Chicago Window Expert

Nobody knows more about windows.

  • By Mark Meshulam
    Architects and engineers, take this course for PDH credit here.

    Types of Windows

    First let’s start with a couple of charts that show window types organized by category, with architectural symbols for these window products:

    Chart of window types: fixed windows, single hung windows, double hung windows, sliding windows and sliding doors

    Window chart showing window types including fixed windows, single and double hung windows, sliding windows and doors. Hung and sliders use sweep type weatherstripping

    Window types: Project-out awning windows, project-in hopper windows, inswing casement windows, outswing casement windows, balcony doors, top hung inswing windows, tilt turn windows, pivoted windows and jalousie windows

    This window chart shows windows types that use compression weatherstripping, including project-out awning windows, project-in hopper windows, inswing casement windows, outswing casement windows, balcony doors, top hung inswing windows, tilt turn windows, pivoted windows and jalousie windows (if weatherstripped). Compression weatherstripping is compressed when the window is closed, creating a tight seal

    Fixed Windows

    The simplest type of window is a fixed window. It has no moving parts and does not open. It usually consists of a single frame, within which glass is installed. Residential fixed windows are sometimes called picture windows.

    Single and Double Hung Windows

    Wood double hung with ropes, pulleys and counterweights

    Wood double hung window showing rope, pulley and counterweight

    Single and double hung windows are named “hung” because the original design involved having the operable sash hung from ropes or chains which then were placed over a pulley at the top of each jamb. The rope or chain then was attached to a counterweight concealed in the jamb, allowing the sash to be easily raised and to stay at the height to which it was raised.

    These days, numerous styles of balances are available, but they usually involve springs rather than counterweights. Double hung windows have the top and bottom sash operable. Single hung windows only operate the bottom sash. The horizontal rail where the upper and lower sash meet is called the meeting rail.

    Because the sash of hung windows slide in the frame, sweep-type weatherstripping is typically used.

    Sliding Windows and Doors

    Sliding windows and doors utilize wheels or rollers at the bottom of the operable portion, known as the sash. The sash travels horizontally in tracks at the top and bottom of the master frame. Usually only one of the sash operate, however when both sash operate, the window is called a “double slide.”

    Due to the presence of a track at the sill of a sliding window or door, water can accumulate in the track. These products must have provisions to “weep” the water to the exterior. This poses a design challenge because weep holes can conduct air infiltration to the interior.

    Because the sash of sliding windows and doors slide in the frame, sweep-type weatherstripping is typically used.

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    Projected Windows and Doors

    This family includes project-out awning windows, project-in hopper windows, inswing casement windows, outswing casement windows, balcony doors, top hung inswing windows, tilt turn windows, pivoted windows and jalousie windows. In architectural drawings, operation of all windows and doors that swing (as opposed to slide) is indicated by dotted lines that converge at an edge of the window, forming a V shape. The point of this V is the theoretical hinge location. I use the term “theoretical’ because the actual hinge is not always positioned at this location. Example: the project-out window has the V pointing to the top, at the theoretical hinge location. If butt-hinges were used, that would also be the exact location. Instead, scissors hinges are used at the sides. These are far preferable because they hold the sash open in any position.

    Project-out awning window showing hinge and hinging symbol

    Project-out awning window showing hinge (left) and hinging symbol (right)

    Image above shows a project-out awning window (center). At left is the scissors hinge that is installed in the jambs (sides), yet at right, the convention for indicating operation with dotted lines formed into a V, point at the theoretical hinge location at the top, as if the hinges were butt hinges as used on a typical door.

    Project-out awning and project-in hopper windows compress their weatherstripping when they are closed, so they use compression-type weatherstripping, sometimes called gaskets. These window types are often combined with a fixed window above or below. A common design in high-rise construction is to place these near the floor.

    Tilt-turn Windows

    This European window design involves a sash that operates in two directions: as an inswing casement (typically for cleaning) and as a project-in hopper (for ventilation). Both functions can be controlled by a single handle. These windows have not really gained a foothold in the US for reasons of cost. Other impediments to their widespread acceptance include having the ventilation near the ceiling, interference with window treatments, and widespread use of window washers who can work from the outside efficiently without needing to gain entry into the living units.

    Jalousie Windows

    This design involves multiple project-out sash, usually comprised of single-thickness (monolithic) glass. They are gang-operated and frequently not weatherstripped. This is a product that is designed for a tropical environment or sunrooms that are only used in summer.

    Window Safety

    Every year hundreds of people, especially children, die and are injured because they either fell from windows or were injured by windows or broken glass, or any combinations of the above. Design professionals cannot be too careful when designing around this serious set of issues.

    Care must be taken to limit the opening of any windows near the floor (preferably ALL windows) to a maximum of 4”.

    All glass at or near walking surfaces, and in or near doors and bathtubs/showers must be safety glass.

    Be sure to check all of the prevailing codes, including local building codes, local amendments to the code, local property maintenance codes, local ordinances and any industry specific guidelines (such as for hotels, hospitals, etc.) to ensure compliance.

    Do not be deterred if, for example, window limits are not mandated in any of the codes. Check with the fire department and life-safety codes to ensure proper egress, and then limit those windows and use safety glass!

    Glazing and Sash – Definitions
    Installing and sealing glass into a surrounding frame is called glazing the glass into the frame. The term glazing also refers to the glass or whatever other material (sheet plastic or metal panels, for example) that may be glazed into a frame or sash.

    A sash is the operable frame that resides within the outer frame, called the master frame. In the aluminum window industry, only operable windows have sash. Sash in projected windows are also called vents.

    In the wood or wood-clad window industry, both fixed and operable windows often consist of an inner and outer frame, and so the frame immediately adjacent to the glass is sometimes called a fixed sash.

    Weatherstrips and Gaskets

    Operable windows can be grouped by the type of weatherstripping they use, either sweep or compression. Weatherstripping is flexible material that runs continuously between operable and fixed elements of a window to repel air infiltration and water penetration.

    Double hung, single hung, and sliding windows and doors all utilize weatherstripping that slides or sweeps between the sash and the frame. Such weatherstripping must be tolerant of shearing movement while still maintaining a seal. Fuzzy piles, sometimes accompanied by a plastic or Mylar fin, are frequently used for this purpose at the perimeter of the sash, or along the frame where the sash meets the frame.

    Sweep-type weatherstripping

    Sweep-type weatherstripping

    Image above left: Sweep-type weatherstripping with Mylar fin (arrow) embedded in the pile for better control of air infiltration. The original trade name, fin-seal, has now been popularized to represent all such products. Right: Sweep weatherstrip without fin. The rigid backer threads into receiving slots that run along the sash or frame.

    Sectional detail of head of aluminum window with weatherstrips shown

    Head of aluminum window with weatherstrips indicated

    In the sectional detail through the top (also called the head) of an aluminum thermally broken double hung window, the master frame has been coded yellow. The sash is blue. The two thermal breaks are gray. The two weatherstrips are colored orange and circled. The weatherstrip on the left is threaded into the sash and weathers (rubs and seals against) against the master frame. The weatherstrip on the right is threaded into the master frame and weathers against the sash.

    All other operable window types utilize compression weatherstripping, such as bulbs or fins. These tend to seal better that sweep-type weatherstrips because they are actually compressed between sash and frame when the sash is locked in the closed position. Compression weatherstrips can also be called gaskets.

    Performance differences between products of sweep vs. compression windows are recognized in industry standards such as AAMA/WDMA/CSA 101/I.S. 2/A440
    Standard/Specification for Windows, Doors, and Unit Skylights.

    Bulb-type weatherstripping used in projected windows, casement windows and balcony doors

    Bulb-type weatherstripping used in projected windows, casement windows and balcony doors

    Image above, at left: Bulb weatherstrip with a rigid backer for threading into a slot which has been extruded into an aluminum or PVC sash or frame. Right: Bulb weatherstrip equipped with a “double-dart” designed to be permanently pressed into a slot cut along the edge of a wood window frame or sash. These weatherstrips are extruded from vinyl, TPEs (thermoplastic elastomers), PVC, silicon, polypropylene and jacketed foams.

    There are literally thousands of weatherstrips available on the market, varying by material, size, thickness, and backer configuration. If replacement weatherstrips are needed, the best way to achieve success is to obtain a sample of the material, measure it using a caliper-type micrometer, detail it and send it to various suppliers, even accompanied by photos of the material. If any suppliers claim to have a matching product, request a sample and compare it with your original.

    Architects and engineers, take this course for PDH credit here.

    Weatherstrip and Gasket Compression

    Weatherstrips and gaskets are designed to be compressed when in service. The window manufacturer will select a particular weatherstrip type for the product, and will decide, possibly in coordination with the weatherstrip manufacturer, to what extent the weatherstrip should be compressed.

    This decision must be made judiciously. If the weatherstrip is compressed too hard, meaning it is too thick for the space in which it resides, the window will be difficult to operate. If weatherstripping is too loose, excessive air infiltration or even water penetration could result. As windows are operated year after year, many weatherstrips will remain compressed even while not under compression. This is called “taking a set” or having a “memory”. Other products may erode over time, loosening the seal. Therefore the choice of weatherstripping material and its initial compression has large ramifications for the functionality of the window product not only when it is new, but also for many years in the future.

    How Windows are Made

    Here we will discuss the frame of the window. Glass, the thing that makes a window a window, will be discussed later. Window frames are commonly made from extruded aluminum, extruded PVC, wood and wood that has been clad on the exterior with aluminum or PVC. A newer product, combining the strength of aluminum with the thermal resistance of PVC, is a fiberglass pultrusion.

    Historically, windows were also made from rolled section or hollow metal steel and there is a market for these products now.

    The material chosen for the window frame, combined with its accompanying manufacturing processes have evolved frame designs that are peculiar to the material.

    Anatomy of an aluminum window
    Above, Anatomy of an Aluminum Window. Sectional detail of a fixed aluminum window shows that the core of the frame (yellow) is a highly detailed series of walls. In commercial windows, these walls are often 1/8” (.125”) thick.

    The extrusion process, along with the formability of aluminum, allows intricate shapes that are highly functional. A thermal break can be added to the frame to reduce thermal conductivity. A separate glazing stop (blue) can be extruded to neatly snap into the main extrusion and hold the glass in place.

    Aluminum is reactive to changes in temperature, and its coefficient of thermal expansion must be considered, especially when designing larger assemblies.

    PVC and fiberglas windows

    PVC (top) and fiberglas (bottom) windows have a similar appearance




    PVC is easily and accurately extruded, but its strength does not compare to that of aluminum. PVC’s softness and low melting point allow the extruding of multiple hollows within a single frame section – something far more expensive in aluminum, which increases the much needed strength of the PVC frame.



    PVC is highly reactive to temperature changes and is available predominantly in white. These factors should be considered when selecting product for a project.



    Sectional details and thermal conductivity of fiberglass windows are similar to extruded PVC, however the fiberglass is much stronger than the PVC.

    Wood double hung window sill

    Sectional detail of a wood double hung window sill


    The wood profiles (there are at least 6 unique shapes in this detail) are milled from larger pieces of lumber, making the fundamental manufacturing process quite different from the extruding processes of aluminum and PVC. Because of this, the sections tend to be solid, rather than being comprised of a series of extruded connecting walls.

    Wood is reactive to changes in humidity. It swells in the presence of moisture and usually shrinks back when moisture reduces. Protecting the wood against weather and moisture is a necessity, and ongoing maintenance is critical.

    Aluminum clad wood window

    Aluminum clad wood window


    In the image at right, wood (right side) is exposed to the interior while protected on the exterior by aluminum cladding. This design, sometimes using a PVC cladding in lieu of aluminum, is commonly used in residential construction.


    Steel windows

    Rolled section steel window (left) and hollow metal steel window (right)

    Above, rolled section steel windows (left) and hollow metal steel windows (right) still have a place in current construction and are both available. The rolled section steel provides some of the narrowest sightlines available. They are favored in historic renovations as well as interior partition applications. Hollow metal sections are frequently used in fire-rated partitions along with hollow metal doors.

    Architects and engineers, take this course for PDH credit here.

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This beautiful scanning electron microscope image of a nickel sulfide inclusion was provided by Jianjun Yang of Xinyi Glass, Dongguan City, Guangdong Province, China
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