Paul Lambert at Hambleside Danelaw explains why ventilation measures are critical for cold pitched roof systems, in the context of warmer interiors.
In 2018, the World Health Organisation (WHO) provided guidelines that strongly recommended homes in temperate or colder climates should be a minimum of 18°C. This is a big step from the average internal temperature of homes in 1970, which was 12°C. Thanks to increased insulation, internal temperatures have climbed.
However it has become clear that merely increasing the amount of insulation results in a whole host of other problems caused by increased moisture in the living space. High levels of airborne moisture can be created through a range of activities, such as a resident boiling water, taking a shower, or drying clothes. It can also be introduced through weather events, such as periods of high humidity or increased rainfall. This increase in moisture also increases the potential to cause damp, mould, and structural damage as a result of condensation forming, and all the resulting issues.
Therefore, alongside the increasing insulation, it is vital to consider ventilation in the roof space, both in new buildings and refurbishment work, to allow for the moisture-laden air to exit.
The Code of practice BS 5250: Management of Moisture in Buildings was updated to recognise the impact that increasing insulation was having on growing moisture levels, and seeks to outline ways to handle this issue. Ensuring adequate roof ventilation is one of these methods of management. It is recommended that all cold pitched roofs have ventilation openings, whereas unventilated designs are not considered in the standard.
Ridge ventilation – high level
With ridge tiles requiring to be mechanically fixed, as outlined in BS 5534, the most common method of introducing high-level ventilation to the roof space is to install a ventilated dry fix ridge and hip system. These systems include fasteners, meeting the requirements for the mechanical fixing needed to secure ridge tiles in place. It is imperative that the ridge ventilation system used meets BS 8612 guidelines (which focus on performance requirements – material durability, resistance, ventilation, etc) that dry fix ridge and hip systems must meet. Not all such kits meet these requirements.
Tile & slate ventilators – low & high level
There are numerous tile and slate ventilators available that are compatible with a wide range of tiles and slate sizes on the market. These vents are installed to replace the roof tiles or slates and ventilate through the roof covering itself. In addition, some tile and slate roof ventilators have the option to be adapted and fully utilised for both soil pipe ventilation and mechanical extraction from bathrooms.
Soffit & over fascia eaves ventilators – low level
The eaves are located where the wall meets the roof, and usually run parallel to the ridge. Designs may or may not include fascias and soffit boards.
An overhang is created to keep rainwater away from the walls where it can discharge, usually into the guttering system. Ventilation openings can be provided along the full length of the eaves, providing low level roof ventilation in line with the requirements recommended in BS 5250.
Where low level ventilation is being provided, it is essential to ensure that the ventilating air path is not blocked by the insulation in the roof, and various products are available to assist with this.
Other ROoF ventilation options Proprietary products such as ventilated ridge systems, tile and slate vents, eaves and soffit vents together with vapour permeable underlays, in combination
with eaves panel ventilators and trays, can also contribute to ventilation provision and are easily incorporated into a well-designed roof. For pitched cold roof systems, much of the ventilation requirements depend on the type of underlay used.
Where non-breathable or high resistance underlay is installed, low level vents should be installed, with a 10 mm continuous opening along the eaves. Roofs spanning more than 10 metres, or over 35° pitch require ventilation equivalent to a 5 mm continuous gap at the ridge line. For roofs with low resistance underlay and a normal ceiling, the ventilation openings required at opposite eaves maybe reduced from the equivalent of a 10 mm continuous gap to a 7 mm continuous gap.
With a well-sealed ceiling, the equivalent eaves opening may be reduced to the equivalent of a 3 mm continuous opening with no ridge ventilation required. Alternatively, the equivalent of a continuous 5 mm ventilation opening may be provided at the ridge with a well-sealed ceiling.
Paul Lambert is field technical support at Hambleside Danelaw