tempered, laminated and certain plastic glazing.

In practice, new French doors from mainstream manufacturers ship with tempered or laminated glass, and the glass is usually marked with a small permanent label, often called a bug, in one corner. The concern is older doors, salvaged doors and site-made doors that may have ordinary annealed glass, which breaks into large sharp shards. Homebuyers looking at an older house with original glass doors can check each lite for a safety glazing mark. A missing mark does not prove the glass is unsafe, but it is a reason to ask questions or consult a glazier.

Replacing a cracked lite in an insulated French door is often possible without replacing the whole door. Our window glass replacement guide covers how glaziers handle insulated glass units and what affects that cost.

Headers and Wider Openings

Converting a standard 36-inch exterior door or a window into a pair of 30- or 36-inch French doors widens the rough opening considerably, sometimes to six feet or more. If the wall is load-bearing, the header over that opening must be sized to carry the roof, ceiling or floor loads above across the new span. An undersized header can sag over time, which shows up as doors that rub at the top center, astragals that no longer align and cracks in drywall above the corners of the opening.

Whether a header needs to be upsized, and what size it should be, depends on the span, the loads above, the lumber or engineered beam used and local code tables. In many jurisdictions this work requires a building permit, and some cases call for an engineer’s input. Our load-bearing wall guide explains how to tell which walls carry load and why temporary shoring matters during the cut.

Interior French doors in non-bearing partition walls still need a header of some kind to frame the opening, but it carries much less load.

Energy Performance and ENERGY STAR Doors

Glass conducts heat far more readily than an insulated door slab, so a wall of French doors changes how a room heats and cools. ENERGY STAR explains that its certified windows, doors and skylights are made by an ENERGY STAR partner, are independently tested and certified by the National Fenestration Rating Council (NFRC), and have ratings that meet EPA efficiency guidelines. For doors specifically, ENERGY STAR notes that criteria are based on glazing level, meaning how much of the door is glass, along with NFRC-certified ratings. The program also notes that even doors with lots of glass have lower glass-to-frame ratios than windows or skylights.

ENERGY STAR’s buying guidance recommends determining the climate zone, using its product finder, looking for the ENERGY STAR label next to the NFRC label, asking contractors to break quotes into labor and materials, and getting quotes from several installers. It also notes that certified products may cost more but that labor should be comparable.

West- and south-facing French doors along the Front Range can get significant afternoon sun. Solar heat gain coefficient (SHGC) ratings on the NFRC label help compare how much solar heat each unit lets in.

What Do French Doors Cost?

Costs vary widely by size, material and brand, so these broad 2026 ranges are only a starting point:

• Interior French doors, prehung pair: often a few hundred to around $1,500 for the unit, plus labor that may run several hundred dollars or more, especially if an opening must be enlarged.
• Exterior French doors, standard vinyl, steel or fiberglass: units commonly start around $1,000 to $2,500 and installed prices often land somewhere between $2,000 and $5,000.
• Premium wood or clad-wood exterior units with multipoint locks: installed prices can reach $5,000 to $10,000 or more.
• Structural changes: widening an opening in a load-bearing wall, new headers, siding and interior finish work, and permits can add several thousand dollars.

Rot repair at the sill, new flooring at the threshold and lead-safe work in pre-1978 homes are common add-ons. For comparison with single-door projects, see our guides to exterior door replacement cost and interior door replacement cost.

What Home Inspectors Look for at French Doors

The American Society of Home Inspectors (ASHI) Standard of Practice lists exterior doors among the exterior items an inspector inspects and a representative number of doors and windows among interior items. At French doors, that inspection often includes:

1. Operation: both leaves open, close and latch without binding, and flush bolts engage.
2. Seals: weatherstripping and astragal seals are intact and contact the door.
3. Glass: cracked lites, fogged insulated units and the presence or absence of safety glazing marks.
4. Sill and threshold: slope, condition, gaps and signs of water entry.
5. Exterior trim and flashing: rot, failed caulk and missing head flashing.
6. Interior clues: stained or cupped flooring, ceiling stains below, and drywall cracks at the head corners.

Our guide to hiring a home inspector explains what a general inspection covers, and the structure and exterior hub collects related guides on framing, siding and doors.

Choosing and Maintaining French Doors

A few practical habits help French doors stay tight and weathertight for decades:

• Choose exterior-rated units for any opening to the outdoors, and match swing direction to exposure and patio layout.
• Insist on a sill pan and shingle-lapped flashing in the installation scope, and ask how the installer will tie into existing siding or stucco.
• Check flush bolts, hinge screws and the strike every season, since a sagging leaf quickly misaligns the astragal.
• Refinish wood doors and frames before the finish fails, paying attention to the bottom rail and the lower corners of the frame.
• Watch for fog between panes, which signals a failed insulated glass seal.

References

• Final Rule to Amend the Safety Standard for Architectural Glazing Materials (16 CFR Part 1201) — U.S. Consumer Product Safety Commission
• Residential Windows, Doors and Skylights — ENERGY STAR
• What Makes It ENERGY STAR? Windows, Doors and Skylights — ENERGY STAR
• Sealing Behind Window and Door Trim Project — ENERGY STAR
• ASHI Standard of Practice for Home Inspections — American Society of Home Inspectors

Seeing stained floors or soft wood by the patio doors of a house you are considering? Reach out for an inspector referral and get the sill, flashing and glass checked before closing.