Structural & Weather Ratings: NAFS, Wind & Impact

Energy ratings tell you how well a window insulates. They say nothing about whether it will stay in the wall in a storm, keep out wind-driven rain, or stop drafts. Those are proven by physical testing, and in North America the core standard is the North American Fenestration Standard, AAMA/WDMA/CSA 101/I.S.2/A440, known as NAFS. This page explains what NAFS tests, when codes require it, how location raises the bar, and how owners and architects specify above the minimum.

What NAFS is

NAFS is a single standard shared by the United States and Canada, published jointly by FGIA (formerly AAMA), the Window & Door Manufacturers Association (WDMA) and CSA Group. It replaced the older AAMA 101 and the separate Canadian A440. The 2017 edition is referenced by the 2021 International Building Code (IBC), the 2021 International Residential Code (IRC) and the 2020 National Building Code of Canada. The 2022 edition is being picked up by the 2024 I-codes and the 2025 National Building Code. In Canada, NAFS is always used with the Canadian Supplement, CSA A440S1, which sets how Canadian design pressures, water test pressures and air leakage levels are chosen.

NAFS covers windows, sliding and swinging glass doors, side-hinged doors, skylights and tubular daylighting devices. It does not cover curtain wall, storefront, sloped glazing or other site-assembled systems, which are qualified differently (see below).

What a NAFS rating tests

To earn a rating, a sample of the product at or above a minimum "gateway" size is tested at an accredited lab. The core tests:

• Air leakage (ASTM E283). Air passing through the closed, locked window under pressure.
• Water penetration (ASTM E547 and E331). Water sprayed on the window while air pressure pushes it inward, as in wind-driven rain. The test pressure is 15% of the design pressure for most classes and 20% for the AW class, with a minimum of 140 Pa (2.9 psf) and a maximum of 580 Pa (12 psf) in the U.S. and 720 Pa (15 psf) in Canada.
• Uniform structural load (ASTM E330). Positive and negative pressure equal to 150% of the design pressure, with no breakage or permanent damage. Skylights are tested to 200%.
• Forced entry resistance, operating force and, for some classes, deflection limits, life-cycle testing and thermal cycling.

Reading a NAFS label

A NAFS rating is written like this:

Class CW-PG50: Size tested 1200 × 2100 mm (47 × 83 in): Casement

• Performance Class is the letter group: R, LC, CW or AW, in increasing order of severity. The class is not tied to building use. A house can use AW windows and a tower can use LC windows if they meet the loads, but the higher classes add stricter testing.
• Performance Grade (PG) is the design pressure the window passed, in psf. PG50 means a design pressure of 50 psf (about 2,400 Pa), structurally tested to 75 psf.
• Size tested matters: a rating applies to the tested size and smaller. A larger window needs its own test or an engineering analysis.

Minimum grades are typical gateway values; confirm against the current NAFS edition for each product type.

Design pressure: how much a window must resist

The required design pressure is calculated for each building, and often for each zone of each façade, using ASCE 7, the structural loads standard referenced by U.S. codes, or Part 4 of the National Building Code of Canada. It depends on:

• Basic wind speed from the wind map for the site and the building's Risk Category. A hospital or emergency shelter is designed for a rarer, stronger wind than an apartment building.
• Exposure category. Exposure B is built-up urban or suburban terrain; C is open terrain; D is flat, unobstructed terrain facing open water. A building on the waterfront can see substantially higher pressures than the same building a few blocks inland.
• Height. Wind speed rises with height, so upper floors of a tower need higher-rated windows than lower floors.
• Location on the façade. Corners and edges see the highest suction. Windows near building corners often need a higher grade than those mid-wall.
• Topography. Buildings on hills, ridges and escarpments get a topographic factor that increases the load. Some mountain areas are designated special wind regions with locally set wind speeds, such as parts of the Colorado Front Range.
• Elevation. Thinner air at high elevation slightly reduces wind pressure, and recent editions of ASCE 7 allow for it.

The structural engineer or architect of record typically issues a design pressure schedule. The window supplier then shows, through NAFS ratings or engineering analysis, that every window meets or exceeds its zone.

When codes require NAFS

Curtain wall, storefront and window wall

Site-assembled and large commercial systems fall outside NAFS. They are usually qualified in three layers:

• Laboratory testing of a representative system to ASTM E283 (air), E331 and E547 (water), E330 (structure), and often AAMA 501.4 (seismic drift) and AAMA 501.5 (thermal cycling).
• Project mockup testing, where a full-size section built with the project's actual details is tested before production, often required on high-rise and institutional work.
• Field testing of installed work, such as AAMA 502 for windows and AAMA 503 for curtain wall, typically at a percentage of openings, to catch installation defects.

How location raises the bar

Hurricane coasts. In the U.S., a wind-borne debris region is the part of the hurricane-prone coast within one mile of the shoreline where the design wind speed is 130 mph or more, and anywhere the design wind speed is 140 mph or more. Glazing there must resist impact from wind-borne debris or be protected by shutters. Glass within 30 ft of grade must pass a large-missile test (a 9 lb lumber missile); higher glass passes a small-missile test (steel balls), followed in both cases by thousands of cycles of positive and negative pressure. This covers most of the Gulf and southern Atlantic coasts, including much of Florida, coastal Texas, Louisiana and the Carolinas.

Florida. Every window sold for a building in Florida needs Florida Product Approval or local approval. Miami-Dade and Broward counties, the High-Velocity Hurricane Zone (HVHZ), design for about 175 mph and 170 mph respectively and require the stricter Florida tests TAS 201, 202 and 203 and, in practice, a Miami-Dade Notice of Acceptance (NOA).

Texas Gulf Coast. In the 14 coastal counties covered by the Texas Windstorm Insurance Association, and parts of Harris County, buildings need windstorm certification for insurance, which requires products evaluated for the area's wind and impact requirements.

New York City and other dense coastal cities. New York City writes its own wind provisions into the NYC Building Code. Its design wind speed is 117 mph for ordinary buildings and 127 to 132 mph for higher risk categories, below the threshold for wind-borne debris rules. But the code maps Exposure D along shorelines facing open water, extending inland 600 ft or 20 times the building height, and the city's tall buildings reach heights where pressures climb quickly. Waterfront towers in Manhattan, Brooklyn, Queens and Staten Island routinely need CW or AW windows at PG50 to PG100 and more.

Tornado regions. ASCE 7-22, referenced by the 2024 IBC, adds tornado loads for Risk Category III and IV buildings, such as large schools, hospitals and emergency facilities, in tornado-prone areas of the central and southeastern U.S. Storm shelters and safe rooms, required for many new schools and emergency facilities in the strongest tornado zones, must meet ICC 500 with its own missile-impact tests. Ordinary houses and apartment buildings are not designed for tornado loads.

Mountains and high elevation. Hilltop and ridge sites can see much higher pressures through topographic effects. At high elevation, insulating glass made near sea level bulges as the outside air pressure drops, which can stress seals and distort reflections. Units shipped to high-elevation sites are often fitted with capillary or breather tubes, or built to account for the pressure difference. Vacuum glass has no gas to expand, so it is not affected in the same way.

Canada. The National Building Code lists the 1-in-50-year hourly wind pressure for every locality. Exposed coastal sites in Atlantic Canada and British Columbia, and the Prairies, have some of the highest values. The Canadian Supplement converts them into required design pressures and also sets the water test pressure from the local driving-rain wind pressure, which is why Canadian water ratings are often higher than U.S. ratings for the same product.

Specifying above the minimum

Codes set minimums. Owners, architects and envelope consultants often specify higher, for good reasons:

• A higher class, typically CW or AW, for durability, stiffness, tighter air leakage and life-cycle testing on institutional, rental and long-hold buildings.
• A higher water rating than the structural grade implies, for exposed sites and tall buildings. NAFS allows a separate, higher water test pressure to be stated.
• Tighter air leakage, important for Passive House and for buildings with energy models that assume low infiltration.
• Project mockups and field testing on multifamily and institutional work, where a single installation detail is repeated hundreds of times.
• Impact glazing outside the code-required area, for resilience, insurance or security.

A typical specification reads: "Windows shall be AW-PG60 per AAMA/WDMA/CSA 101/I.S.2/A440-17, with a water test pressure of 12 psf, field tested per AAMA 502."

Our products

Our A-Series hurricane line carries Florida Product Approvals across twelve systems, with design pressures up to ±115 psf, and two systems also carry Miami-Dade NOAs for large and small missile impact. For our F-Series fiberglass windows, ask us for the air, water and structural performance data for each system and size. Send us your design pressure schedule and we will confirm the rating for every window on the project. Contact us.

Summarized from NAFS, the IBC, IRC, ASCE 7, the NYC Building Code, the Florida Building Code and the National Building Code of Canada as of 2026. Requirements vary by edition and local amendment; the design professional of record determines project design pressures.