Home Cold Non-Vented Roof

Cold Non-Vented Roof

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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, some 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.

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Vapour Control Layer (VCL)
All non vented roofs must incorporate a VCL such as high performance Permavent Reflect. A VCL is usually positioned just behind the plaster board. BS:5250 recommends that a VCL should to be a minimum 500 gauge polythene (plastic DPM) sheeting and all corners and laps should be minimum 75mm and taped with 'Tactape'. With a warm roof it is acceptable to tape the joins of a foiled insulation with the appropriate foiled tape ensuring all joins and peripheral edges are sealed. Vapour will travel through or manifest itself in the joins of foil backed plasterboard if not detailed correctly. Given the difficult nature of vapour sealing foil backed plasterboard we do not recommend its exclusive use as a VCL. Care should be taken with cables and pipes that could break the VCL.

Air Sealed Roofs
If the dwelling is to be air tested then all laps and joins should be carefully taped and peripheral edges sealed. All pipes and apertures should be sealed and an air tight loft hatch used.

Air Permeable Roof Coverings.
As vapour travels through Permavent it must then be released to the atmosphere. Most roof coverings (concrete, plain tiles etc) allow what is termed 'fortuitous ventilation' and are classed as air permeable roof coverings. This means that air can travel around them and vapour will not get trapped in the batten space between the breather membrane and tiles causing 'interstitial condensation'. Some metal sheet and man made slate roof coverings may not allow sufficient air circulation and the batten space should therefore incorporate additional ventilation such as counter battens with ridge and eaves ventilation.

 

Newsflash

Following the success of the Queen’s award winning Easy Verge, here at Permavent we are proud to announce the launch of the recently developed Easy VergeTrim. This latest addition to the Easy Roof System has been specially designed to meet the requirements of the Scottish and refurbishment markets.