The Tyvek sealed warm roof system: retro-fit

Principles

As much as 50% of construction activity is in work to existing buildings, including adapting and modifying them to suit new uses and upgrading them to improve their performance. Building Regulations now require material changes to existing buildings to comply with standards of thermal performance.

Building conversions and home extensions often involve a loft conversion to provide accommodation within a roof, requiring thermal insulation at rafter line. If the roof is to be stripped and re-covered then a Tyvek^® underlay can be installed as recommended in previous sections of this brochure; otherwise the insulation must be applied from inside the loft

The depth of existing rafters will determine how much insulant can be placed between them; if depth is limited it may be necessary to add rigid board insulation across their internal face to achieve the required U-value (0.30W/M².K max.) and to limit thermal bridging, alternatively battens can be applied to the underside of rafters to provide extra depth.

Converting an unheated loft space to heated occupied living spaces increases internal moisture levels, and increases the risk of condensation occurring within the newly insulated roof, especially if the existing roof covering contains a bituminous felt underlay.

It is recommended to maintain a ventilated space of 5Omm beneath the bituminous felt to encourage removal of condensation and to install a Tyvek^® REFLEX breather membrane to protect the new insulation.

Use of DuPont Tyvek^® breather membranes in loft conversions can ensure optimum thermal performance with no risk of condensation.

Use a layer of Tyvek^® REFLEX, silver side facing downwards, drawn taut, and secured to the sides of the rafters using small battens, then place fibrous insulant between the rafters (see illustration below). An internal layer of rigid insulation board - such as extruded polystyrene or polyurethane - with a higher vapour resistance than mineral fibre, applied across the face of the rafters will minimise their thermal bridging effect.

A Tyvek^® SD2 air leakage barrier/ vapour control layer should then be applied stapled to the rafters followed by the internal finish of plasterboard. Installing the plasterboard onto counterbattens will provide a useful void for services such as electrical wiring.

In larger roofs where the rafters are supported by purlins on cross walls or trusses, Tyvek^® REFLEX may be fixed and sealed to the face of the purlins.

The entire purlin depth may then be filled with fibrous insulation - which will normally give sufficient thickness to exceed the required U-value.

For improved air tightness an internal air leakage barrier of Tyvek^® SD2 should be applied, followed by the internal finish fixed to counterbattens.

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