Loft Conversion Ventilation

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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 than 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 a 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 be the tiles themselves.

Other Things To Consider About Loft Conversion Ventilation

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.

For more info see the related pages below

The Roof And A Pre-Build Survey

Loft Conversion Ventilation And The Thermal Envelope

The Specifications For Your Project

Package Deals For Converting Your Loft

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