A Guide to Window Classes, Codes and Glass Types (And Why They Are Important for Window Replacement Projects)
When selecting windows for window replacement projects, it’s important to consider the ultimate goal of your windows. For many, energy-efficiency is a significant concern. High-efficiency windows can lower heating and cooling expenses. When specifying windows, glass type can determine energy-efficiency, while performance class designations identify the level best suited for the building. You may have other priorities for your window selection, like sound dampening or thermal comfort. Since projects budgets allow for the purchase of windows only once, the wisest choice is not always based on price alone, but instead the long-term performance.
Understanding Window Classes and Building Codes
The American Architectural Manufacturers Association (AAMA) standardizes window according to performance grades, distinguished by pounds per square foot (psf) of pressure (or wind loads) and the corresponding wind velocity:
- R class, 15 psf, designed for 77 mph wind speed.
- LC class, 25 psf, designed for 100 mph wind speed.
- CW class, 30 psf, designed for 109 mph window speed.
- AW class, 40 psf, designed for 126 mph window speed.
Choosing a window class depends on the application and performance required. Generally, the higher performance grades, the more capable the window is of withstanding greater operating force, deflection and structural loading – meaning that the CW and AW class are usually selected for commercial projects.
It’s also important to know the applicable building code before you choose a window class. Requirements for structural stability typically cover window frame, glass, anchorage and substrate attachment. You can find the current building codes for your state on your state’s Department of Labor and Industry website. Be sure that the window class you choose will also meet the requirements for your building code.
Understanding Window Glass Types
The type of glass may be dictated by building codes, along with the number of glazing options available to improve energy efficiency. You’ll also want to select a glass type based on the desired application – some glass types are more resistant to force and deflection.
Annealed glass is a raw glass that’s sometimes also called standard glass. It has a tendency to break into large, sharp pieces which means it’s limited by many codes. This is used when cost is the highest concern, not strength, safety or efficiency.
Heat-strengthened glass is glass that has undergone controlled heating and cooling to improve its strength. It’s roughly twice as strong as annealed glass, but can still break into large, dangerous shards so it’s still limited by code. Like annealed glass, this is usually a choice when low-cost is the only priority for glass.
Tempered glass undergoes controlled heating and rapid cooling, which makes it roughly four times as strong as annealed. It is significantly more shatter-resistant and breaks into small pieces when it does shatter, meaning its far less likely to cause injury. Tempered glass is a popular choice for buildings, although tempered glass only improves safety – not thermal comfort or impact-resistance.
Dual-glazed windows are made by combing two layers of glass. In some cases, the space between the layers is filled with a material. Commonly, it’s left empty and then filled with an inert gas to help increase the glass’ resistance to energy transfer. Temperature transmission is reduced by introducing an airspace and creating a thermal break in the glass. Although triple glazing is also an option, dual-glazed is often the most cost-effective choice.
Laminated glass is created by combining two layers of annealed glass together with vinyl. The vinyl layer helps hold the glass in place if it’s broken, causing less damage. This makes it a popular choice in commercial applications.
Low-e or Low-Emissivity Glass
Low-e glass uses a thin coating to limit the amount of ultraviolet and infrared light that passes through the glass. This helps increase performance of the windows and make them more energy-efficient.
Beyond window classes and glass types, you’ll also want to consider your glazing, coating and filming options. These additional steps can help increase your glass’ resistance to damage and ability to insulate.
St. Cloud Window manufactures custom made, high-efficiency aluminum window and door products for new building construction, commercial window replacement, historic window replacement and projects where high-performance acoustic attenuation sets the standard. Not sure which high-efficiency windows are right for your project? View our full product line or contact us for more information.