Types of roof

Cold non–vented roof

This most modern type of roof has only become possible through the recent introduction of breather roofing membranes with superior vapour permeability such as a performance leaders like Permavent, Permavent-MAX & Permavent-ECO . Although cold non–vented is not the best performing type of roof as recent changes to part L1 of the building regulations, requiring increased levels of insulation, have made it the most cost efficient against performance. By eliminating active ventilation the roof space it is therefore 25% more thermally co–efficient requiring less insulation. The only problem with this type of roof is that it will only work effectively when installed as a complete system using good working practice.

A cold non–vented roof requires that the roof space is isolated from all but residual amounts of vapour by means of a vapour control barrier  from non–permeable sheeting such as Permavent–DRY. The use of this control barrier, fixed to the joists for example behind the plasterboard, will prevent the ingress of vapour through the ceiling.

Permavent should be laid on the roof in conjunction with a counter–batten to allow eaves to ridge ventilation over the top. The laps and peripheral edges of Permavent must be stuck and sealed using Permavent-TAPE (double sided butyl based tape). The type and style of insulation may vary quite considerably and all manufacturers will supply relevant information in accordance with the latest building regulations.

Cold vented roof space

This is the most common roof type in use even though it has most problems and is, hence, the most inefficient. You must provide adequate ventilation to this roof space (building regulations F2) as well as a high degree of insulation (building regulations L1) to create sufficient separation between habitable room space and roof space, usually at ceiling level.

In order for condensation not to cause problems, it is critical that the roof space is ventilated thus dispersing any vapour that is present. Old roofs that have no under felt are draughty thus giving good ventilation, but the loss of a tile would cause a leakage; Cold vented roof has also got a very high heat loss ratio.

Recent history has seen the introduction of a bitumous membrane (1F) to combat heat and tile loss problems but the introduction of loft insulation at ceiling level together with inadequate ventilation has, in essence, created this condensation problem. Even when a roof has eaves ventilation the wool insulation is often jammed into the eaves thus restricting the ventilation or conversely the insulation is left short creating a thermal bridge in the corner of the ceiling in the room below, resulting in mildew. Both of these problems are a breach of the building regulations and are difficult to correct remedially when faced with local enforcement.

Cold vented roofs will continue to be widely used throughout this country through existing properties that need re–roofing because of practical problems would make the job not cost effective.

Warm roof

This is the best performing of the roofs and is suited to all applications although it is the most expensive one and does not lend itself so well to remedial applications because it often requires a re–roof or the original roof level needs to be raised. Because the insulation is on the outside of the building (like an overcoat) this illuminates the main roof design problem of interstitial condensation because there are no voids for the vapour to condensate in.

The main use of this form of roof is when the roof space is habited as an original design (mansard style) or when the ceilings are vaulted or of course when design necessitates a flat roof.

Only vapour permeable membranes with a vapour resistance of not more than 0.25MNs/g such as Permavent-0.15MNs/g^-1, Permavent-MAX-0.15MNs/g^-1 or Permavent-ECO-0.11MNs/g^-1. Permavent should be laid directly onto the insulation (usually phenolic boards or multi-layered quilt type of insulation). Wet or damp rafters should not be closed in and of course there should be no voids in insulation. Building regulations (L1) make such a requirement for insulation that it resultedthat in rafters being considered as a thermal bridge and require for insulation to span over an exposed edge. Permavent breather membranes should be laid onto the insulation with a counter batten to allow for eaves to ridge ventilation and wind driven moisture runoff.

Hybrid roof (habitable roof space)

This type of roof is most commonly used in loft conversions as any type of membrane can be used and because either there is no original roof membrane or it is a non–permeable one. This system should incorporate effective use of a vapour control barrier such as Permavent–DRY installed directly underneath the insulation. The ventilation at eaves level must be connected to the ventilation at the ridge via a min 25mm gap (BS5250:2000) between the membrane and the insulation although good building practice has established a 50mm gap as the norm. Non–permeable membranes can be prone to problems because any problems with the vapour barrier could result in interstitial condensation (especially on long rafters). The resulting condensation would then run down the insulation boards to the join and leak through to damage the finished ceiling. Some specifies have, in recent years, designed a warm roof on new build with a 25mm vented space (in essence a hybrid) between the insulation and the breather membrane but there have been reports of interstitial moisture forming and leaking through the joins in the insulation, the BSi now recommends permanent sealing of all insulation joins in this system. This means that the use of a warm roof with Permavent laid directly on, and then ventilated, is now more preferable. Permavent used in conjunction with the Permavent–DRY vapour control would be a permanent and effective use of the hybrid system.

Scottish Practice

Scottish practice of using sarking boards is compatible with our Permavent System providing the boards are of not more than 100mm wide and maintain at least a 2mm gap between them. Permavent can be made at many levels of permeability but increasing its permeability would make it less durable or even not waterproof like some other membranes. Permavent has been designed using all considerations to provide passive roof ventilation whilst still having excellent performance as roofing underlay with all the rigours of installation that are required. (For more details see our INFO & ACCREDITATION sections)