How to Ventilate a Non-Breather Roof Underlay

Nowadays, breather membranes as roof underlay are becoming more and more popular. However, many older houses still use non-breather underlay. However, there’s no need to fret about replacing your underlay. Some handy felt lap vents can help give your loft a breath of fresh air without having to make major changes.

The reason breather membranes became more popular than non-breather membranes is that while both resist water – providing backup for the tiles on the top of the roof – breather membranes are permeable. However, a roof is also at risk of water vapour from within the house – it can cause condensation. Condensation leads to rotting, rotting leads to mould, and both lead to serious damage to the house’s integrity as well as making it less fit for human habitation. Making sure that the roof is properly ventilated is therefore of the highest concern.

But while breather membranes allow water vapour to leave, but not enter, non-breather membranes allow no air penetration in either direction. This can leave the roof vulnerable to condensation. But felt lap vents can allow air to go through the underlay, sidestepping this issue.

Manthorpe Felt Lap Vent

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You will need:

• Some Felt Lap Vents
• That’s it (really)

All you need is some felt lap vents and they can be installed without any need for a saw, ladder, or even a utility knife. Count the number of rafter bays in your loft, on both sides of your roof. Divide that number by two and you have the amount of felt lap vents that you need. 

Simply take the vent and look for the lowest or second lowest overlap in the roofing felt in that specific rafter bay. Rotate the vent around so that the clips are facing you and pointing downwards. Then push the vent into the overlap so that the clips hang it around the roof felt. Do this for every other rafter bay in the roof. 

It’s just that simple – a toolless installation that can be accomplished by any able bodied person. But it can make a huge difference to your roof, and is a must for those with non-breather membrane roof underlays. In addition to felt lap vents, which provide ventilation for underlay, you should also ensure  ventilated in other layers. Roof tile vents and soffit or fascia vents can be used in order to achieve a completely ventilated loft.