Regulations: Underfloor Ventilation in Northern Ireland

Introduction

Providing a building with adequate and effective underfloor ventilation is an essential foundation for the prevention of moisture and damp and the build-up of dangerous gases, such as radon. For your convenience, we have created a sub-floor ventilation calculator (see below) and outlined extracts from the following regulations:

Building Regulations (Northern Ireland), Technical Booklet C (October 2012).

NHBC Standards.

BS 5250:2021.

For detailed guidance, refer to the full versions of the regulations and standards linked above.

Technical Booklet C: Site preparation and resistance to contaminants and moisture

Suspended Timber Ground Floors

5.13(c): A suspended timber floor next to the ground should be constructed as follows:

With a ventilated air space measuring at least 75 mm from the ground cover to the underside of any wall plates and at least 150 mm to the underside of the suspended timber floor (or insulation if provided).

Two opposing external walls should have ventilation openings placed so that the ventilating air will have a free path between opposite sides and to all parts. The openings should be not less than either 1500 mm²/m run of external wall or 500 mm²/m² of floor area, whichever gives the greater opening area.

Any pipes needed to carry ventilating air should have a diameter of at least 100 mm.

Suspended Concrete Ground Floors

5.16(b): A suspended concrete ground floor should meet the requirement to resist the passage of moisture from the ground if it incorporates:

Where there is a ventilated air space below the floor it should measure at least 150 mm clear from the ground to the underside of the floor (or insulation if provided).

Two opposing external walls should have ventilation openings placed so that the ventilating air will have a free path between opposite sides and to all parts. The openings should be not less than either 1500 mm²/m run of external wall or 500 mm²/m² of floor area, whichever gives the greater opening area.

Any pipes needed to carry ventilating air should have a diameter of at least 100 mm.

NHBC Standards

5.2.10 Damp-proofing and ventilation

Ventilation should be provided to precast and timber suspended floors. This is generally provided by ventilators on at least two opposite external walls, with air bricks properly ducted in accordance with Chapter 6.1 ‘External masonry walls’. Where this is not possible, suitable cross ventilation should be provided by a combination of openings and air ducts. Ventilation should not be obtained through a garage.

Sleeper walls and partitions should be constructed with sufficient openings to ensure adequate through ventilation. If necessary, pipe ducts should be incorporated in adjoining solid floors, separating walls or other obstructions. Where underfloor voids adjoin ground bearing floors, ventilation ducts should be installed.

Void ventilation should be provided to whichever gives the greater opening area: 1500mm² per metre run of external wall or 500mm² per m² of floor area.

In the case of timber floors, ventilators should be spaced at no more that 2m centres and within 450mm of the end of any wall.

BS 5250:2021 – Management of moisture in buildings

F.4.2 Suspended concrete floors and F.4.3 Suspended timber floors

In the case of dwellings, subfloor ventilation should be provided by vents not less than 1,500 mm²/m run of external wall or 500 mm²/m² of floor area, whichever is the greater.

Make a sub-floor ventilation calculation

Total Floor Area (m²):
Run of Exterior Wall (m):

Results

General notes:

Position ventilation clear of hard landscaping and vegetation.

Where flooding is a risk, consider using Rytons Periscope^®Vertical Extension to raise the height of the Periscope^® by up to 1m above ground level, this may prevent low-level flooding and protect the property for longer against higher flood water. Note: Flood water deeper than 1m should be allowed into the property to relieve pressure on the walls.

Specify Rytons underfloor ventilators by name. Alternative products may have lower rates of ventilation which could leave the sub-floor under ventilated. Find NBS specifications here.

Rytons Periscope^® Underfloor Ventilator with Multifix^® Air Brick is a plastic unit and therefore is not suitable for use within the external walls of high-rise buildings requiring non-combustible materials. For a compliant alternative, see Rytons A1® Fire-rated Periscope® Underfloor Ventilator.

Rytons Underfloor Ventilation Range

Achieve compliant sub-floor ventilation with Rytons Periscope^® Underfloor Ventilators and range of accessories. We are always happy to assist you with ventilation calculations and product selection – call our technical sales office on 01536 511874 for advice.