What is background ventilation under Part F?

Background ventilation provides continuous low-level air exchange through trickle ventilators (small openable slots in window frames). ADF Table 5.2a specifies minimum equivalent areas (EA): 5000 mm² per habitable room and 2500 mm² per wet room (kitchen, bathroom, WC). Trickle vents should be controllable by occupants but designed to be left open.

How do I calculate the whole dwelling ventilation rate?

ADF Table 5.1a gives the minimum rate based on bedrooms: 1 bed = 14 L/s, 2 bed = 21 L/s, 3 bed = 28 L/s, 4 bed = 35 L/s, 5 bed = 42 L/s. Also calculate 0.3 L/s per m² of floor area and use whichever is greater. This is the total ventilation rate for the whole dwelling, provided by background ventilators and/or mechanical systems.

What is equivalent area (EA) for a trickle vent?

Equivalent area is a standardised measure of the airflow capacity of a background ventilator, measured in mm². It represents the area of a sharp-edged orifice that would pass the same airflow. A trickle vent with 5000 mm² EA provides approximately 5 L/s at 1 Pa pressure difference. EA is marked on the ventilator and tested to BS EN 13141-1.

What is purge ventilation?

Purge ventilation is the ability to rapidly ventilate a room — for example, after decorating, spillage, or smoke. ADF requires an openable window (or external door) with an opening area of at least 1/20th of the room floor area in each habitable room. This is separate from background and extract ventilation requirements.

Do I need trickle vents if I have MVHR?

Generally no. System 4 (MVHR) provides both supply and extract ventilation mechanically, so trickle ventilators are not normally required. However, purge ventilation (openable windows) is still needed in habitable rooms. If the MVHR system is turned off, the dwelling would have no ventilation without trickle vents, so some designers include them as a failsafe.

TradeCalculator

Background Ventilation Calculator — Part F Trickle Vents

Calculate minimum whole dwelling ventilation rate, trickle ventilator equivalent areas per room, and purge ventilation openings per Approved Document F Table 5.1a and 5.2a.

Dwelling Size

Determines whole dwelling vent rate

Bedrooms

5000 mm² EA each

Living Rooms

5000 mm² EA each

Kitchens

2500 mm² EA each

Bathrooms

2500 mm² EA each

WCs

2500 mm² EA each

How We Calculate This

This calculator determines background ventilation requirements per Approved Document F (2021) System 1 (background ventilators with intermittent extract).

ADF Table 5.1a — whole dwelling rates

1 bedroom: 14 L/s
2 bedrooms: 21 L/s
3 bedrooms: 28 L/s
4 bedrooms: 35 L/s
5 bedrooms: 42 L/s
Or 0.3 L/s per m² floor area (whichever is greater)

ADF Table 5.2a — trickle vent EA

Habitable rooms (bedrooms, living): 5000 mm² EA each
Wet rooms (kitchen, bathroom, WC): 2500 mm² EA each

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.