RIBA Core Curriculum
- Design, construction and technology
- Legal, regulatory and statutory compliance
- General Awareness
Condensation, caused by water vapour coming in to contact with cold surfaces which then forms water droplets, can be harmful to buildings and cause issues such as mould and mildew growth. This course examines how to control and avoid excessive condensation from building up within a roof structure by providing passive airflow from the outside of a building and by limiting airflow from within the living spaces.
In this section
- Cold roofs with impermeable underlay
- Cold roofs with vapour-permeable underlay
- Warm roofs with impermeable underlay
- Warm roofs with vapour-permeable underlay
- Examples of well-sealed ceilings
High Resistance (HR)
Low resistance (LR)
In practice, adding high level ventilation creates a far more efficient mechanism for air movement than eaves ventilation on its own.
Air passing over the ridge encourages air circulation through the eaves vents.
The problem with eaves to eaves only ventilation is that it requires more external air movement and the path between opposite eaves is frequently blocked by objects that the homeowner puts in the loft.
It is also known that air passing from eaves to eaves tends to pass close to the ceiling only and does not circulate within the higher roof space.
Cold roofs with vapour permeable underlay
For cold roofs with vapour permeable underlay, if a building has a well sealed ceiling then 3mm eaves ventilation is required.
If a building has a normal ceiling then 7mm eaves ventilation is required.
In practice, a commercially available 10mm eaves ventilation system would normally be used.
National House Building Council, usually known as the NHBC, standards require 5mm ridge ventilation where a vapour permeable underlay is installed without eaves ventilation.
This does not apply to underlays that are third party assessed as being vapour and air permeable.
For buildings larger than dwellings, the NHBC Standard does not provide specific examples; it simply states that in larger or more complex roofs it might be necessary to provide additional vents at high level. If in doubt, it is wise to include high level ventilation.
Good practice for any building over 10 metres wide or over 35 degrees in pitch, is that high level ventilation is used.
Warm roofs with impermeable underlay
Warm roofs with vapour permeable underlay
Example well-sealed ceiling details
It is generally accepted that installing a totally convection-tight ceiling is unattainable. BS 9250 was written to give designers and builders practical ways of installing a well-sealed ceiling – in other words, to provide something that is possible to do, given current construction methods.
BS 9250 provides example details of how to create seals at junctions, such as the junction between the wall and ceiling. In the following example, it shows the use of adhesive or sealant to seal the gap behind the plasterboard. If a continuous seal is not in place here, then there is potential for a lot of air flow behind the plasterboards, which will reduce the energy efficiency of the building.
Key points for the control of roofspace condensation
* Examples of roof coverings requiring addition ventilation include some fibre cement slate products (but check with the manufacturer) and metal tile and sheeting products.
You’ve reached the end of the CPD. To make sure you’ve taken on board the key learnings of this course, please fill out the quick multiple choice Q&A below. This will certify that you have completed the CPD and provide you with an email certificate, which, if the course is accredited, you can share with RIBA.