The Tyvek^® sealed warm roof system

Principles

Whilst Tyvek^® non-ventilated warm roofs have less air leakage and are more energy efficient than conventional warm roofs, they are still subject to air infiltration at laps, perimeters and penetrations. Air movement through those gaps results in significant heat losses. The Tyvek^® seated warm roof system both controls the risk of condensation and minimises the heat losses caused by air infiltration. Eliminating air movement substantially improves energy efficiency.

The Tyvek^® sealed warm roof system consists of a Tyvek^® roofing underlay fully supported by the thermal insulation, sealed at all laps, abutments and penetrations, with counterbattens to support the tiling battens clear of the underlay. To gain maximum benefit a Tyvek^® SD2 air leakage barrier/vapour control layer should be installed behind the internal lining. Adopting the Tyvek^® sealed warm roof system will:

> control the risk of condensation
> minimise heat losses, including those caused by air infiltration
> maximise use of the building volume including room-in-the-roof or cathedral roof construction
> improve the structural stability of the roof

The Tyvek^® sealed warm roof system is the most energy efficient and cost effective system for pitched roofs.

The Tyvek^® sealed warm roof system eliminates air leakage and reduces whole house energy consumption by 25% by eliminating convection heat losses associated with traditional roof ventilation, as extensively demonstrated by the BRE and the BBA.

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 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.

Design

In the Tyvek^® sealed warm roof system Tyvek^® SUPRO Plus is laid directly onto the insulation with all laps and joints sealed  and secured in place with counter battens nailed through to the rafters. The insulation may fill the space between the rafters, or cover them, presenting a flush surface which fully supports the underlay. Tyvek^® does not 'tent' when laid directly in contact with insulation or any other type of support.

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 warm 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 roof covering.

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

Counterbattens:
The use of a fully supported Tyvek^® underlay and counterbattens ensures the tiling battens are held clear of the underlay which offers 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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