The Tyvek^® non-ventilated cold roof system

Principles

The sole justification for ventilating beneath the underlay is to off-set the risk of damage by condensation to the roof structure and the insulation. However, the introduction of external air between insulant and underlay:

> reduces the effectiveness of fibrous insulation
> promotes warm air leakage from the building into the loft
> increases air infiltration into the heated building
> introduces dirt, dust and insects into the loft
> can frequently introduce external moisture into the building.

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 the air movement offers substantial gains in energy efficiency.

BBA certificate 99/3635 (fourth issue) states that, for non-ventilated roofs using a Tyvek^® underlay ... "the risk of condensation is equivalent to, or less than, that attending current conventionally ventilated cold roof systems".

The certificate also states "In conventionally ventilated roof constructions, energy loss by ventilation can account for up to 25% of the total beat loss through the roof. The Tyvek^® non-ventilated roof system will substantially reduce this mechanism of beat loss".

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 99/3635 (fourth issue) covers the use of Tyvek^®  breather membranes in non-ventilated pitched roofs of any plan, any shape or any size under all types of roof coverings including tight jointed and fully interlocking tiles and synthetic slates.
Using a Tyvek^®  underlay in accordance with the certificate will enable a roof to meet all the requirements of the Building Regulations.

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Installation

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

Eaves:
There is no need to provide any vents into the loft at eaves, because a Tyvek^® non-ventilated roof system does not rely upon air movement beneath the membrane to avoid damage by condensation.

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^® SUPRO underlay 5Omm onto the gable or flying rafter.

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

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

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

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