Ventilation Rates

Standard Airtightness Method

Where neither a measured air permeability or design air change rate is known, the whole-building air permeability is assumed based on BS 12831 Table B.6 default values.

Property type	Air Permeability @ 50 Pa (m³/h.m²)
All properties	12

NOTE

Refer to BS EN 12831-1:2017 for more information.

Ventilation Systems

Where a mechanical ventilation system exists, the supply and extract ventilation rate will vary based on the type of system.

The supply air temperature will be the design external air temperature in all cases except MVHR, where the heat recovery percentage will be applied relative to the extract air temperature.

The data is derived from Standard Assessment Procedure (10.2) Section 2.6.1 and converted to a airflow volume based on the total internal air volume of the ventilation zone.

Ventilation System	Supply Flow Rate	Extract Flow Rate
Mechanical Ventilation (MV)	0.5 ACH	0.5 ACH
Mechanical Extract Ventilation (MEV)	-	0.5 ACH
Mechanical Ventilation with Heat Recovery (MVHR)	0.5 ACH	0.5 ACH
Positive Input Ventilation (PIV)	20 m³/h	-

NOTE

Refer to Standard Assessment Procedure (10.2) for more information.

Minimum Room Air Change Rates

For each heated room, the default minimum air change rate (ACH) used is determined by the room type (habitable vs non-habitable).

Additionally, ANY room that has no external envelope area (e.g. fully enclosed rooms such as landings) will use a zero minimum air change rate.

This data is derived from CIBSE Domestic Heating Design Guide (2026) Table 2-18.

Room Type	Minimum ACH (1/h)
Bathroom	0.5
Bedroom	0.5
Bedroom with en-suite	0.5
Bedroom/study	0.5
Breakfast room	0.5
Cloakroom/WC	0.5
Dining room	0.5
Dressing room	0.5
Family/breakfast room	0.5
Games room	0.5
Hall	0.5
Internal room/corridor	0.0
Kitchen	0.5
Landing	0.5
Lounge/sitting room	0.5
Living room	0.5
Shower room	0.5
Store room	0.5
Study	0.5
Toilet	0.5
Utility room	0.5

NOTE

Refer to CIBSE Domestic Heating Design Guide (2026) for more information.

Infiltration Rates of Ventilation Devices

When unknown, the infiltration rate of ventilation devices will be determined by the device type based on the table below. This data is sourced from Standard Assessment Procedure (10.2) Table 2.1.

Ventilation devices that are classified as an Air Terminal Device (ATD) according to the BS 12831 definition will increase the infiltration rate of the room that the device exists in.

Ventilation Device Type	Infiltration Rate (m³/h)	Air Terminal Device
Chimney or flue (closed fire)	10	No
Chimney or flue (solid fuel burner)	20	No
Chimney or flue (other heater)	35	No
Chimney (open)	80	No
Chimney (blocked)	20	No
Flueless gas fire	40	Yes
Intermittent extract fan	10	No
Other open flue (vertical duct)	20	No
Passive vent	10	Yes

NOTE

Refer to Standard Assessment Procedure (10.2) for more information.