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What is an Astragal?

Tuesday February 27th, 2024
When diving into a DIY door or window project, you’ll soon realize these everyday objects have multiple components. Each component adds functionality, increases insulation, protects from the outdoor elements, or enhances aesthetic value. Such is the case with an astragal—this important door and window part has multiple functions, and comes in a variety of styles, including: 

8 Types of Astragals on Doors

  1. Overlapping Astragal
  2. Flat Bar Astragal
  3. T Astragal
  4. Z Astragal
  5. Split or Meeting Astragal
  6. Brush Astragals
  7. Acoustic Astragals
  8. Fire-Rated Astragals 
Astragals aren’t a well-known door or window part. As such, this is likely a new term for you, which is why we’ll dive into everything you need to know about astragals, including their unique function in doors and aesthetic value in windows. Because there are different types of astragals, we’ll discuss each in length so you’ll know which door astragal is best for your project. 

What is an Astragal?

Astragals have differing uses in doors and windows. However, this article explores the astragal’s use in doors more so than in windows because door astragals have more variances and functions than window astragals. 

A window astragal is another term for a window grille, which are the wood or metal bars dividing the window plane into four or more sections. For windows, astragals are more of an aesthetic touch, and are reminiscent of historical architecture. Door astragals have a much different purpose. 

A door astragal, also referred to as astragal molding, is a vertical piece of door hardware used to completely seal the thin space between the door edge and the door frame. In a double door system, astragals seal the gap between the two doors or leaves. This unique door hardware increases security and privacy while protecting the interior from drafts, moisture, and other outdoor elements. 

Astragals aren’t on all doors—they are most common on exterior double doors, such as French doors, as well as storm doors, commercial steel doors, and fire rated doors. 

Most door astragals are made of metal (aluminum), but can also be wood or heavy-duty plastic, such as polypropylene or vinyl. How an astragal functions and affects the door depends on the type of astragal, which we’ll dive into more below. 

What is the Difference Between a Mullion and an Astragal? 

Mullions and astragals perform similar functions in double doors and single doors, as they both seal the clearance gap between the two doors or the door and the door frame. However, they differ in their location. 

Mullions are distinct from astragals because they are attached to the door frame, whereas astragals are attached to the door itself. In a double door construction, the mullion is the fixed bar between the two doors attached to the frame. They are most prevalent on commercial steel doors and sliding doors. 

8 Types of Astragals on Doors

There are a few different types of door astragals, and they differ in shape, function, and attachment style. The type you choose will depend on your needs and door performance preferences. 

Before deciding which type you need, you may want to brush up on your knowledge of the different parts of a door so you can better understand how an astragal is mounted. 

1. Overlapping Astragal 

An overlapping astragal is the most common type of exterior double door astragal. As such, it is one of the main astragal categories, and within this category are a few specific astragal types. 

An overlapping astragal door seal is mounted on the active door of a double or French door system. When the doors are closed, the overlapping astragal on the active door will overlap the other door, sealing the gap between the doors on the exterior surface. 

The “active” door is the operating door, which must open first for the other “inactive” door to open. In this case, the active door is the door most frequently used. If both doors are open, the inactive door must close first, followed by the active door, so the doors properly latch into place. If both doors are active, a door coordinator is required for proper door function. 

Overlapping astragals are highly secure, as they cover the gap on the exterior/secure side of the doors. Because of this placement, the astragal helps prevent tampering with the latch bolt, making the doors difficult to break into. 

The downside of overlapping astragals is that the active door must open first, and it has to be an outswing door. Certain commercial door hardware codes also prohibit overlapping astragals or have specific requirements for installation. If you’re working on a commercial door, it’s important to research your region’s building codes to ensure overlapping astragals are within the code. 

2. Flat Bar Astragal

One of the most common overlapping astragals used in residential buildings is the flat bar astragal. The simple design of this door astragal consists of a flat piece that is mounted to the outside surface of the active door via screws. 

The flat bar extends out from where it is screwed into the active door so that when the doors are closed, the gap between them is sealed. 

The flat bar astragal is considered standard and can be wood, aluminum, or vinyl. This door astragal is best for residential and low-traffic areas. 

3. T Astragal 

A T astragal is a specific type of overlapping astragal, and is shaped like a “T” with a thin, vertical edge that sticks out from the center of the astragal. When closed, the extruding astragal piece compresses between the two doors, effectively closing the gap between them and creating a continuous seal. 

As an overlapping astragal, a T astragal is mounted to the exterior surface of one of the doors on a single active double door system. They are available in aluminum, wood, or heavy-duty vinyl.  

This is a great French door astragal option, as it smoothly closes the gap between the doors, and the wood, aluminum, or metal can be painted to match the door’s style. 

4. Z Astragal

A Z astragal is another type of overlapping astragal, but it differs in that it is often mounted to the inactive door, rather than the active door, in a double door system. This astragal is shaped like a “Z,” with the hardware completely covering the door’s edge using a piece that extends out to seal the gap between the doors. 

Because the astragal covers the length of the door, it will come with holes prepped for the strike plate, flush bolt, and screws. It can also be mounted to the active door if needed. If mounted to the active door, it will come prepped for the locks, strike plate, and screws. 

The Z astragal is highly durable and secure, making it a great option for enhanced security. Because the strike plate is part of the entire astragal piece, it is extremely difficult to break the strike plate, making the Z astragal the preferred choice to prevent break-ins. 

5. Split or Meeting Astragal 

A split or meeting astragal functions differently than the overlapping astragal, and is the other astragal commonly used for exterior double doors. A split astragal, as the name implies, is comprised of two pieces of hardware that attach to each door leaf. 

Both astragal pieces have a gasket that meets in the middle when the door is closed. These gaskets close firmly together, sealing the doors and preventing moisture, air, smoke, or insects from entering the building. 

That said, how the two pieces fit together in the door gap depends on the specific type of meeting astragal you purchase. Just as with overlapping astragals, there are a variety of double door astragal types within the split astragal category. 

Some meeting astragals join together via spring-adjusted seals or with magnetic strips. Thoroughly reading the product description of the meeting astragal you’re researching will help you identify how it functions. 

Because of the design of meeting astragals, they aren’t as visible from the outside of the door, creating a seamless look. They are also ideal in situations where you want both doors to be active. In other words, both doors can open and close independently of each other unless there is other double-door hardware that prevents it. 

The downside to split or meeting astragals is that they are slightly less secure than overlapping astragals. However, they are necessary in situations where an overlapping astragal won’t work, such as for openings with vertical rod exit devices on each door leaf or push/pull door pairs. 

6. Brush Astragals 

Brush astragals can be a type of split or meeting astragal where the doors seal via sturdy bristles coming together. 

Typically, brush astragals are used on single or double doors, and are mounted on the top or bottom of the door. They are used to seal the gap between the door and the header, as well as the door and the ground. They are especially common for large exterior storm doors, garage doors, and exterior barn doors. The thick bristles prevent air, moisture, debris, and critters from entering an interior space. 

Brush astragals are common for exterior and interior barn doors as they close the “barn door gap.” This gap is a small section between the door jamb and the barn door edge. Mounted brush astragals add privacy and sound reduction for interior barn doors. 

7. Acoustic Astragals 

Acoustic astragals contain a thick gasket that is used to seal the gap between double doors. Because of the thick gasket, acoustic door astragals are specifically used to seal out noise, hence their name. They are extremely effective at sealing out drafts as well. 

Acoustic astragals are designed as both overlapping and meeting astragals, depending on whether there are two active doors or one active door. They are commonly used in commercial buildings, especially government-funded buildings, such as schools and hospitals. 

8. Fire-Rated Astragals 

Certain astragal types are specifically designed for fire-rated doors.  A fire rated door is a door that is made specifically to withstand fire for a certain amount of time. They provide a buffer to slow the fire from spreading, so long as the doors are closed from one room to the next.

Because of their specific function, fire rated doors have a number of hardware requirements to ensure they can withstand heat and fire. As such, certain astragal types are intentionally designed for commercial fire rated doors, including steel doors and hollow metal doors. 

Fire-rated astragals are extremely heavy-duty, and typically made out of aluminum. They are designed to meet fire rating standards and comply with IBC, NFPA 80, and NFPA 252. 

If you’re purchasing an astragal for double doors in a commercial setting, be sure to check whether you need a fire rated astragal to ensure you stay compliant with fire rating codes. 

Door Hardware 101: Astragals for Energy Efficiency, Security, and Protection

An astragal is an uncommon door hardware piece that is key for effectively sealing the gap between double doors. 

With the right astragal for your double doors, you can prevent drafts, moisture, smoke, and insects from entering your home. In a commercial setting, astragals are ideal for increased security, noise reduction, and fire safety. 

There are a variety of astragals, but by understanding each of their unique structures and advantages, you can choose the perfect astragal type for your door DIY projects.  

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