Condensation in roofs has become more of a problem as we introduce more insulation and eliminate draughts in our homes. It is often linked to older style impermeable roofing felt which has been around since the 1930s although breathable membranes, which were introduced in the 1980s, are not immune in some circumstances.
Both of these materials perform a number of functions:
- To provide temporary weatherproofing to the building before the main roof covering is installed (membranes can normally provide a couple of months of protection to direct exposure to the elements)
- A secondary barrier to rain and snow which is driven under the roof covering by the wind
- Prevent potential pests like insects and birds from entering the roof through small gaps.
Despite its benefits, older style roofing felt can contribute to an unwanted side effect – condensation in the roof which can lead to the rotting of timbers and damp appearing on ceilings. Roofing felt is impermeable to water so it will not allow moisture to escape from the roof.
Breathable membrane does allow water vapour to escape from the roof space but if other circumstances are working against it then it may not be sufficient on its own to prevent condensation.
Nowadays condensation problems are well understood and most new homes are designed with suitable ventilation to prevent condensation occurring. This is covered for house designers by BS 5250:2011+A1:2016 (Code of practice for control of condensation in buildings). In the British Standard, reference is made to a well-sealed ceiling, which means tradesman installing plumbing and electrical services, as well as ceiling plasterboard, have worked in a way which prevents air from finding its way from the home into the roof space. For a normal ceiling (not a a well-sealed one) using a vapour permeable membrane then there should be a 7mm gap along the eaves in the roof space to allow adequate ventilation.
In properties 20 to 80 years old condensation can be a potential problem, particularly with the recent drive to insulate houses by putting thick layers of thermal insulation in lofts and to reduce air infiltration by stopping draughts. The loft insulation means the roof space is colder than before which encourages condensation in the roof. Installation of a wood burning stove has also been cited as a cause as the wood burner seems to provide a “thermal” in the house, helping all the moist air to rise faster.
What Causes Condensation In The Loft Or Roof Space?
Warm air can contain more moisture than colder air. Warm moist air enters the roof space through gaps in loft hatches or where bathroom plumbing or other services go through the roof space. When this warm moist air hits the cold air in the roof space, water droplets condense on the nearest cold surface – the roofing felt. It can be hard to identify the exact spot causing problems as once water condenses it can then run down the rafters and form damp spots away from the original source of the drip.
Condensation is most likely to be a problem during cold weather when the loft is at its coldest and doors and windows in the home are closed. It is less likely to be a problem in summer months unless there is a more serious problem.
The solution to condensation in roofs is straightforward. Prevent moist air from entering the roof space and ensure there is adequate ventilation and air flow in the roof space to carry any moisture laden air to outside the roof space.
Preventing warm moist air from entering the roof space:
- Consider how to reduce moisture in the house with improved extraction in the kitchen, bathroom and utility room
- Ensure that the loft hatch has a suitable seal to prevent air rising into the roof space and is well insulated.
- Avoid having gaps in bathroom ceilings and make sure that any shower extractor fan is not discharging into the roof space.
- If you have one, ensure the central heating cold water tank in the loft is covered with a lid, so no evaporation can occur.
- Consider how recessed lighting in ceilings and other light fittings can allow moist air into the roof space.
Cooking, boiling the kettle and bathing/showering are some of the biggest culprits for generating moist warm air (as well as people breathing!). Washing and drying clothes can also add to air humidity. It is important to have adequate air extraction in these areas so the moist air is cleared to the outside of the house quickly, rather than being allowed to cause condensation and mould problems.
Kitchen extractor hoods and wall or ceiling mounted bathroom/toilet extractors can play a part. If mould and condensation is a problem more generally then a Mechanical Ventilation Heat Recovery System (MVHR system) may be worth a thought if a full house refurbishment is being considered. The benefit of a MVHR system is that it recovers the heat from the extracted air and uses it to heat incoming fresh air from outside.
Adequate ventilation in the roof space
- Check that any recently installed thermal insulation has not covered any ventilation holes in the eaves.
- Consider if items stored in the loft are reducing air flow across the roof space and reducing ventilation.
- If you have enclosed soffits then consider adding eaves vents to improve air circulation (be aware that old roof soffits may contain asbestos though). These can be installed from the outside using a ladder or sometimes internally from the roof space.
- Consider adding some form of roof ventilation. Roof vents which replace tiles can be installed but due to cost this is much better done as part of a re-roof. To add eight vented tiles into a roof space would cost around £650-£700 (made up of 8 x vented tiles @£30 + access tower @£150-£200 + £260 labour ( 1 day for 2 people). To do this as part of a reroof would cost around £250.
On new houses you can often see roof vents installed in the roof. These are generally for extractor fan outlets rather than for ventilating the roof space.
A local roofer will be able can advise on the best steps to take to improve the roof space ventilation and avoid condensation in roofs. Provided safe access to the roof space is available then most of these actions are fairly low cost (under a couple of hundred pounds).