The Manthorpe Felt Lap Vent is a refurbishment product designed to increase the amount of ventilation into existing roof spaces with a lapped felt/membrane construction. The product is intended to be fitted retrospectively to properties that have had additional loft insulation recently installed. The vent will help to combat the problem of condensation build-up caused by the extra insulation by adding additional ventilation into the roof space. The product will not meet the requirement for roof space ventilation solely on its own; it will only provide airflow through the felt laps into the batten void. The rate at which water vapour is then removed from this batten space is determined by the permeability of the roof covering, see BS 5250:2011, H.4.1. Other measures may need to be in place to ensure that the adequate amount of ventilation is provided to the roof space. The specifier should also be aware that the opening of felt laps on the property may adversely increase the wind uplift load on the tiles/slates and affect the driving rain resistance of the roofing felt, for more information see BS 5534:2003, 5.8.1 & 220.127.116.11
Initially, it is recommended that one ventilator be placed into the laps in every other rafter bay, down at low level (as near to the eaves as possible) and on both sides of the roof space to ensure a cross-flow of ventilation. In extreme circumstances, additional ventilators can be positioned in the vacant felt laps to increase the overall amount of ventilation provided. Only one product should be positioned into a single felt lap. If additional vents are required, then more vents can be placed into the other felt laps within the same rafter spacing at higher intervals. The product can be installed in roof spaces with rafter spacings ranging from 400mm to 600mm. Each unit will provide 3,000mm² of free airflow into the batten void. For further information on the recommended ventilation requirements for roof spaces refer to BS 5250 Code of Practice for Control of Condensation in Buildings.
Orientate the G630 so that flat face is facing upwards and the curved face with the felt clips is facing down.
Push the tapered front edge of the vent into the lap of the felt, once the front edge is in; pivot the vent up flat against the felt forcing the lap apart.
Push the vent down into the felt lap, ensuring the flat top is in line with the underside of the battens.
When the vent is almost in position, the felt clips will slide over the lower lap of the felt. Ensure that all 3 clips are properly located before proceeding.
Once correctly installed all that will be visible of the vent is the top edge with the stated airflow of 3,000mm² and the 3 felt clips.
Working across the roof space, slot the vents into the appropriate felt laps between joist openings to give the required amount of ventilation.