Window Ratings: Understanding the Different Ratings and Why Location Matters
When choosing windows for a construction project, you will often see a variety of ratings and certifications listed on the product. Terms such as Design Pressure (DP), Performance Grade (PG), U-Factor, Solar Heat Gain Coefficient (SHGC), and impact resistance can be confusing if you are not familiar with what each one means.
The important thing to understand is that there is no single “best” window rating for every location. Window requirements vary based on factors such as climate, wind speed, building height, proximity to the coast, exposure to severe weather, and local building codes. Don’t forget, no matter which window you choose Rescom Products has all the accessories you need for a successful installation!
Design Pressure (DP) Rating
One of the most important structural ratings for windows is the Design Pressure (DP) rating. DP measures how much wind pressure a window assembly is designed to withstand and is expressed in pounds per square foot (psf).
A window might have a rating such as DP ±30, DP ±50, or DP ±70. The positive number represents pressure pushing inward on the window, while the negative number represents suction pulling the window outward.
Higher DP ratings are generally required in areas exposed to stronger winds. For example, a single-story home in an inland area may require considerably less wind resistance than a high-rise building or a home located directly along the coast.
The required DP rating can also vary from one window to another on the same building. Building height, window size, corner locations, and exposure to open terrain or water can all affect the wind pressure an opening must withstand.
Performance Grade (PG)
Performance Grade (PG) is another rating commonly found on windows. Rather than measuring only structural wind resistance, PG represents a broader level of performance that incorporates requirements for things such as air infiltration, water penetration, structural performance, and other tests.
A window with a higher PG rating has generally demonstrated a higher level of overall performance under the applicable testing standard.
This is particularly important in regions that experience heavy rain, high winds, hurricanes, or other severe weather conditions.
Impact Resistance
In hurricane-prone areas, you may also see windows described as impact-rated or hurricane-rated.
Impact-resistant windows are designed to withstand the impact of wind-borne debris. Testing can involve impacts from objects such as a large piece of lumber, followed by repeated pressure cycles.
Certain areas designated as Wind-Borne Debris Regions require exterior openings to be protected with either impact-resistant products or approved impact-resistant coverings. In Florida, these requirements become especially important in coastal and high-wind areas.
Impact resistance and DP rating are not the same thing. A window can be impact-resistant but still need to meet a specific DP requirement for the building where it is installed.
U-Factor: Important for Energy Efficiency
Not every window rating relates to hurricanes or structural performance.
U-Factor measures how easily heat passes through a window. Unlike some other ratings, lower is better.
A lower U-Factor means the window provides better resistance to heat transfer. This is particularly important in colder climates, where keeping heated indoor air from escaping is a major concern.
In warmer climates such as Florida, U-Factor is still important, but controlling solar heat entering the building becomes especially important.
Solar Heat Gain Coefficient (SHGC)
SHGC measures how much solar radiation passes through a window and contributes heat to the building.
SHGC is expressed as a number between 0 and 1. Lower numbers mean less solar heat enters through the glass.
This rating becomes especially important in hot, sunny regions such as Florida, Texas, Arizona, and other southern states. A window with a lower SHGC can reduce the amount of heat entering the building, potentially reducing the workload on the air-conditioning system.
In colder northern climates, however, a somewhat higher SHGC can sometimes be beneficial because solar heat entering through the windows can help warm the building.
In other words, the ideal window specification for Minnesota isn’t necessarily the ideal specification for Florida.
Why Window Ratings Change by Geography
The biggest reason window specifications vary throughout the United States is that buildings experience very different environmental conditions.
Northern climates generally place greater emphasis on preventing heat from escaping. Low U-Factor windows are therefore particularly valuable.
Hot southern climates place greater emphasis on controlling solar heat gain. SHGC becomes an important consideration, along with U-Factor.
Coastal areas may require substantially greater resistance to wind, water, and wind-borne debris.
Hurricane-prone regions can require impact-resistant windows, higher DP ratings, and specific product approvals.
High-rise buildings can require higher structural ratings because wind pressures generally increase with building height.
Florida Is a Good Example
Florida provides a good illustration of why window ratings can vary significantly even within the same state.
A window installed in an inland, low-rise residential neighborhood may have substantially different requirements than one installed in a beachfront condominium.
South Florida is an especially demanding environment. Miami-Dade and Broward Counties are within Florida’s High-Velocity Hurricane Zone (HVHZ) and have some of the country’s most demanding requirements for exterior windows and doors. Products used in these areas may require appropriate Miami-Dade approvals or other qualifying approvals in addition to meeting the required structural and impact criteria. (Impact Doors Miami)
Even within a hurricane-prone area, the required DP rating isn’t necessarily identical for every opening. A window on the middle of a wall can experience different pressures than a window near a building corner, and a window on an upper floor can experience different wind loads than one near ground level.
One Rating Doesn’t Tell the Whole Story
When comparing windows, it is important not to focus on a single number.
A properly specified window may need to satisfy several different requirements:
- DP Rating – Resistance to structural wind pressure
- PG Rating – Overall tested performance
- Impact Rating – Resistance to wind-borne debris
- U-Factor – Resistance to heat transfer
- SHGC – Control of solar heat gain
- Air Leakage (AL) – Resistance to air passing through the window
- Water Resistance – Ability to resist water penetration
- Visible Transmittance (VT) – Amount of visible light passing through the glass
The right combination depends on the building, climate, location, local building code, and intended application.
Choosing the Right Window for the Job
The best window isn’t necessarily the one with the highest rating in every category. Instead, the goal is to select a product that meets or exceeds the requirements for the specific project without paying for specifications that aren’t necessary.
For example, a window designed for a mild inland climate may not be appropriate for a coastal hurricane zone. Conversely, using an extremely high-performance hurricane window in a location with relatively mild weather may add unnecessary cost.
Before selecting a window, contractors, builders, architects, and homeowners should consider the local building code, required wind loads, climate zone, building height, exposure, and energy requirements.
Understanding window ratings makes it easier to select the right product for the right location—and ensures that the window is providing the level of performance the building actually requires.
If you’re sourcing windows or other construction products, it’s also important to verify the specific product approval and installation requirements, rather than relying solely on a general rating printed in a catalog.
