What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
No local equivalenttrickle vent
also background ventilator, night vent, slot vent
Required in most new and replacement windows, sized in equivalent area in square millimetres per room, and the commonest reason a replacement window job needs more than a like-for-like swap.
Australia
Close, not identicaltrickle vent
also window vent, background ventilator
Present in the vocabulary and far less commonly required, since the climate and the construction tradition lean on openable windows and on mechanical extract.
United Statesyours
No local equivalent(no common term)
also window vent, fresh air vent
Not part of standard practice. Background ventilation is delivered by a mechanical system — an exhaust fan running continuously, or a balanced ERV or HRV — and the envelope is sealed rather than deliberately perforated.
Canada
No local equivalent(no common term)
also make-up air inlet, passive inlet
As the United States, with balanced mechanical ventilation effectively mandatory in new housing, so a passive hole in the window is contrary to the design intent.
The governing standard, by market
United Kingdom
Approved Document F / BS EN 13141-1
Means of ventilation, including background ventilator equivalent areas per room, and the performance testing of externally and internally mounted air transfer devices.
United States
ASHRAE 62.2
Ventilation and acceptable indoor air quality in residential buildings, delivered mechanically rather than by envelope openings.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Why does Britain put holes in its windows when everyone else seals them?
- Because the two traditions answered the same question with different technology. As houses were draughtproofed, the incidental air exchange that had always removed moisture disappeared, and something had to replace it. North American practice replaced it with a MECHANICAL system — continuous extract, or a balanced heat recovery unit — and sealed the envelope so that system is actually in control of the air. British practice replaced it with controllable PASSIVE openings, distributed through the rooms, backed by intermittent extract in the wet rooms. Each is coherent on its own terms. The failure mode is mixing them: a British house that has been sealed up without background ventilators has no path at all, which is the condensation and mould complaint in a great many modern homes, and it is also why fitting an extractor fan without providing replacement air does not fix it.
- What is equivalent area, and why not just state the hole size?
- Because the hole size does not tell you how much air passes through it. A ventilator is not an open aperture: it has a grille, a hood, an insect mesh, a closing flap and a path with bends in it, and each of those resists flow. EQUIVALENT AREA is the measured answer — the area of a sharp-edged orifice that would pass the same airflow at the same pressure difference — so it is a performance figure derived from a test rather than a dimension read off a drawing. That is why a ventilator's equivalent area is always smaller than its visible slot, why two products of the same apparent size can differ substantially, and why a specification states equivalent area in square millimetres per room rather than naming a product. Substituting a similar-looking vent is a change to the ventilation of the room.
- Do they make the house cold, and should they be taped shut?
- They cost a little heat and closing them costs more than that. The airflow through background ventilators is small — the design intent is a continuous trickle, not a draught — and its heat cost is real but modest against the heat lost through the glass beside it. What closing them costs is the moisture path: the water vapour a household generates has to leave, and if it cannot leave through the ventilators it leaves by condensing on the coldest surface available, which is usually a window reveal, an external wall corner or a cold bridge. So a taped-over vent is very often the hidden cause of a mould complaint on the wall next to it. The correct response to a vent that feels draughty is usually to close its flap in the coldest weather and open it again, which is what the flap is for.