Condensation in Loft Conversions and How to Avoid It

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The risk of condensation when converting the loft is very high but happily it can be easily avoided with proper insulation and planning.

The reason condensation is such an issue when converting the loft is quite simple.

Adding a new room in the roof space will mean that there will now be lots of moist air that will look to escape from the roof.

This moist air will reach the structural timbers (because warm air rises) and turn into damp. As can be imagined this will lead to all sorts of trouble with rotting timbers.

Preventing condensation in the loft

Thankfully there are simple steps that can be taken to eliminate the problem of condensation.

The first step is to install a vapour control layer. Basically this acts as a barrier to minimise the amount of moisture that escapes into the roof.

The vapour control barrier also helps to prevent heat loss so as well as avoiding condensation the barrier combines with the insulation to increase the energy conservation in loft conversions.

And that, ultimately, will reduce your heating bills. Your builder will be able to give more advice on this.

Installing a vapour control layer

The vapour control layer can be installed on the roof, walls and ceilings to make a fully insulated shell in the new loft room.

Once the insulation has been installed the vapour control layer is then added.

The simplest and easiest vapour control layer to use is a polythene sheet.

It is important to remember that the VPL is fitted to the room side of the insulation and must be done so before any plaster boarding is done.

There are some different types of vapour control layers that can be used to eliminate condensation in loft conversions and typically these are foil covered insulation boards. The foil on the board essentially fulfils the same role as the polythene sheet.

Ventilation in lofts and loft conversions

Maybe not the most exciting of subjects but ventilation in loft conversions is hugely important and must be carefully considered by the homeowner if you are planning to convert your loft.

Without adequate ventilation the moist air that manages to penetrate the vapour control layer will cause damp within the timbers of the roof structure.

Why ventilation is an issue

To discover why ventilation is so important to avoiding condensation in loft conversions we need to consider the original purpose of the roof space.

Lofts were of course made to be well ventilated, in fact they were meant to be as draughty as possible whilst remaining watertight. They were built this way to allow any damp or condensation to easily escape.

The insulation laid down on the loft flooring would protect the rest of the house from the draughts in the loft by creating a sealed unit that would trap the heat within the first two stories of the property.

This was known as creating a ‘cold roof’ and is a very effective way of avoiding problems with condensation.

But issues arise with loft conversions because building a new room in the loft removes that cold roof and unless the now lost ventilation is replaced then condensation will become a serious problem with damp penetrating the roof timbers and causing rot.

To ensure that moist air is allowed to escape a ventilation path must be created. The building regulations will insist that a 50mm gap or air space is left below the roof tile underfelt.

Added to this there must be some way that cold air can enter the roof before being expelled (this helps the airflow) so small openings of 25mm should be made in the eaves and the ridge of the roof. This doesn’t apply to those older properties that don’t have underfelt as in that case the air freely flows through the roof tiles themselves.

This ventilation path allows the air to circulate and then escape from the roof. This prevents the moisture in the air having time to condense and to attack the timbers.

Of course once this ventilation path is made the old problem of draughts returns. This is fixed by installing loft insulation between the rafters.

Condensation in the loft space

Whether you’ve converted the loft or not, condensation can still be an issue. Condensation is caused by excess moisture in the air – and this can be a result of cooking, bathing or other everyday activity.

The easiest way to reduce condensation is through better ventilation. We’ve mentioned the vapour barrier but you can also reduce condensation by increasing the flow of natural air in the loft. In other words, open a window. Opening your rooflight will increase air flow and let the moisture escape.

If you haven’t yet converted your loft, or have just installed flooring without a window, check that you haven’t blocked the roof vents. You can add extra vents if the problem continues.

Why lofts suffer from condensation

Lofts are designed to be cold and draughty. The job of the loft is to allow the rising warm air to escape from the property. Once we convert the loft or even put down flooring we naturally enough block the vents which cause the draught. This traps the moist air and causes condensation.

Modern roofs

If your house is a new build or a new roof has recently been added to the property then it is quite likely that the worries about ventilation and condensation in loft conversions become mute.

New roofs use a technology that means that under felt isn’t required or fitted. New builds use special ‘breather membranes’ that allow vapour to escape and also usually have insulation boards pre-installed above the roof rafters. This completely negates the need for additional ventilation in loft conversions.

If condensation in the loft conversion remains an issue

If a vapour barrier or more ventilation fails to eradicate your condensation problems it’s time to call in the professionals.

A surveyor will be able to conduct a thorough survey of your home and will be able to identify the root cause of your condensation.

Bringing in a surveyor shouldn’t usually be necessary. Curing the condensation in your loft conversion could be as simple as opening a window.

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