Window limiters: what the four-inch rule means.

A limiter stops an operable window at four inches. A small child cannot pass through a gap that size, and the window still lets in air. On the residential floors of a mid-rise building it is one of the cheapest safety items on the window schedule. It is also one of the easiest to get wrong.

What the rule is for.

In the United States, according to the Consumer Product Safety Commission, window falls send about 3,300 children aged five and under to hospital emergency rooms each year, and kill about eight. Most of those windows were simply open. A limiter takes that away. The sash opens far enough to ventilate the room and no farther.

We call it a vent limiter. You will also see it called a limiter, a restrictor or an opening control device. They all mean hardware that holds the clear opening to four inches or less.

Where it is required.

The model building codes set the limit as a sphere test. Where a limit applies, a four-inch sphere must not pass through the opening. In the IBC it is section 1015.8. It covers operable windows in residential occupancies, Groups R-2 and R-3, where the bottom of the clear opening is less than 36 inches [914 mm (36″)] above the floor and more than 72 inches [1829 mm] above the ground outside. Either a 4-inch [102 mm (4″)] sphere must not pass, or the window carries a device that meets ASTM F2090. Check the section in the edition adopted for the project: New York City's building code applies its own version to one- and two-family houses and, for apartment buildings, defers to the Department of Health. Some cities add their own rules on top. New York City is one: its Department of Health approves the devices that may be used, and an approved device is stamped.

Beyond the code minimum, these are the places owners most often ask for limiters:

• Every operable window in the dwelling units above the ground floor.
• Children's bedrooms and play areas.
• Common rooms and amenity spaces with low windows.
• Student housing and hotels, where the operator wants every window limited whatever the code minimum is.

The conflict to settle early: escape windows.

Some windows are also emergency escape and rescue openings. A limiter that cannot be released would block the way out.

There are two kinds of limiter:

• Fixed. Holds the sash at four inches, always. Suitable where the window is never an escape route.
• Releasable. Holds the sash at four inches until an adult releases it. Then the window opens fully. It resets when the window is closed.

The Vistaza limiter is a steel stop built into the sash. It is approved by the New York City Department of Health as a window fall-prevention device, and every limiter carries the approval stamp, so an inspector can check it on the window. It cannot be released by hand. Releasing it takes a special tool, which stays with the building's staff, so a resident or a child cannot open the window past four inches. For the same reason it is never fitted to an emergency escape window: an escape window ships without a Vistaza limiter.

Where an escape window must also be limited, the IBC allows only a window opening control device that meets ASTM F2090: the window must still open from inside without keys or tools, and once released it must give the full escape opening. That is a different device from the Vistaza limiter. Mark the escape windows on the schedule before the limiters are assigned, not after.

Mark the escape windows first. Then assign the limiters.

How the limiter is built in.

On a Vistaza window the limiter is on the side of the sash, at the lock side, between the sash and the frame jamb. It can be mounted along the bottom of the sash instead. Vistaza does not put it there, because at the bottom it is hard to reach to engage, to check and to release.

The four inches is the gap between the open sash and the frame. The part has to suit the sash, so that the gap is four inches or less on the window as built, not only on paper.

Every operable Vistaza window comes with a four-inch limiter as standard. It is built into the sash at the plant and shown on the shop drawings, so there is no hardware to add on site.

A limiter is not a lock or safety glass.

Each of these does a different job, and they often get mixed together in a specification.

• Limiter. Controls how far the window opens. It is there to prevent falls.
• Lock. Controls whether the window opens at all. It is there for security. Vistaza can supply a keyed handle: with the key out, the handle does not turn and the window stays shut.
• Safety glass. Tempered glass breaks into small, dull pieces. Laminated glass stays in place when it breaks. Glass near the floor is usually where the code asks for one of them.

Ventilation with a limited window.

Four inches still ventilates. If the ventilation design needs more air than that, or needs air while the window is shut and locked, solve it somewhere else. Do not solve it by leaving the limiter off.

Engaged before handover.

A limiter only counts as a limiter when it is fully engaged. During installation the limiters are released, so the sashes can swing open for fitting, adjusting and cleaning. Before the building is handed over, every one has to be engaged again and checked. A limiter that is fitted but not fully engaged is not a fall-prevention device.

Fitted is not enough. Engaged, and checked, on every limited window.

A checklist for the schedule.

1. Every operable window comes with a limiter. Mark any opening where the limiter must be left out or released, and say why.
2. Who engages and checks every limiter before handover to the owner, the contractor or the resident, and who signs that it was done.
3. Who holds the release tool after handover.

Limiters sit with the other options when you choose a window. The place to confirm them, opening by opening, is the shop drawings.