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Anatomy of a Double Hung Window

The double hung window, in which both the upper sash and lower sash are able to move, has become the standard for buildings of all kinds. It’s worth focusing on the components of these windows, whether the unit in question is a vinyl window, aluminum window or wood window.


It’s easy to underestimate the number and variety of parts that go into an average double hung window. Every piece of the complete window anatomy has a part to play in letting light in while keeping the elements out, and generally making a building hospitable.


Learning more about these parts — and the products used to support them — can help you excel when it comes to window manufacturing and installation.



What goes into a double hung window?

What are the key elements in a double hung window, and what do they do?


Before delving into the products available to protect windows and keep them in top condition during manufacturing, transport and beyond, it’s worth exploring the anatomy of the window itself.


The following are some of the essential parts of a typical double hung window.

  • Frame: Window frames are the strong, rigid outer sections that surround double hung windows. When installed correctly, a window frame takes weight evenly, keeping the entire window secure.
  • Sash: The sash resides inside the frame and moves on the balance track. This is the rectangular part of the window that holds panes of glass, and it moves up or down when the window is opened.
  • Insulated Glass (IG) Unit: The glass section in a modern window isn’t just a single sheet of glass — it’s a sealed unit made up of multiple panes of glass, either double or triple, that provide better insulation and noise prevention than single pane windows. The glass may have a Low E (low emissivity) coating to help increase the energy efficiency of the IG unit. Sometimes the space between those glass panes contains gas such as argon or krypton, so they’ll transfer less heat.
  • Balance Track and Balance Pocket: The balance track is the groove on the sides of the frame that allows sashes to move up, down and tilt in. The balance pocket contains the balances that allow the sash to function. This pocket, or the hollow inside of the track, is technically part of the outside of the house as air from outside reaches this part.
  • External Muntin Bars: Most window panes today are no longer divided into quarters (divided lites) — each sash contains a single insulated glass panel or unit. The simulated divided light (SDL) effect is created by muntin bars. These parts may be placed on the outside surface of the glass as external muntin bars.
  • Internal Muntin Grids: When divided lights are simulated by a grid in between the panes of the IG unit, that is referred to as an internal muntin grid or a grid between the glass (GBG).
  • Nailing Fin: Only used in new construction, not window replacement applications, a nailing fin is a thin strip around the frame which allows builders to attach the windows to the building during installation.
  • Sill and Weep Holes: The sill is the ledge below a window, which the whole assembly rests on. The sill contains “weep holes,” which allow accumulated water to flow out.

Double Hung Window Diagram with Parts by LAMATEK*some parts may not be available on all double hung windows. For example, SDL muntin bars and GBG muntins are an either/or option in that they are not used together on the same window.


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Products to protect double hung windows

Once you’ve refreshed your knowledge of window components, it’s time to consider the materials used in and around the windows themselves.


For these parts of a window, there are a variety of foam, tape and film offerings available to cushion, seal and protect them during production, shipping, installation, and finally daily use.

  • Surface Protection Film: A removable protective film can keep IG units, windows, and other delicate surfaces like hardware safe and clean during manufacturing and installation. PROTEK film can also cover parts, like external muntin bars for example, during painting.
  • Foam Plugs (Chimney Blocks): These sit inside the balance pocket within the balance track to make sure air doesn’t leak into a house from outside.
  • Foam Shipping Shoes: Foam shoes can be secured to a window frame with straps or wrapping while windows are in transit. This prevents damage to the window and nailing fins caused by impacts, movement, and shifting that can occur in shipping and handling.
  • Foam Shipping Corners: These blocks of foam protect and cushion a window’s frame and nailing fin corners in transit. Using foam corners is a better alternative to screwing a protective piece of wood to the window, because breaking that wood off can damage the fin.
  • Setting Blocks: At least two setting blocks are placed around all four sides between the insulated glass unit and the window sash. They ensure IG units never touch the vinyl of the window assembly. This creates space for thermal expansion and drainage, as well as cushioning that protects against stress cracks, especially when windows are opened and closed over time.
  • Glazing Tape: This highly engineered tape is designed to seal the insulated glass unit inside the sash and prevent air and water leaks.
  • Sealing Gaskets: Custom-cut gaskets are designed to seal up any and all gaps between mechanically fastened joints and parts on the window assembly. Any window that is screwed together (aluminum, wood windows and many vinyl windows) could have gaskets in their corners.
  • Shipping Pads: Sometimes referred to as transport or separator pads, shipping pads come in a variety of materials for specific use cases. When manufacturers store or ship sheets of glass outside of an assembly, glass pads keep those sheets separated to prevent them from scratching or breaking one another. Foam shipping pads also create space between windows and window frames in storage and transport.
  • SDL Muntin Tape: This tape is designed to permanently attach simulated divided light bars to the outside of an IG unit (ie: not between two glass panes) on both the exterior and interior facing side of a building.
  • Reticulated Filter Foam: Without any sort of protective barrier , weep holes could allow debris and insects into the window system. Filling the gap in the sill with open cell foam prevents dust, bees and other insects from entering the window structure without impeding the drainage function of the weep hole itself.


Manufacturing and transporting double hung windows

The modern double hung window is a complex, multi-part system. Every aspect of the window experience, from creating a reliable barrier against the elements to promoting energy efficiency and providing natural light, is shaped by the materials and components that go into making it.


To keep quality high and costs low, it pays to have access to industry-leading foam, tape, and film products when your company assembles or transports windows. Damage to windows can cost time, money or even affect your reputation, whether the unit in question is a replacement window or a new window slated to be installed in a building under construction.


Contact LAMATEK to learn more about sealing and protecting double hung windows, and to find the right solutions for your exact needs.

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