The Tyvek^® non-ventilated warm roof system

Principles

The adoption of warm roof construction has many benefits:

> the whole building volume can be used, either as room-in-the-roof or cathedral roof construction
> the stress upon the building structure from thermally induced movement is reduced
>  the use of rigid board insulation increases the stability of the roof.

Conventional warm pitched roof construction includes a bituminous felt underlay to resist wind uplift upon the roof covering and to provide a secondary defence against wind-driven rain or snow. Because bituminous felt has a high water vapour resistance, moisture rising through the roof structure will be trapped beneath the underlay and can condense, damaging the roof structure and reducing the thermal performance of the insulation. BS 5250 recommends the provision of a ventilated air gap, not less than 5Omm deep, between the insulation and the felt. If the insulation fills the depth of the rafters, counterbattens are used beneath the underlay to provide that air gap.

Whilst air movement will usually remove moisture from beneath the underlay, in some weather conditions it may increase moisture levels by drawing in saturated external air. The uncontrolled flow of air greatly increases heat losses, both by conduction through the insulation and by convection due to air leakage through gaps in the construction.

DuPont Tyvek^® membranes are engineered to have a low vapour resistance: when used as an underlay they permit moisture to migrate freely to atmosphere, removing the need for ventilation beneath the underlay.  Eliminating air movement substantially improves energy efficiency.
 
The performance of a warm roof will be improved by adopting the Tyvek^® non-ventilated warm roof system.

Roof coverings of tiles and natural slates do not form an impervious airtight layer; they will give natural venting of the batten space sufficient to allow moist air to disperse to atmosphere without the need for vents, saving time and money and improving the appearance of the roof.  

KEY
a  water vapour from inside the building passes easily through the vapour open Tyvek^® underlay

b  the windtight Tyvek^®  underlay prevents air penetration

c  wind-driven rain and snow which penetrate the outer roof covering are deflected by the watertight Tyvek^® underlay

BBA certificate 94/3054 (third issue) covers the use of Tyvek^®  vapour open membranes, unsupported or fully supported, in pitched roofs of any plan, any shape and of any size.
Using a Tyvek^®  underlay in accordance with the certificate will enable a roof to meet all the requirements of the Building Regulations.

Design

If the insulation only partially fills the space between the rafters, the underlay will be draped across the rafters and unsupported.

Use Tyvek^® SUPRO installed in the same way as a bituminous felt underlay and secured with stainless steel staples or large headed galvanised clout nails.

If the insulation fills the space between the rafters and/or covers them, you may use Tyvek^® SOLID as a fully supported underlay, securing it beneath counterbattens.

A Tyvek^® non-ventilated warm roof does not rely upon air flow beneath the underlay to remove the risk of condensation, there is no need to maintain an air gap between the Tyvek^® membrane and the insulation. You can therefore use the full depth of the rafters to achieve the insulation value required by the Building Regulations.

Installation

Eaves:
To avoid water ponding behind the fascia of closed eaves and to prevent prolonged exposure of the membrane to ultra-violet radiation, use a Tyvek^® Eaves Carrier. Stop the membrane short of the fascia and lap it onto the Eaves Carrier.

Pack closed eaves with fibrous insulation to reduce thermal bridging.

Verge detail:
Lap the Tyvek^® underlay 5Omm onto the gable or flying rafter.

Ridge/ hip detail:
Extend the Tyvek^® underlay 15Omm from each slope and overlap it at the ridge/ hip line. There is no need to provide vents at the ridge.

Valley detail:
Extend the Tyvek^® underlay from each slope by 15Omm beyond the valley centre line and seal the lap using Tyvek^® Butyl Tape to form a watertight seal.

Abutrnents:
Turn up the Tyvek^® underlay beneath flashings not less than 5Omm.

Penetrations:
Seal around all penetrations using Tyvek^® Butyl Tape.

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