Felt Lap Vents

Felt lap vents increase the amount of ventilation into existing roof spaces and lofts. They are an affordable solution for those looking to reduce the build-up condensation in their roofs and attics. They are especially useful in properties that have poor ventilation and that are susceptible to damp as a result.

It’s important to combat poor ventilation in loft spaces because, if let untreated, items stored in the attic could become damaged from the damp, and it could penetrate the insulation, causing it to get wet. This can be a tricky issue to fix. Furthermore, in the very worst cases, persistent dripping can cause damp patches to appear on your ceiling.

Much like other products we sell, homeowners can easily install lap vents themselves. This means you don’t need to worry about hiring and paying for a specialist roofer to do the job for you. With proper ventilation, you can also avoid the costs associated with treating damp and replacing items that have been ruined as a result of mould.

You can shop the range below, but if you can’t find exactly what you need, please contact us and we’ll be happy to help and advise further.

Buy Felt Lap Vents

Manthorpe Felt Lap Vents

As low as

£1.77 per vent (incl VAT.)

Easy to Fit

Affordable Solution

Prevents Condensation

Frequently Asked Questions

How many felt lap vents do I need?

Luckily, you don’t need a calculator for this product. From the installation guide, it is recommended to place one ventilator into the laps in every other rafter bay down at a low level (as near to the eaves as possible) and on both sides of the roof space to ensure a cross-flow of ventilation.

How do I fit felt lap vents securely?

Installing felt lap vents is an easy process that homeowners can complete without any professional assistance. You first want to make sure you can physically install them without any barriers or obstructions, so this may mean clearing a space and moving objects out of the way so that there are no safety hazards.

You don’t need to buy felt lap adhesive because the vents slot into felt laps of the roof. To find out more detail on how to install them, we recommend reading our felt lap roof vent installation guide below. If you’re still struggling, please call us on 0333 006 3838 and our team will be happy to talk you through it.

Why does condensation happen?

Condensation happens when warm air clashes with cold surfaces, or simply when there’s too much humidity in your home. This is prevalent in winter when your heating system comes on in the cooler hours of the mornings and evenings.

Felt lap vents won’t entirely prevent condensation as they do not meet the requirements for attic ventilation as a solitary tool. They are designed to be used in roof spaces that have recently had additional insulation installed. They provide airflow to the batten void, allowing air to circulate and water vapour to dissipate.

When used in combination with proper ventilation materials and permeable roof coverings, roof lap vents are an effective tool for reducing condensation, providing adequate protection to your home.

Felt Lap Vents Fitting Instructions

The Manthorpe Felt Lap Vents are 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 &

Step 1

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.

Step 2

Orientate the G630 so that flat face is facing upwards and the curved face with the felt clips is facing down.

Step 3

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.

Step 4

Push the vent down into the felt lap, ensuring the flat top is in line with the underside of the battens.

Step 5

When it 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.

Step 6

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.

Step 7

Working across the roof space, slot the vents into the appropriate felt laps between joist openings to give the required amount of ventilation.