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
- Building Regulation requirements
- British Standard 5250: 2011
- Traditional Scottish practices
Building Regulation Part C sets out what must be achieved; i.e. roofs should be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial Condensation’. It then goes on to say that one way of meeting its requirements is by following the guidance given in BS 5250.
Clause 6.2 c
‘Roofs should be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial Condensation’
‘A roof will meet the requirement if it is designed and constructed in accordance with BS 5250’
BS 5250 is one way of complying with the requirements of Building regulation Part C.
There is some very good advice in this Standard; for example, remember that building occupants may not always use the building on the way it was intended, so err on the side of caution and provide robust solutions.
Example use case: A family with several young children may generate more condensation than a single person and may push the system beyond its limits, particularly in winter, if they are at home all day and want to keep the house very warm, with no draughts – so they keep the windows closed – and are hanging clothes on radiators to dry etc.
As with Building Regulation ‘C’, BS 5250 recognises that temporary condensation may occur during adverse climatic and internal conditions (e.g. when it’s very cold outside with little or no air movement, warm indoors with no windows open).
Obviously, any condensation cannot be severe enough to cause damp or staining on internal surfaces and must disappear before damage to the structure can occur
BS 5250: 2011 key summary
BS 5250: 2011 key summary
Whilst following the guidance given in BS 5250 is one way of complying with Building Regulation C, an alternative approach is to follow a recognised third party certification of a product or process.
For example, some vapour permeable underlays are also classed as ‘air permeable’ and as such the manufacturers claim that these can be installed without roof space ventilation. However, it is important that the guidance given in the BBA certificate is carefully followed.
BS 5250 Table H1 shows the minimum recommendations for low level ventilation requirements for a cold roof.
Regardless of whether an impermeable or vapour permeable underlay is used, some ventilation is required at low level.
Similarly, regardless of whether a normal ceiling or well-sealed ceiling is installed, some ventilation is still required at low level. Albeit less if the underlay is vapour permeable and there is a well-sealed ceiling.
BS 5250 does allow the use of high level ventilation instead of low level ventilation where the underlay is vapour permeable and there is a well-sealed ceiling.
It also important to note that if using impermeable underlay, additional high level ventilation should be provide if the roof pitch exceeds 35 degrees or the span exceeds 10 metres.
Best practice, unless you can be very sure of the long term integrity and efficiency of your design, is to install eaves ventilation in a cold roof construction, even with a vapour permeable underlay.
On a roof where plywood or other similar boarding has been used, this must be treated as an impermeable underlay with regard to ventilation requirements.
But where softwood sarking is used, typically 150mm wide with small gaps between each board, this can be regarded as air permeable and so used in conjunction with a low resistance underlay it can be treated for design purposes as a low resistance underlay.
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.