Timber frame: Design Guidance

Control of condensation is vital for timber framed buildings. To prevent the build up of moisture which can lead to condensation the designer must:

1. make provision for extracting moisture at the point of generation (e.g. bathrooms and kitchens)
2. prevent moisture from entering the construction, means of a vapour control layer (VCL)
3. allow moisture within the construction to pass to atmosphere, by using materials with high vapour resistance on the warm side of the insulation and those with low vapour resistance on the cold side of the insulation. To avoid problems which may arise because of the high vapour resistance of the plywood conventionally set on the outside of the studs, the designer may specify materials with a lower vapour resistance, site the plywood on the inside of the studs or specify a high performance VCL.

Condensation risk analysis to EN ISO 13788 should be carried out for all projects.

There is a risk of thermal bridging at the floor/wall junction: where Ballytherm Insulation is being applied across the face of the studs it should be extended as far as the surface of the floor.

It is difficult to form a continuous layer of insulation at intermediate floors, but some control of thermal bridging may be achieved by packing Ballytherm Insulation between the wall headers and the floor perimeter (figure 40).

To avoid thermal bridging at doors and windows Ballytherm Insulation must be butted tightly against horizontal and vertical members. Door and window reveals may also be lined with 20mm thick Ballytherm boards.

At the wall/roof junction continuity of insulation may be achieved by fitting Ballytherm Insulation between the rafters to fill the gap between the wall plate and the underside of the roof insulation (figure 39).

Design guidance
Fixings which penetrate wall insulation, such as wall ties or screws, will reduce the thermal performance of the insulation layer: the use of stainless steel fixings or plastic ties and fixings with small cross-sectional areas will minimise that effect.

Where flues from boilers and other heating appliances penetrate the walls precautions must be taken to avoid the hot pipe coming into contact with the insulation. There should either be a 25mm gap between the insulation and the flue - provided by a pipe sleeve - or a combination flue should be specified.

Sitework
Installation sequence (Ballytherm Insulation between studs)

1. Cut Ballytherm Insulation to size.
2. Set between studs.
   
3. Fix VCL and internal finish.

Installation sequence (Ballytherm Insulation across the face of the studs)

1. Pack fibrous insulation between the studs
2. Set Ballytherm Insulation across the face of the studs and tack in place.
3. Tape board joints with Ballytherm metalised tape.
4. Fix battens through Ballytherm Insulation to studs.
5. Fix plasterboard to battens.

Points to watch

• Seal around services to reduce air leakage.