Manthorpe Felt Lap Vent (Box of 50) The Manthorpe Felt Lap Vent is a refurbishment product designed to increase the amount of ventilation into existing roof spaces. Usually fitted retrospectively to properties with additional loft insulation recently installed, however the Felt Lap Vent will help to combat the problem of condensation build up in any roof space with a lapped felt/membrane construction. The product will not meet the requirement for roof ventilation solely on its own, it will only provide airflow through the felt laps. Other measures may need to be in place to ensure that the adequate amount of ventilation is provided for the roof space. Product Benefits Can be fitted at joist centres ranging from 400mm to 600mm 3,000mm2/m airflow Easy to fit, the tapered lead in helps to slot the product between the felt laps The 3 felt clips help secure the product in place even if the edge of the felt is degraded