This fact was recognised by the NHBC a couple of years ago when an increasing number of complaints regarding condensation-related damage in roofs drew attention to the problem. The organisation produced guidance in 2011 to remind builders that ventilation, at eaves and particularly at ridge level, is the recommended system for cold roofs.
The science is relatively straightforward: the rates of moisture removal that can be achieved via ventilation (convection) are far greater than can be achieved by diffusion, the method that most vapour permeable underlays rely on for their function.
The starting point is effective eaves-to-eaves ventilation. A gap is created between the insulation and underlay at the eaves so that air comes into the roof space and out the other side.
Introducing this type of low-level ventilation is a relatively straightforward process as part of a new roof construction and it can also be retrofitted to solve a recognised problem. Redland provides a number of component options to integrate this ventilation into the roof system.
While eaves-to-eaves ventilation is a good starting point it will not necessarily solve the condensation problem on its own. This is because it is dependent on wind to create the through-draft necessary to take the water vapour out of the roof space and factors such as lack of wind or building orientation can reduce the effectiveness of this method of ventilation.
Moreover, for steeply pitched roofs a through-draft from eaves to eaves may not be totally effective in removing pockets of stagnant moist air trapped in the apex.
Consequently we always recommend a combination of low and high level ventilation for cold roofs where a vapour permeable or non-breather underlay is in use. The only exception is where a well-sealed ceiling can be achieved: in this case with a vapour permeable underlay the roof can function with just high level ventilation.
The high level ventilation product, situated either at or near the ridge of the roof (perhaps in the second course of tiles down) is an effective form of passive roof ventilation in all weather conditions: it forms a stack effect, creating a draw that is not dependent on wind.
We published our own technical guidance document on this issue in 2011 and the conclusions are in accordance with both NHBC recommendations and the British Standard BS 5250.
Returning St Joseph’s School to its former glory
Generations of school children have attended St Joseph’s Catholic Primary School in Willesden so when a fire destroyed its roof, it was a blow to both the current students and its community.
The roaring twenties - and 40's - sees BMI take shape
With BMI UK & Ireland’s iconic BMI Redland brand celebrating 100 years of concrete tile manufacture in the UK, making it the oldest concrete tile maker in the country; the company, ahead of its formal celebrations later in the year, continues to look back over its 180-year heritage.
BMI's training facilities claim academy status-12 09:59
BMI UK & Ireland has relaunched its National Training Centre as the BMI Academy. The UK’s first ever dedicated roof training centre, the BMI Academy – now a multi-site operation, with its hub in South Cerney, Gloucestershire – offers best-in-class training and comprises possibly the most comprehensive learning resource in the country. Not content with just being the UK’s first dedicated roof training centre, the BMI Academy looks forward to many more firsts as it aligns with the other training academies across BMI Group’s global operations.