Ventilation in loft conversions




ventilation in loft conversions

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Lofts were never meant to be lived in; it was designed as a cold ventilation space. So, when we convert the loft, we need to create a new ventilation path or the loft conversion will be permanently damp.

Why is ventilation so important in loft conversions?

If you’ve not considered ventilation before , don’t worry. You’re not alone. The need for proper and efficient ventilation is often overlooked when the first plans are drawn up to convert the loft.

But, the reason that ventilation is so important is because when the use of the roof space is changed it alters from a ‘cold roof’ to a ‘warm roof’.

The original purpose of any loft was to act as a cold ventilation space that would expel any damp caused from rain or condensation.

This cold roof space was then effectively sealed from the rest of the house by laying down layers of insulation material.

Of course, when a loft is converted, that barrier of insulation must be removed to the new roof level ceiling. And it is there that the new loft conversion ventilation path must be placed.

If there is no ventilation above the new loft room then eventually the timbers will begin to rot because of dampness.

Making a ventilation path

Before the new course of insulation is fitted a 50mm air space has to be left below the underfelt of the roof tiles.

This creates a ventilation path that will allow air to be expelled from the roof space before condensation forms and causes dampness.

However, on its own a ventilation path will not afford adequate protection.

To encourage a proper air flow there should be small gaps or holes cut along the length of the eaves of the roof. This allows the air to enter from one side before exiting from the opposite side of the roof.

If the roof is very steeply pitched it may be necessary to also install vents in the ridge; this would in effect form a vacuum which would suck up air from the eaves.

But, this won’t be necessary in older houses that don’t have underfelt beneath the roof tiles as the air will naturally be expelled by the tiles themselves.

Other things to consider about ventilation in loft conversions

Timber stud walls are used extensively in loft conversions to create storage space along the eaves. The interiors of the cupboards will obviously be well ventilated but they may be sitting on insulation from the bedroom ceilings.

This can cause black mould but won’t be a problem as long as plastic ducts are fitted above the soffits. This allows the eaves to be ventilated but also retains that important insulation.

To allow adequate air flow where skylights have been fitted, ventilation holes should be drilled into the rafters on each side of the window.


As with everything else the use of new technology makes many of the old rules obsolete and this is true enough in the case of loft conversion ventilation.

Most new roofs are now built with insulation boards installed above the rafters. This creates a warm roof which doesn’t need any further ventilation.

The use of new breather membranes as underlay in the roof also means that the need to create ventilation paths is greatly reduced when converting the loft.

Types of ventilation in loft conversions

Good ventilation is crucial to maintain the air quality when converting the loft.

With loft conversions a lot of effort and time is spent fitting vapour barriers so that the moist air can’t attack the new timber in the roof space or the wooden structure of any dormers that have been added.

But all this means that unless adequate ventilation is provided the new loft room will suffer from damp and condensation.

To prevent this the homeowner has several ventilation choices which should be considered when the first fix electrics are installed.

How much ventilation should a loft conversion have?

As much as possible is the easy answer and this can be a mix of background, rapid, and extraction ventilation systems.


An easy option. Basically, background ventilation is performed by the small air vents in windows.

This is a continuous process that can lead to draughts. To prevent this the vents in the window frames should be around two metres from the floor.


Sometimes referred to as rapid purge ventilation this is no more complicated than opening a window! All windows in habitable rooms must be openable so that any foul air can be quickly expelled by opening the window.

There are a few specific requirements here though. The casement of the windows must be equivalent to at least 5% of the room’s floor area or, where skylight windows have restricted opening, 10%.


Extractor fans can be installed very easily into most rooms at the first fix electrics stage and are simple to operate.

The three loft ventilation methods outlined above apply to any habitable room such as a living room or bedroom but, as you would expect, loft conversion bathrooms are a special case.


Unsurprisingly ventilation is more important in the bathroom than anywhere else in the home.

Hot baths or showers cause a lot of condensation and this obviously has to be dealt with before the moisture soaks into the new walls and ceilings.

Understandably the building regulations are more stringent for bathrooms than for the rest of the property but they are still very easy to comply with.

In bathrooms extractor fans must be fitted. The minimum specification for a fan is 15 litres per second (of air) and must have an overrun of 15 minutes (i.e., they must continue working for 15 minutes after they have cleared the air.

This overrun period can be ignored if there is a vented opening window in the bathroom.

One safety point that should be mentioned is that extractor fans are of course wired into the mains so ensure they are positioned well away from where they can be accidentally touched by a wet hand.

Why is ventilation important for a loft conversion?

Without adequate ventilation the new loft room will suffer terribly from damp. Eventually this dampness will begin to rot the new timbers which were used to build the conversion.

Can you over ventilate a loft?

Surprisingly, yes it can. If there is too much ventilation, vents in the roof for example, you increase the risk of rainwater getting into the roof and causing damage. Of course, the converse side of this is that if you don’t have enough ventilation any moisture won’t be able to escape, and you’ll have problems with condensation.

How do I stop condensation in my loft conversion?

It can be as easy as opening a window. As we’ve said throughout this article ventilation in loft conversions is so important, and good ventilation will prevent any issues with condensation. We go into this in much more detail in this article: Condensation in Loft Conversions.

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