Condensation in the roof space

Condensation in the roof space

The recent cold weather has made condensation issues in the home, particularly in the roof space, more obvious. This article explains why there is condensation in the roof space and what can be done about it.

Moisture that is generated in the home can manifest in two ways. Visible condensation/moisture can be seen, for example, on the inside of a bathroom window. Invisible interstitial condensation occurs in the fabric of the building, the result of which can sometimes be seen as black mould. 

Condensation moisture should not be confused with a leak or water ingress that is often distinguished by tide lines or effervescent salt-like stains.

What is condensation?

Water and air are closely connected and constantly interact with each other, the level of water in the air is constantly changing depending on temperature and atmospheric conditions. A puddle can dry or evaporate, this warm moist air will rise, and then cool high in the sky where it turns into a cloud which ultimately falls back down as rain. All warm moist air will always try to escape to the atmosphere.

In the home a pan of water can be boiled dry but this does not mean the water disappeared. By heating the water it atomised into tiny molecules of moisture (the steam that rises from the pan) and moved into the atmosphere. The warm moist air rises, and when it cools it turns back into water, for example when it contacts the cold surface of the bathroom window or on the outside of a cold tin from the fridge. 

These molecules of moisture are very small and can easily travel through brick, plasterboard and wood (which gets wet) but cannot travel through dense products like metal, glass and plastics. 

Causes of moisture in the home

Warm moist air can be generated in the home in many ways such as boiling the kettle, cooking dinner, showering, drying clothes and surprisingly even when we breathe. Therefore, the more people in the home, the more moisture is generated. Often this moist air can also contain bacteria, such as in a sneeze or cooked food.  

Simple temperature changes can also generate moisture, such as from an uncovered water tank in the loft. This example is seen when a lake or open water can 'steam' on a very cold day because the water is warmer than the air.

Why does the home have condensation?

This warm moist air rises and is naturally attracted to any of the cold areas of the building through convection. For example, more condensation is often seen on northern or more shaded walls, but can also be seen behind a wardrobe in cold corners that are not heated.

Moisture in a room can travel through a wall when trying to escape to the atmosphere. As this happens it can cool inside the brick and turn back into water, which can sometimes make the wall damp, this is called interstitial condensation. The bacteria in this moisture can cause the black mould. 

More commonly, moisture is visible at a thermal bridge – a section or area where there is no insulation – for example at the lower corners of the roof space or at the eaves over the wall plate, often the upper corner on the outside wall of a bedroom. This 'thermal bridge' often attracts the most moisture travelling up through the home and can encourage black mould growth.

Why is condensation in the roof space?

There are many ways that moisture can enter the roof space. In older properties there is often an old wooden loft hatch outside the bathroom door. When the bathroom door is left open after a shower, this moist air can get 'sucked' into the loft space.

Other openings such as recessed light fittings and brick cavities that are open at the top when a wall is drying in the sun, are in effect small moisture chimneys. Uncovered cold water tanks generate moisture. 

Alternatively, moisture can also pass through the plasterboard ceiling and into the roof space, especially when there is insufficient insulation. 

This image shows how the warm air rises to the highest point of the roof and has melted the frost.

Prevention at the source

Sudden cold weather in the UK is creating the perfect conditions for condensation, where our homes are often highly insulated, windows are closed, and the heating is on high. Roofers are receiving more calls than ever before due to these perfect conditions for condensation problems.

Unfortunately, there is no miracle cure for clearing moisture, older houses are draughty and this exchange of air would often clear the condensation. Modern homes have sealed double glazing and much higher levels of insulation that can trap moisture. Many homes often unknowingly rely on the coming and going through the front door to exchange the air trapped in the home.

The best way is to clear the moist air is at the source. Many people will remember when a parent would open the bedroom window in the morning to 'clear the air'. Nowadays modern trickle ventilators on windows are an effective way of helping with moisture in each room. Mechanical vents such a cooker hood or bathroom extractor fans should always be used when cooking or showering.

Easy methods of controlling condensation in the home are closing the bathroom door after a shower. Opening windows a little can really help. Many windows have a security 'vent' option so the window can be locked slightly open. Highly efficient houses can use complex heat exchangers and passive ventilation systems that are perfectly designed to cope with such problems. 

A sealed loft hatch can be fitted or ensure the old hatch has a seal around it and is insulated somehow, it’s always best to have lots of well installed loft insulation. Water tanks should be insulated, and a lid or cover should be well fitted.

Most importantly clothes should not be dried on radiators or a clothes horse as this generates lots of moisture, try to ask yourself where the moisture in those clothes goes when they dry, often drying clothes on the radiator will cause the radiator to rust and offers an indication as to habits of the homeowner. 

Why is there moisture on the underside of the breather membrane?

If moisture in the home is not properly contained or disposed of then it will rise up through the house and ultimately arrive in the roof space. When the moist air arrives in the roof space it is immediately attracted, through convection, toward the colder outside surfaces of the roof where the first point of contact is the breather membrane.

Breather membranes have a MVTR (Moisture Vapour Transfer Rate) that states how much of this generated moisture or vapour can pass through a m2 of the membrane in 24 hours. This is important because in the same way as a funnel, too much moisture can overload it, it still works, it’s just overloaded.

Example of condensation on the membrane

Another element that affects the performance of breather membranes is the weather which makes the membrane very cold. As the warm moist air entering the loft rises and contacts the underside of the cold membrane, it's temperature then suddenly drops as it tries to pass through the cold membrane causing it to condense on the underside in the same way that cold air instantly forms on the outside of a cold drink. 

The same type of cold condensation can also happen when the warm air tries to pass through cold roll out ridge ventilators.  

Excluding sealed systems such as warm roofs etc, when the moist air does pass through the breather membrane it must be released to the atmosphere, because of this the fortuitous ventilation of the roof covering must also be considered. Air open roof coverings such as plain tiles offer high levels of ventilation above the membrane whereas metal sheets or manmade slates will limit the passage of this moisture to the atmosphere. Therefore, the batten void above the membrane must always be adequately ventilated so the moisture can properly escape to the atmosphere. 

Breather membranes are an excellent solution in closed systems or warm roof detailing but in open rafter details in high moisture, modern homes they can often be overloaded. 

To reduce the risk of condensation in the roof space

To avoid condensation through excessive humidity in the home or roof space the moist air generated must be cleared. Warm air will always rise to the high level. Most breather membrane manufacturers recommend additional free air ventilation to the loft space both at the eaves (by the guttering) and at the ridge to avoid the risk of membrane overload.

Free air high level roof space ventilation is the most effective way of releasing the warm moist air trapped in the roof space. A free air ridge vent such as the Mayan RealRidge is not only highly effective at releasing high level moisture but the action of wind over the apex can generate a vacuum that can also draw the condensation out. 

Condensation is not the fault of the roofer, but he is often blamed for condensation in the roof space. Adequate high level free air ventilation is the best way of preventing those call backs.

This image shows an experiment that illustrates the increased ventilation provided when using the free air vented RealRidge, compared to a woven polyester roll out system with half round ridge. Watch what happens in the full experiment here