The Tyvek^® sealed cold roof system

Principles
Whilst Tyvek^® non-ventilated roofs will not suffer from condensation problems, have less air leakage and are more energy efficient than conventional cold roofs, they are still subject to air infiltration at laps, perimeters and penetrations. Air movement through those gaps results in significant heat losses. Adopting the Tyvek^® sealed cold roof system will both control the risk of condensation and minimise the heat losses caused by air infiltration.  Eliminating air movement substantially improves energy efficiency.

The Tyvek^® sealed cold roof consists of a Tyvek^® roofing underlay laid taut across the rafters with all laps, abutments and penetrations sealed and with counterbattens to support tiling battens clear of the underlay. To gain maximum benefit a Tyvek^® air leakage barrier/ vapour control layer should be installed above the internal ceiling.  

Extensive tests of the Tyvek^® non-ventilated roof system by BRE and BBA have shown:
> 74% reduction in air leakage
> 25% reduction in whole house heat losses.

In the average dwelling this would produce:
> 11 tonne reduction in C0² emissions per dwelling over a 50 year period
> 1,000 kWh reduction in electrical heating consumption over a full heating season

with lower risk of condensation than would occur in a conventional roof using bituminous felt and a ventilated loft.

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.

BBA certificate 9913635 (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 heat loss". 

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.

Product selection -
For Tyvek^® sealed cold roofs DuPont recommends the use of Tyvek^® SUPRO Plus as an unsupported roofing underlay with Tyvek^® SD2 as an air leakage barrier/ vapour control layer.

Design

In the Tyvek^® sealed cold roof system Tyvek^® SUPRO Plus is laid over the rafters with all laps, joints and penetrations sealed to form an airtight vapour open barrier.

Condensation risk:
To minimise the risk of damage by condensation all roofs should be designed according to the basic principles described in BS 5250: the design should:

> provide sufficient heating and thermal insulation
> reduce airborne moisture by extracting it at source
> reduce air filtration
> place materials with higher vapour resistance closer to the inside and those with lower resistance > closer to the outside.

In the Tyvek® sealed cold roof system, Tyvek® SD2 - with a vapour resistance of 10.5MN.s.g­¹ (Sd=2.0m) - provides an ideal internal air leakage barrier/ vapour control layer at ceiling level, whilst Tyvek® SUPRO Plus - with vapour resistance of 0.16MN.s.g­¹ (Sd=0.03m) - provides a vapour open, airtight, watertight underlay beneath the tiling.

In all cases we recommend condensation risk analysis be carried out using the method set out in Appendix D of BS 5250: 1989.

Counterbattens:
The use of counterbattens ensures the tiling battens are held clear of the Tyvek^® underlay which is laid taut. Counterbattens offer several advantages:

> rain, snow and condensate can drain out freely
> a naturally vented batten space outside the underlay is assured
> debris arising from slating/ tiling is not trapped in the batten space.
> good construction, with a continuous airspace, is easier to achieve at valleys, hips, eaves and
> penetrations

Achieving an effective seal:
To achieve a sealed roof, all laps in the Tyvek^® underlay, all penetrations and all junctions to adjoining construction must be sealed.
Tyvek^® SUPRO Plus includes a self-adhesive acrylic strip which provides a robust seal at laps in the underlay. To support the sealed laps when laying Tyvek^® SUPRO Plus parallel to eaves, cross noggins may be added for extra security.
Use Tyvek^® Butyl Tape to seal the Tyvek^® underlay at the roof perimeter and at all penetrations.
At eaves use a suitable mastic sealant to seal around noggins or soffit boards.

As an alternative to conventional installation, Tyvek^® membranes can be installed vertically, from eaves to eaves over the ridge. This has the advantage of allowing laps to be formed on rafters where they are secured by counterbattens and horizontal laps are eliminated.  

Installation

Eaves:
To obtain an airtight seal at open eaves, fix noggins tight against the wall between the rafter feet and seal the junction. Extend the Tyvek^® underlay approximately 5Omm beyond the noggins and fix with staples or nails. Seal the Tyvek^® to the noggins with Tyvek^® Butyl Tape and seal the noggins to the wall with mastic sealant.

At closed eaves use Tyvek^® Eaves Carrier to avoid water ponding behind the fascia, and prevent prolonged exposure of the underlay to ultra- violet radiation. To achieve an airtight junction seal the 
underlay to the eaves carrier with two strips of Tyvek^® Butyl Tape. Seal the soffit board to the fascia and to the wall with mastic sealant. 

Ridge:
Lap the Tyvek^® underlay 15Omm over the ridge from each side and seal the laps with Tyvek^® Butyl Tape.

A clear drainage path should be maintained to allow rain, snow and condensate to drain to the eaves and to encourage moisture dispersal above the underlay. One solution is to raise the valley boards above the underlay by setting them into the ends of the counterbattens and supporting the centre on a thinner batten.

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