Wet Loft? Try Lap vents

Written by Daniel Woodley. Fact checked by Paul Farley. Published to Blog on the 2nd November 2023.

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We are fast approaching that time of year again.

Every winter, people go into their lofts to put away items they won’t need until next year or to take out the Christmas decorations.

And because it’s colder outside in winter, the underside of the roof is often covered in condensation – that’s moisture created by the occupants of the home that has risen into the loft space and condensated on the cold surfaces.

When I worked as a roofing contractor, I would get dozens of calls in late November or early December when folks go into the loft to get those decorations out of their boxes.

Interestingly, I would get another batch of phone calls in early January, as this is when people go back into the loft to put the Christmas decorations away.

If it was a particularly cold winter, I would get hundreds of calls and emails.

In this short blog post, I’ll show you a way you can get rid of loft condensation yourself – provided it is safe for you to go into your loft.

These plastic vents open up the felt so air can get into the loft.

Lap Vents: A Simple Solution

Lap vents are plastic trays that you can insert into the felt overlap in the loft, this opens a small gap for air to flow through.

As air flows into the loft, it picks up the moisture and takes it outside.

Improving the airflow into a loft is the quickest, easiest and cheapest way to remove excess condensation.

When I worked as a roofing contractor, I would install dozens of loft and roof vents each year, sometimes hundreds.

I know from experience that in 99% of loft condensation cases, loft lap vents will cure the problem.

I will use a bathroom as a comparison: What do you do after you shower and there’s condensation over the walls and windows in your bathroom? You open a window. Why? To vent the room so the moisture can escape.

Loft vents do the same thing in your attic.

I’ve never had issues fitting these lap vents, they just slide into place

Caveats

Before you go off and buy these vents, there are a few caveats you need to know:

1) They only work in lofts where the roofing felt is visible between the timber rafters.

2) I have seen cases where the roof felt is so tight that the vents won’t fit – I suggest pushing a finger between the felt overlap to see if there’s enough room for a vent.

3) Most lofts have vents at the eaves (the lowest part of the loft), but if these are blocked with loft insulation, loft flooring boards or boxes of possessions, the airflow into the loft will be reduced – I suggest unblocking any existing lap vents before buying more. I have seen cases of severe loft condensation being cured by simply opening up existing vents.

4) There are other causes of loft condensation and alternative products to consider (see link at the bottom of this page).

How to Install The Loft Lap Vents

I’ve never found it challenging to install these vents, in fact, getting into the loft is harder than fitting the vents!

(Don’t forget safety; people have fallen through ceilings so only fit these yourself if you can work safely in the loft)

These vents just slip into the narrow gap between the two layers of roofing felt.

No tools are needed, just slide them into place:

Me sliding the vent into place

The Number of Vents You’ll Need

There’s no set number of vents a loft will need, as this depends on:

• Size of the loft.
• Level of condensation.
• Number and size of existing vents.

I have installed these and similar vents in dozens of lofts and based on my experience and as a very general rule of thumb:

Back-to-back terraced house: 5-7 vents

Small terraced house: 10+ vents

Semi-detached house: 15+ vents

Small detached house: 20+ vents

Large detached house: 30+ vents

By creating a small gap, air can flow into and out of the loft

Workarounds/Problem Solving

Over the years, I have seen a few consumers experience issues with these vents, here are the solutions that I’ve found work very well:

Problem: The vent is too wide and won’t fit between the roof timbers.

Solution: I’ve used a fine saw to cut the vents down to size, they then fitted into the gap between the roof rafters.

Problem: The felt is too tight, and the vent won’t fit.

Solution: I have cut the vents in half before and only put half a vent into the gap, it then fitted easily.

Problem: I still see condensation after installing the vents.

Solution: You may need more vents. Also, I have seen a few cases where on calm, no-wind days, the condensation persisted. These vents rely on a breeze to force air into the loft and pull out the moisture. When the wind picked up, the condensation eased.

Don’t forget: It’s normal to see a little bit of condensation after you’ve created moisture in the home (cooking, bathing, showering), especially when it’s very cold in the loft. I’ve found it always clears after installing the lap vents and waiting for a breezy day. 

How Long Does it Take For The Lap Vents to Work?

I’ve installed these vents on homes where it then cleared the condensation in a matter of hours (just like opening a bathroom window to clear condensation after a bath).

I’ve also seen cases where it has taken several days to ease, from experience I believe these are the factors that determine how long it takes to work:

• How many vents are installed.
• If it’s a windy or calm day.
• How cold the loft is.
• How much moisture is making its way from the home into the loft.

Other Causes of Condensation, Alternative Products and Further Reading

Based on my experience, loft vents solve condensation problems in 99% of cases, but there are other reasons why moisture may build up in a loft, such as:

• Structural changes were made to the loft.
• Poorly constructed loft.
• Leaks.
• Insulation, boxes or flooring disrupting the airflow in the loft.
• Excess condensation in the home below due to poor home ventilation (no window vents etc).

I have previously published a more in-depth guide to condensation in a loft and that would be a good place to start if you are still having issues after installing lap vents.

This page looks at the cost of getting a professional to install loft vents for you.

YouTube Videos

I have published two YouTube videos that cover the topic of loft condensation and loft ventilation.

In-depth video:

Here’s a quick 30-second YouTube short: