Condensation in loft

My Video

Watch me fit this product in my loft and listen to me explain how and why condensation forms in the first place:

What Causes Condensation in Lofts?

Every occupant of your home produces moisture from:

• Breathing.
• Sweating.
• Cooking.
• Bathing.
• Drying clothes, etc.

Warm air pulls this moisture upwards and into the loft where in theory, it shouldn’t cause a problem.

However, when the loft is cold (from late autumn into winter and spring), the underside of the roofing felt and the timbers will be cold.

As the moist, warm air comes into contact with the cold surfaces in the loft, it condensates, just like it does on car windscreens in the winter or on cold taps in the house.

Next time you have a hot bath, look at the cold mirror, window or tap; condensation forms here just as it does in the loft.

The Solution

I’ve found the most effective way to cure loft condensation is to install ventilation so a breeze can enter the loft and pull the moisture out of the loft into the atmosphere (this method works for most lofts in the UK but not “warm roofs”).

There are several ways to achieve this, the most common being loft and roof vents:

Roof vents cost more and should be installed by a professional from outside the roof 

How to Install Loft Vents

Installing these loft vents couldn’t be easier; I’ve installed them myself and seen DIYers do it as well. The vents simply slip in between the felt overlap, as shown in my image below:

No tools or specialist equipment are needed to fit these vents, and you don’t need access to the exterior of the roof either.

I’ve fitted these dozens of times, and here’s my general rule of thumb guide:

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

Don’t forget, it can take some time before the moisture clears, especially if there’s no breeze.

I Found 1500 Reviews Online – Here’s How Others Rate These Vents

As part of my research into the best products to get rid of loft condensation, I looked at plenty of reviews and ratings online for this and similar products.

I know these vents work but I wanted to see how others rate them.

As of July 2022, I found 1650 reviews online.

91% of consumers rated these loft vents either 4 or 5 stars out of 5 which is very high.

Only 2% of reviewers rated the vents 1 star out of 5 which is very low.

I think these stats speak for themselves and I couldn’t find any issues worth reporting or investigating further.

11 Questions About Loft Condensation Answered

During my 17 years working as a roofing contractor, I was often asked questions about loft condensation; here are the answers to the most common questions:

Will My Heating Bills Go Up If I Install Ventilation? 

No, in most lofts, the insulation is placed on the loft floor. Your heating bills won’t be affected if the vents are installed above this.

I Never Had Condensation Before, Why Has It Formed Now?

Condensation forms in colder conditions, and it could be that you only entered the loft when it was warm, and none was present.

Also, if you add extra insulation to your loft, the warmth will stay trapped in the home below, meaning the felt and timbers in the loft will be colder. The colder the surfaces in the loft, the more condensation will form.

Boarding out a loft can also cause condensation for the same reason – by trapping heat in the home, the loft becomes colder and more susceptible to condensation.

Condensation In My Loft Comes and Goes. Why?

This is normal and, in most cases, is nothing to worry about. You should only be concerned if it persists and causes mould, dampness or rot.

The roof or loft vents require a breeze to pull the moisture out of the loft, so, on calm days, you may experience more condensation.

You may also notice more condensation when the outside temperature drops – it’s perfectly normal and should disappear when it warms up and should ease when there’s a breeze, provided you have sufficient ventilation.

Do Double Glazing Windows and Cavity Wall Insulation Cause Loft Condensation?

Old drafty timber windows allow moisture to escape from the home, and if you replace them with airtight modern windows, the moisture levels in the home and also the loft will increase, and condensation is more likely to form.

Cavity wall insulation keeps homes warmer for longer, and as warm air holds more moisture than cold air, you may notice a slight increase in loft condensation after installing cavity wall insulation.

Why Am I Experiencing Loft Condensation In The Summer, When It’s Warm?

This is not normal and should be investigated further.

It could be caused by:

• A roof leak.
• A pipe leak.
• Loose or faulty extractor fan or fan ducting.
• Hot water tanks in the loft failing (broken lid, defective thermostat or heating element etc.).

Is It Normal For a Loft To Smell Musty?

Lofts shouldn’t be damp or smell musty and I’ve found that such issues are usually a sign of a roof leak, ventilation problem or other faults that will need investigating.

Can I Install Vents To a “Warm Roof”?

A “warm roof” is when the insulation is placed directly under the roof tiles and the loft is usually the same temperature as the rest of the house.

If your loft insulation is placed on the floor of the loft, you have a “cold roof” and the loft will usually be colder than the rest of the home in the winter.

Loft lap vents should only be installed on cold roofs and never to warm roofs.

I Have Condensation In My “Warm Roof”. What Should I Do?

This is usually due to missing insulation under the roof tiles which causes cold spots where condensation forms. This should be investigated by a professional roofer if you can’t see any obvious failure of the insulation.

Will Loft Vents Cause Leaks?

The felt on most roofs is overlapped and not airtight, these vents just open the overlap slightly to create an airflow into the loft. If correctly installed, there is no way that these vents can cause a roof leak.

Are There Any Alternatives To Loft Vents?

Roof vents are just as effective but must be fitted from the exterior of the roof, and access equipment such as scaffold might be needed.

Mechanical ventilation, such as an extractor fan, would be effective, but I’ve rarely seen them in lofts and they consume electricity.

Will A Vapour Barrier Help?

A vapour barrier is a plasticky felt-like material that is laid above the ceiling plasterboard and is designed to let air and heat into the loft while blocking moisture/vapour.

A correctly installed vapour barrier can stop moisture from getting into the loft in the first place, but I know from experience that they are difficult and costly to retrofit into an existing home and are best suited to new builds, extensions and major renovations.

An incorrectly installed vapour barrier can cause condensation spots on the ceiling and I have seen cases where the condensation in the home went up after the barrier was fitted.

Modern types of vapour barriers allow some moisture through but not so much that condensation will be a problem in the loft.

In my opinion and from feedback from past customers, 99.9% of loft condensation cases can be resolved to an acceptable level by improving the ventilation rather than retrofitting a vapour barrier.

Loft Condensation Checklist

When I worked as a roofing contractor, I received dozens of phone calls every winter from customers with loft condensation issues.

Here’s a checklist I’ve created for anyone experiencing loft condensation:

• Is it unusually cold outside? It’s normal to see some condensation in the loft during cold snaps.
• Is it warm outside? It’s very unusual to see condensation in a warm loft. Check for roof leaks, burst pipes or faulty hot water tanks, lids, thermostats or heating elements etc.
• Is it unusually calm with no breeze? Wind is needed to flush out moisture via vents. If it’s unusually calm, wait a day or two and see if the condensation clears on its own.
• Double-check your loft insulation isn’t blocking any existing vents at the corners of the loft (where the roof meets the loft floor, there should be a gap).
• If you have boxes and possessions in the loft, make sure they aren’t blocking the existing vents.
• Have you recently upgraded your loft insulation or boarded out your loft? Your house may be warmer but your loft will be colder and more susceptible to condensation, try adding a few loft vents to improve ventilation.
• Are you creating excessive amounts of moisture in the home? Showering or bathing without opening the window or using an extractor fan can cause moisture levels to increase, as can washing clothes on radiators etc. The condensation will only form when the temperature in the loft drops so it may not be obvious that the lifestyle of the occupants is contributing to the problem.
• Do you have downlights in the ceiling? Can you install downlight covers to stop moisture from escaping into the loft (try these, they’re popular and sold on Amazon).

Further Reading

We’ve also published a price guide for ventilation options, including tile vents, eaves vents and flat roof ventilation. Explore our roof and loft ventilation price guide here.

Are you seeing condensation on your windows? This is often the result of high humidity levels in the home; some of this moisture will make its way into the loft. Discover how to get rid of window condensation here.

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