Paragraph 9 Specifications for permanent vents
Every permanent vent type ‘A’ shall consist of a sound-attenuating purpose-made opening or duct which is designed to allow the passage of air between the room and the external air at all times, and shall have an external cowl or grille for protection against the passage of snow, rain and vermin. It shall be so constructed that when installed in a cavity wall exposed to the weather, the weather resistance of the cavity is retained and the acoustic performance of the vent is protected.
The effective area of the air path through the permanent vent shall be ascertained by the method set forth in paragraph 8(g) above, save that for the words “inlet fan ventilator system” in that paragraph there shall be substituted the words “permanent vent”.
When installed, the permanent vent shall be securely fixed to the wall and the junction between the vent and the face of the wall shall be fully sealed by means of compressible strip, mastic packing, cover strips or other equally effective means, and where the vent has an air supply duct traversing a cavity wall the junction between the supply duct and the internal leaf shall be fully sealed by suitable means to prevent the passage of air to or from the cavity.
Every vent shall be so constructed and installed as not to present a fire hazard.
Every permanent vent type ‘A’ shall—
have an effective area, calculated in accordance with sub-paragraph (2), of not less than 3,250 square millimetres; and
be so constructed that the sound pressure level difference ascertained by the method set forth in paragraph 8(h)(ii), complies with the requirements therein set forth.