Why is there a need for roof ventilation?
As modern construction techniques continue to place more emphasis on energy conservation, minimising the heat loss from buildings, insulation thicknesses have increased and air tightness is constantly improving. Although this is a way of achieving good energy performance certification, there is now far less opportunity for water vapour to be dissipated through fortuitous ventilation in the property, and therefore a higher chance of condensation build up and potential damage.
Few new buildings contain open fireplaces, single glazed windows and solid floor construction has become the norm. The increase in levels of moisture being created has gone hand in hand with more heating and greater insulation levels creating greater temperature differentials within the fabric of the building.
Water vapour in the air causes a vapour pressure, the warmer the air, the greater capacity there is to contain moisture. The greater the moisture content, the higher the vapour pressure becomes. This vapour pressure acts in all directions and will cause the water vapour to pass through the smallest of gaps to anywhere where the vapour pressure is lower until equilibrium is achieved.
Pitched, cold and warm roof ventilation
First of all, what type of construction is the roof? You may have heard of the terms cold roof and warm roof construction, but what do these actually mean?
In pitched roof construction, the insulation can be placed horizontally above the ceiling level forming a ‘cold roof’; it can be inclined above, between or below the rafters forming a ‘warm’ or ‘hybrid’ roof, or positioned in the roof structure in different ways where a ‘room-in-roof’ is to be created that can result in warm and cold roof construction types. Cold roof voids are generally always required to be ventilated; however, where vapour permeable (or Low Resistance, LR) underlays are specified, the levels required may be reduced. The enclosed void of a warm pitched roof may not require to be ventilated, but this is usually subject to the requirement to install a fully sealed Air and Vapour Control Layer (AVCL) in conjunction with an air-open roof covering.
Understanding the roof construction of any new build or refurbishment is essential to determine what products are required for the job.