Trickle vents, explained.

A trickle vent is a small slot through the head of a window frame, with a cover that opens and closes. It lets a steady, low flow of outside air into the room while the window stays shut and locked. It is also called a background ventilator. It is a simple part, and it touches several of the window's ratings.

What it does.

Tight windows do not leak much air. That is the point of them. But a tight room still needs fresh air, and it needs a way to lose moisture. Without either, the air goes stale, humidity rises, and condensation and mold follow.

A trickle vent supplies that air in a controlled way. It works with the window closed, so it works when nobody is home, overnight, and on floors where the windows carry opening limiters.

There are two kinds:

• Fixed. Always open. It gives a constant flow that the resident cannot shut off.
• Adjustable. The resident slides or tilts the cover to open or close it.

Where it goes.

The vent sits high on the window, in the upper part of the frame. Air that comes in at that height mixes with the room air before it reaches anyone, so it is not felt as a draft.

There are three ways to fit one:

Method	How it is fitted	Worth knowing
Integrated	Built into the frame when the window is made	The neatest result. It appears on the shop drawings
Slot vent	A slot is routed through the frame, with a cover inside and a hood outside	The usual adjustable type
Surface-mounted	Fixed to the face of the frame, inside or outside	Mostly a retrofit

On a new building the vent should be ordered with the window and routed at the factory. Cutting slots into finished frames on site puts the frame and its drainage paths at risk.

When to use one.

• Sound-rated facades. Acoustic glass only works with the window closed. If residents must open the window to get fresh air, the acoustic rating is lost every time they do. An acoustic trickle vent gives them the air without opening the window.
• Rooms that make moisture. Bathrooms, kitchens and laundry rooms. A vent gives humid air a way out and replacement air a way in.
• Replacement work. If the old windows had vents, the ventilation design may depend on them. The new windows should provide at least the same vent area, unless the mechanical design has changed.
• Windows with limiters. A window held to four inches still ventilates. A vent adds air that does not depend on the window being open at all.

Whether a building needs trickle vents is a question for the ventilation design, not for the window maker. The mechanical engineer decides if the window is part of the fresh-air path, and how much open area each room needs. US codes do not ask for trickle vents by name. The mechanical code lets a room be ventilated naturally, through openable area to the outdoors (4 percent of the floor area in the International Mechanical Code), or mechanically, and apartments built to the energy code's air-tightness limits are usually ventilated mechanically. The window schedule then carries that vent size, room by room.

What a vent does to the ratings.

A vent is a hole through the frame, made on purpose. It is common to read that modern vents have no real effect on heat or noise. That is too generous. Here is the straight version.

• Sound. Sound takes the easiest path. An open slot is easier than any glass. A standard vent will pull down the rating of an acoustic window, which is why acoustic vents exist.
• Air leakage. An open vent passes air. That is its job. What matters is how tight it is when closed, and whether the project's air-tightness test is run with the vents open, closed or sealed. Settle that before the test.
• Energy. The vent adds ventilation heat loss while it is open, and the energy model should count that as ventilation.
• Water. The outside hood has to shed wind-driven rain. Ask how the vent performed on the window in the water test.
• Security. The slot is too small to reach through, and the window stays locked. That is an advantage over a window left ajar.

A vent is a hole made on purpose. Size it, place it, and count it.

A checklist for the schedule.

1. Which rooms need vents, and the open area for each, from the mechanical engineer.
2. Fixed or adjustable.
3. Standard or acoustic. On a sound-rated elevation, assume acoustic.
4. Where it sits in the frame, and the color inside and outside.
5. How the vents are set during the air-tightness test and the field water test.
6. Where the vent appears on the shop drawings, so that it is approved with the window.

Trickle vents sit with the other options when you choose a window. If the vents are going on a noisy elevation, read the sound page first.