A Practical guide to roof space ventilation

In standard building construction, there are two methods we normally use to control condensation build up in the roof space; ie we prevent water vapour from reaching the loft space, or remove it once it gets there, before it builds up to harmful levels. Much has been written previously about roof space ventilation and the control of condensation, though not often from the roofing contractor’s viewpoint. Using a series of examples, I will explain what the roofer needs to consider when installing a roof to prevent harmful condensation, or, in other words, how to follow the guidance given in BS EN 5250: the British Standard Code of practice for control of condensation in buildings & in Ireland following Building regulations part F–  f2 “the control of condensation in roofs ” & S.R.82 Irish code of practice for Slating & Tiling.
It is important to make the distinction between vapour-permeable and air-permeable underlays. In this article, I will deal with vapour-permeable underlays, as defined in BS 5250. Air-permeable underlays are currently not defined in BS EN 5250; therefore, it is important to refer to their Agrément Certificate and manufacturer’s installation instructions. They generally require less, or no additional roof space ventilation.
The ceiling type is a fundamental part of the building design and influences the decisions on how or whether the roof space should be ventilated. Therefore, if the roofer has responsibility for ensuring the roof is constructed in accordance with BS EN5250 (as opposed to simply following a pre-determined design), it is important to obtain the technical specification of the ceiling (in writing if possible). In new build-constructions, the ceiling should be ‘well-sealed’ as defined in BSEN 9250: the British Standard Code of practice for design of the airtightness of ceilings in pitched roofs. This means that there is an efficient air and vapour control layer and all joints, junctions and penetrations through the ceiling are sealed to prevent as much air and water vapour from passing through the ceiling as possible.

Cold roof with eaves ventilation

In our first example, I will consider a domestic cold roof; ie a pitched roof below 35 degrees with the insulation laid over the horizontal ceiling.
BS EN5250 recommends minimum ventilator sizes at eaves of 3mm for a well-sealed ceiling or 7mm for a non-sealed, or normal ceiling. In practice, though, such vent sizes are not available and standard 10mm soffit or over-fascia ventilators should be used.

Alternatively, if it is known that the ceiling is well-sealed, then it is possible to ventilate the roof space at the ridge, without eaves ventilation, using, for example, a dry fixed ridge system.
This can be a good solution for constructions where it is difficult to provide eaves ventilation. Remember to finish the underlay with a gap at ridge to allow an air flow from the roof space.

These are minimum recommendations; combining eaves with ridge ventilation is more efficient in removing water vapour-laden air from the roof space, particularly for buildings wider than 10 metres or roof pitches of 35 degrees and above.
If there are any doubts about the type of ceiling, then install eaves ventilation. As a general rule, new build properties should have a well-sealed ceiling, but in re-roofing work on existing buildings the ceilings almost certainly will not be well-sealed.

Warm roof with a well-sealed ceiling
In our second example, we have a warm roof; ie one where the insulation is between, or below or above the rafters, or a combination of all three.
If the ceiling is well-sealed, then it is not necessary to provide any roof space ventilation when using a vapour permeable underlay

Warm roof with an unsealed ceiling
If the ceiling is not well-sealed, then it is necessary to provide a 50mm deep cavity in each rafter void from eaves to ridge between the insulation and underlay. If the insulation finishes level with the tops of the rafters or is on top, it will be necessary to use counterbattens to provide the cavities. The cavities must be ventilated with a 25mm continuous ventilator at eaves and 5mm

continuous ventilation at ridge. If there are any obstructions, such as roof windows or dormers, there needs to be 5mm ventilation below the obstruction and 25mm ventilation above the obstruction.
This can all lead to an unnecessarily complicated roof structure, therefore there are clear advantages to having a well-sealed ceiling when construction warm roofs.
Batten space ventilation
Be careful not to confuse the roof space (the space between the insulation and underlay) with the batten cavity (the space between the underlay and roof tiles). As water vapour escapes through the vapour-permeable underlay it enters the batten cavity and is dissipated out through the tiles. Most tiles and slates are sufficiently air-open to allow this to happen naturally. However, some close-fitting products, such as fibre cement slates, may not be sufficiently air open, in which case, the batten cavity must be ventilated at eaves and ridge. In case of doubt, consult the tile or slate manufacturer.

In summary
• Seek information on the type of ceiling – remember this can have a significant role in controlling condensation and determines the level of ventilation required.
• Ridge ventilation combined with eaves ventilation provides a much more efficient system for removing condensation in a cold roof.
• If a warm roof does not have a well-sealed ceiling, it is necessary to provide a 50mm continuous gap between each rafter void, ventilated at eaves and ridge.

https://www.rubixbuildingproducts.com/ventilation-membranes/

By John Mercer roofing consultant