Frequently Asked Questions

Q1: What is Permavent?

A1: Permavent is a three–layered, using non–woven polypropylene either side of a micro porous functional film for use as a roofing underlay or as a house wrap. Permavents structure is sufficiently fine to prevent liquid water penetration when in use, but not too fine to prevent transfer water vapour (molecules of water in air) out of the roof space. Permavent's market leading specification is widely used throughout Europe. Permavent is designed using the latest technology, incorporating all the requirements of the harsh northern environment.

Q2: Why use Permavent?

A2: You must guard against the damaging affects of condensation in a roof space because modern homes have eliminated drafts; hence there is less dispersion of the vapour (steam, air bourn moisture) that every home is generating in greater quantities. The roof space has become a buffer between the warm home and the cold outside and the vapour will travel through many materials (but most commonly the loft hatch) and will form condensation as it cools on the coldest areas (usually the under felt). This condensation can also contain bacteria and, if left unchecked will instigate mould growth that can manifest in many ways and could seriously damage your home.

Q3: Do I have to use Permavent?

A3: For non–vented roofs a good quality 3 layered polypropylene (or polyethylene) permeable membranes with a vapour resistance of not more than 0.25MNs/g such as Permavent (0.15MNs/g) is a requirement of the building regulations. However, it should not be too low as to risk leakage.

Q4: How can I combat vapour?

A4: Vapour is present in all homes especially the months following construction. Building regulations Part F make adequate provision for this through window ventilators and mechanical ventilation to bathrooms and kitchens. Vapour will travel through masonry, through plasterboard ceilings and especially through the loft hatches and lighting and pipe apertures.

The use of Permavent–dry vapour barrier is essential with the cold non–vented and hybrid roofs and provides excellent protection against vapour when all peripheral edges and joins are sealed. It is then important that a sealed and insulated loft hatch is used and all lighting (recessed lights would cause many problems if not detailed properly) and pipe–work apertures are sealed well. Water tanks should be covered.

Q5: Why shouldn't I just use a cheap underlay?

A5: Bitumous roofing felts are heavy, bulky, easily damaged during installation on vented roofs and become brittle through time; they also create so–called `plastic bag' effect with vapour problems. Other cheap underlays cannot perform adequately and often cause problems.

Permavent is a very well suited product to the re–roofing market. It is light and easy to use with a typical usage ratio of 1 roll of Permavent to 5 rolls of bitumous felt it is surprisingly cost effective as a percentage of the new roof. Its tough design will virtually eliminate installation damage. With special additives it is U.V stable for 3 months that enables a delayed covering of the roof tiles it will stay watertight and perform well for many years. Although roofs that require active ventilation work well, they must always maintain this, and non permeable membranes do not lend themselves to future changes to the home. Permavent is a highly adaptable product that will help to create a healthy home whilst being cost effective.

Q6: Do I need to provide additional ventilation?

A6: Both the cold–vented and hybrid roofs can be installed using any type of roofing membrane. Both types of roof must be ventilated at eaves level (10000mm/m² equivalent to 10mm continuous on smaller roofs or 25000mm/m² equivalent to 25mm continuous) and must create cross ventilation to eliminate un–vented voids. This sometimes requires connecting to ventilation at the ridge (5000mm/m² min equivalent to 5mm continuous) but may also be achieved with gable vents in the case of lean–to or tall roofs. Ridge ventilation is a requirement underneath the membrane in hybrid roofs and above the membrane on warm and cold non–vented roofs.

Eaves and ridge ventilators that are above the membrane are the subject of some debate as they are known to reduce interstitial condensation to the underside of some slates but do not allow for the fortuitous ventilation of some tiles (such as plane tiles). Wind and atmospheric pressure can adversely affect the performance of breather membranes when in addition to fortuitous ventilation. These for and against discussions have caused much debate. The BSi advocate vents but the BBA have stated that they are satisfied if third party compliance is shown in the favour of no vents. In all cases long rafter lengths require special attention (such as larger counter battens) and advice from the tile or slate manufacturer should be sought.

Q7: What type of roof do I have?

A7: There are 4 basic groups of roof type:

Please folow these links for detailed descriptions.


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