Strength: classes, design pressure, water and air.

A window on the ninth floor sees more wind and more driven rain than one on the second. North American test standards turn that exposure into a class, a grade and a short list of pressures. This guide explains what each one means, how it is tested, and what to check before a rating goes into a specification.

One standard, two parts to the rating.

Windows, doors and skylights in North America are rated to the North American Fenestration Standard, known as NAFS. Its full title is AAMA/WDMA/CSA 101/I.S.2/A440. It is written jointly by the US and Canadian industry bodies so that one rating works on both sides of the border. AAMA, the body most people associate with it, is now part of FGIA. The standard is revised every few years, and the 2022 edition is the most recent. The edition that counts on a project is the one its building code references, which can be older: the 2022 New York City Building Code still cites the 2011 edition.

A NAFS rating reads like CW-PG65. The letters are the performance class. The number is the performance grade.

NAFS covers structure, air, water and forced entry. It says nothing about energy. Energy is rated separately, by NFRC. A window for a mid-rise building needs both sets of numbers, and neither one predicts the other.

The four classes.

Each class sets a minimum the window must reach before it can carry the letters at all.

The standard calls these minimums the gateway requirements. For each class it fixes a minimum performance grade, a minimum design pressure, a minimum structural test pressure and a minimum water test pressure. It also fixes a minimum test size, so a manufacturer cannot earn a class on a small sample. A window that passes the gateway can then be tested at higher pressures to earn a higher grade within the same class. That is how a CW window becomes CW-PG65 rather than CW-PG30.

The class tells you how hard the test was. The grade tells you how far the window went.

What the grade number means.

The grade is tied to the design pressure, in pounds per square foot. Design pressure is the wind load the window is rated to carry in service, pushing in and pulling out. A PG65 window was tested for a design pressure of 65 psf in both directions.

Three other pressures hang off that number.

• Structural test pressure. The window is loaded to 1.5 times the design pressure and must come through without breakage or lasting damage. A PG65 window is loaded to about 97.5 psf.
• Water test pressure. Water resistance is tested at a set fraction of the design pressure: 15 percent for the R, LC and CW classes and 20 percent for AW. There is also a cap, which depends on the edition: 12 psf in the US under the older editions, 15 psf under NAFS-22. A higher grade means the window was sprayed at a higher pressure without leaking.
• Air leakage. Measured at a fixed test pressure and reported as airflow per square foot of window.

A grade is earned only when the same product passes all of these. A window that carries 65 psf structurally but leaks at the matching water pressure does not get PG65.

How each test is run.

Air: ASTM E283.

The window is sealed into a test chamber. A pressure difference is held across it and the air passing through is measured. The result is reported in cubic feet per minute per square foot of window area. Lower is better. Fixed windows leak almost nothing. Operable windows depend on their gaskets and on how firmly the hardware pulls the sash against the frame.

Water: ASTM E331 and ASTM E547.

ASTM E331 is the laboratory water test. Water is sprayed evenly over the outside face of the window while the air pressure outside is held higher than the pressure inside. That copies wind pushing rain at the building. The spray and the pressure are held for 15 minutes while a technician watches the inside face and records any water that gets through. ASTM E547 is the cyclic version. The pressure is switched on and off in cycles, which is closer to gusting wind.

The test looks at leakage through the manufactured window. It does not judge the wall around it, and it does not address long-term effects on sealants or on insulating and laminated glass.

Structural: ASTM E330.

ASTM E330 loads the window with uniform static air pressure. Before the test, an operable window is opened and closed several times and checked for adjustment. Gauges are fixed to the framing members. The load is applied, held for 10 seconds and released. Deflection under load and any permanent set are recorded. The sequence is then repeated in the other direction, because wind pulls on a facade as well as pushing on it.

Deflection.

Deflection is how far a framing member bends under load. Too much bending works the glass seals, distorts the sash and opens the gaskets. NAFS makes a deflection limit mandatory for the CW and AW classes. Under the uniform load test a member may not deflect more than L/175, where L is its unsupported span. R and LC windows have no mandatory limit. This is one real reason to ask for CW or AW on a mid-rise building, not just a higher number.

Deflection is a different matter from optical distortion in the glass. Tempered and heat-strengthened glass carries a slight waviness from the rollers in the furnace. It is inherent to heat-treated glass and is not a defect. Narrow or odd-shaped units, altitude changes and temperature swings can also bow the panes of an insulating unit a little.

Forced entry, operating force and durability.

NAFS also tests resistance to forced entry and checks that operable units open and close within set force limits. The AW class adds life-cycle testing, which opens and closes the unit many times to show that the hardware keeps working after long use.

Choosing a class: four mistakes.

1. Choosing by building type alone. The class names suggest building types, but the standard treats those as guidance. Window size, building height, wind exposure and budget all matter. The design pressure should come from the wind-load calculation for the project, by zone and by height.
2. Choosing arbitrarily. Writing AW into every specification adds cost with no benefit on a sheltered six-story building. Writing LC on an exposed site can leave the corner windows under-rated.
3. Choosing by grade alone. An LC-PG50 window and a CW-PG50 window carry the same design pressure. Only the CW window was held to a deflection limit and the heavier-use tests. Specify the class and the grade together.
4. Applying NAFS classes to products NAFS does not cover. Curtain wall and storefront are tested under their own standards. Asking for a NAFS class on those products sets an expectation nobody can verify.

Laboratory tests and field tests.

Everything above is laboratory testing. It rates the product. A field test checks the installed window, including the joint between the window and the wall.

• ASTM E1105 is the field water test. A chamber is fixed to the inside of the installed window, the pressure inside the chamber is lowered, and a calibrated rack sprays the outside. Procedure A holds a steady pressure for 15 minutes. Procedure B cycles the pressure on and off for at least 15 minutes in total. The practical difference from E331 is simple: in the lab the specimen is fixed to the chamber, and in the field the chamber is fixed to the specimen.
• AAMA 502 is the specification for field testing newly installed windows and doors. It sets out how to combine the air and water field tests and includes wording that can be dropped into a project specification. The specifier still has to state which openings, how many, and at what pressures.
• AAMA 503 is the matching specification for storefronts, curtain walls and sloped glazing.
• AAMA 501.2 is a hose test with a calibrated nozzle. It is a quick check for joints in fixed glazing. It is not meant for operable windows.

Field testing is carried out by an independent testing agency, not by the window manufacturer. Ask the agency for its current accreditation certificate. On larger projects, AAMA 502 suggests testing at several stages: around 5, 50 and 90 percent of installation. Early tests matter most. They catch a flashing or sealant detail while it can still be changed across the rest of the building.

A field test is not a second product certification. If the window already holds a laboratory rating, a field failure usually points to the installation or the surrounding wall.

Is uPVC strong enough?

The question comes up because people confuse uPVC with ordinary plastic. The "u" stands for unplasticized. The material contains no plasticizers, so it is rigid and stays rigid. Where the span or the wind load calls for it, steel reinforcement is placed inside the profile chambers. The frame is a composite: the steel carries the load, and the uPVC provides the insulation, the weather seal and the finish.

The proof is the same as for any other frame material. A uPVC window is loaded under ASTM E330, sprayed under ASTM E331 and measured against the L/175 limit. If it earns CW or AW, it has passed the same test as an aluminum window with that rating. High Performance uPVC is judged by the report, not by the material.

How to read a rating before it is specified.

• Ask for the report number and the laboratory. A class without a report is a claim.
• Check the product type. A fixed window and a casement from one series rarely share a rating. Each is tested on its own.
• Check the tested size. The rating applies up to the size that was tested. Larger units need engineering review or another test.
• Check all the numbers, not just the grade. Design pressure, water pressure and air leakage each appear in the report.
• Ask whether the class is the highest certified or the everyday one. A manufacturer may hold a top rating on one configuration and sell most units at a lower, still adequate, class. Both are legitimate. The specification should say which one the project needs.

Where Vistaza fits.

Vistaza's windows and doors are tested to NAFS by Intertek, and every class above is tied to a report number. The highest certified rating is the Vi6 casement at AW-PG100: a design pressure of ±100.25 psf, water at 15.04 psf and air leakage of 0.06 cfm/ft², under report S3614.03-525-44. The Vi6 fixed window is certified at AW-PG75. In the Vi5 series the fixed window is certified at AW-PG70 and the casement at CW-PG50, and the sliding door is CW-PG50. Each rating belongs to its own type and tested size, so check the table above, or the strength page, against the window schedule type by type.