Understanding Windows – Buying Guide for Homeowners: Anatomy and Key Metrics

In this series, we’ll learn the essential aspects of windows, including anatomy, key performance metrics, and buying tips to enhance your home’s functionality and aesthetic. Learn everything homeowners need to know about how to choose the right windows for your home.


Windows are not merely openings in the wall; they connect the indoors to the outdoors and bridge the gap between nature and your personal living space. For a long time, homeowners have had trouble understanding windows and the window-buying experience. At Andluca, we aim to demystify windows and the purchasing process in order to harness their full potential, thereby enhancing both functionality and the aesthetic appeal of your home. We want to make windows more efficient, innovative, exciting, and approachable.


Windows 101 - Anatomy of a Window:

To fully grasp how windows can be optimized for specific needs, let’s explore their fundamental components:

Window Diagram

Frame: The frame serves as the structural backbone of a window, securely encasing the glass and anchoring the entire assembly within the wall. It is available in a variety of materials, including vinyl, wood, aluminum, and fiberglass. Each material offers unique benefits in terms of efficiency, insulation, maintenance, and durability.

Sash: The sash is the movable part of the window; it holds the glass panes and can be designed to slide up, down, or sideways. Properly functioning sashes are crucial for ease of operation and for enhancing the window’s sealing capabilities. Some window styles (i.e., picture) don’t have any movable elements but might still have a sash that is locked shut to match the style and look of other windows.

Pane: The glass itself can significantly influence the window’s performance. Options range from single to double to triple-glazed, with each step up improving insulation. Advanced coatings and gases between panes can further enhance energy efficiency.

Jamb: The vertical part of the frame that runs along the sides of the window. It forms the structural support where the window sash (the part of the window that holds the glass and can move) fits and slides or pivots. In addition to supporting structural and operational roles, jambs contribute to the window’s overall thermal insulation by forming part of the barrier between the interior of a building and the external environment.

Mullions, muntins, and grilles are often used interchangeably but are not the same thing.

Mullion: The structural elements that serve as vertical dividers between units of a window. They can be found in both traditional and modern window styles, and they often bear structural loads. Mullions separate and support the window into sections when multiple frames are joined together, such as in bay and bow windows.

Muntin: The small bars or strips of wood, metal, or plastic that divide a single window sash or frame into smaller panes of glass, known as lites. Historically, muntins were used to join smaller panes of glass together into a larger configuration, a technique necessitated by the limitations in glass production methods of the past. Today, muntins can be either structural(holding individual pieces of glass in place) or purely decorative.

Grilles: The decorative elements that simulate the look of muntins but do not actually hold or separate the panes of glass. Instead, grilles are placed over a single large pane to give the appearance of multiple lites. They can be mounted on the surface of the glass or enclosed between double-glazed windows for ease of cleaning. Grilles offer the traditional look of multi-pane windows without the need for multiple small panes. Thus, they simplify the window cleaning process and often enhance thermal performance.

This diagram from The Spruce highlights the difference between muntins and mullions.

Key Metrics - Understanding SHGC, VLT, and U-Factor:

There are three key metrics to consider when one is deciding on a window based on its performance:

Solar Heat Gain Coefficient (SHGC): measures the amount of solar radiation that can pass through a window. It is expressed as a number between 0 and 1. A lower SHGC means less solar heat is transmitted, which is particularly beneficial in warmer climates to reduce cooling loads. Conversely, a higher SHGC might be more suitable for colder climates, as it helps harness solar energy to warm the interior naturally.

Visible Light Transmittance (VLT): refers to the amount of visible light that a window allows to pass through. It is measured on a scale from 0 to 1, with 1 representing the highest possible level of light transmission. High VLT might be desirable for spaces requiring more natural light, as it creates a brighter, more inviting environment. However, in very sunny climates, too much VLT without appropriate glare control can lead to excess brightness and heat. This highlights the need for a balance based on the climatic conditions and the building’s orientation.

U-factor: indicates how well a window can insulate. It measures the rate of heat transfer from the inside of a building to the outside. This factor is also presented as a number between 0 and 1. A lower U-factor indicates better insulative properties. Thus, windows with a low U-factor are more effective at keeping the interior warm in the winter and cool in the summer, thereby reducing heating and cooling demands.

R-value is a key metric for quantifying the insulation capabilities of materials in walls, floors, etc. SHGC and U-factor are specific to windows. The U-factor is equal to (1/R-value).

Choosing the Right Window Components:

Selecting the right window involves balancing these metrics with your local climate and the architectural style of your home. In colder regions, a low U-factor is essential, while in sunnier spots, a lower SHGC can prevent unwanted heat gain. The style of the mullion and frame should complement the architectural integrity of your home to enhance both its character and its resale value.



With this knowledge, homeowners can approach window selection with confidence, understanding how each component contributes to the window’s overall performance and appearance. By making informed decisions, you can significantly impact your home’s energy consumption, security, and aesthetic appeal. A well-chosen window not only saves on energy costs but also elevates your living environment, proving that every detail in home design matters.

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