Design Requirements

The BSi documents – 5250:2002 Code of Practice for Control of Condensation in Buildings and BSi 5534 Code of Practice for Slating and Tiling are widely recognised by controlling and professional bodies as an authoritive work with direct references to breather membranes and their use. Although these documents are open for interpretation we have used some of the main points along with our own findings below:

• The type of roof covering and subsequent design should always be well thought out if the roof is to remain healthy.
• People don't open their windows as much as they should (for security reasons, heat loss etc.] and yet modern homes have more en-suite and showers. It is important to remember that lifestyles change and in twenty years time the house will not be used as it is now. Permavent
  will cope with all these changes.
• To reduce the Vapour (moisture) that will affect the loft space this moisture must be extracted at source (extractor fans, window vents).
• Even with a fully ventilated roof, complex design will create ventilation voids around valleys, dormers etc. Permavent will cope with all types of roof design.
• Good detailing of the vapour control layer is essential to reduce the amount of vapour entering
  the loft space; loft hatches and pipes entering the loft should be positioned away from the
  bathroom or well detailed and sealed to reduce vapour.
• Ensure that the top of the brickwork cavities are closed to avoid vapour from the brickwork etc, from entering the loft.
• Correct use of the cold non-vented roof style will reduce the heat loss in the roof space by up to 25% by eliminating the movement of cold air.
• Traditionally ventilated roofs should ensure that any ridge ventilation is smaller than the eaves ventilation to avoid the reverse affect.
• The use of Permavent will avoid the wind noise (flapping] that the first generation of polyethylene
  breathers suffered from because of their stiffness.
• It is possible for a membrane with too low vapour resistance to suffer from the reverse affect. This is where the moisture or dew that forms under the slates from the night before can return through the breather when the morning sun heats the slates and vapour occurs. Tried and tested Permavent has the balance right.
• BSi 5250; 8.4.2.1 .d states a preference toward the use of vapour permeable roofing underlay.