Timber frame is the second most popular construction method for homes in England and Wales, with 15% of UK housing output. It is the most popular in Scotland with 75% of market share.6 There are different types but they all use a timber frame as structural support, timber sheathing and insulation between or outside timber studs. Timber frame is best suited to low-rise development of one to four storeys, but solid timber panels and engineered timber can be used to build apartments up to 10 storeys. A vapour barrier is required to prevent moisture entering the wall structure from the inside. A breather membrane is required to protect against external moisture and an outer leaf of cladding protects from weather and solar radiation.
The timber frame is usually manufactured off-site to different levels of completion – either as an open or closed panel. The panels are taken to site and erected by the supplier or sub-contractor. Some contractors and sites will favour ‘stick construction’ over prefabricated panels, but off-site manufacture of panels or modules can enable better performance.
There are a wide range of types of timber frame, with different types of insulation, moisture barrier, airtightness and erection methods.
The typical methods are shown in Figure 5.1, below.
Timber is approximately five times more conductive than insulation, so more timber results in poor thermal performance. Figure 5.7 demonstrates how an extra layer of insulation should be installed inside or outside the frame to minimise thermal bridging.
The rest of this chapter highlights good practice detailing for timber frame with emphasis on thermal performance. The location of these junctions are shown in this section drawing through a typical house. The most significant external envelope details affecting heat demand are drawn with good practice airtightness and continuous insulation where practical. Heat loss is calculated and psi-values provided where useful for SAP calculations.
The door must line up with the insulation underneath and timber frame insulation in the adjoining wall. The screed acts as the airtightness in the floor which is taped to the door frame.
Ensure the ends of floor joists are fully insulated and airtight with the correct sequencing. Thermal bridging can be reduced by adding an extra layer of insulation to the outside of the timber frame.
This heat flux diagram shows heat flow through the external wall at intermediate floor level (see Detail 5.2). This is normally an area of significant heat loss, and so the design must allow for insulation in the floor cassette or externally to prevent thermal bridging. This detail has a psi-value of 0.067 W/m.K, which is a 52% reduction in heat loss compared to the default value of 0.14 W/m.K.
The temperature factor is above the critical value of 0.75, and so there is no risk of condensation or mould growth. Please refer to Appendix 3 for further information.
SAP Appendix K Reference | E7 |
---|---|
psi-value | 0.067 W/m.K |
temperature factor | fRsi = 0.94 |
approved value | 0.07 W/m.K |
default value | 0.14 W/m.K |
Thermal and acoustic performance must be considered in a party wall, so non rigid insulation is preferable. An additional service zone could be added to improve airtightness which improves both acoustic and thermal performance.
There are three psi-values that should be calculated for a window: the jamb, cill and lintel. There are significant reductions in heat loss when the psi-value is calculated for each detail. Insulating the window reveal results in a significant improvement in the thermal performance. Moving the window in line with the SIP panel will further improve thermal performance.
The three psi-values that account for the performance of the window junctions in this case are significantly better than the default value.
SAP Appendix K Reference | E2 Lintel | E3 Sill | E4 Jamb |
---|---|---|---|
psi-value | 0.069 W/m.K | 0.028 W/m.K | 0.067 W/m.K |
temperature factor | fRsi = 0.92 | fRsi = 0.92 | fRsi= 0.91 |
approved value | 0.3 W/m.K | 0.04 W/m.K | 0.05 W/m.K |
default value | 1.0 W/m.K | 0.08 W/m.K | 0.1 W/m.K |
Canopies over doors often require additional structural steel which needs to be properly insulated or thermally separate from the thermal envelope. Recessed doors are a point of increased heat loss area and are difficult to insulate. Ensure adequate insulation and sequencing is agreed to achieve performance.
This heat flux diagram shows heat flow through a recessed external door junction. This is normally an area of significant heat loss and so the design must allow for insulation in the floor cassette to prevent thermal bridging. This detail has a psi-value of 0.071 W/m.K, which is a 78% reduction in heat loss compared to the default value of 0.32 W/m.K.
SAP Appendix K Reference | E20 |
---|---|
psi-value | 0.071 W/m.K |
temperature factor | fRsi = 0.88 |
approved value | N/A |
default value | 0.32 W/m.K |
Continuous insulation around a timber frame is important to ensure no thermal bridging through the timber.
This heat flux diagram shows heat flow through the flat roof and external wall junction. The top of the wall has a significant amount of timber for structural reasons, which increases the heat loss. The internal insulation has reduced the amount of heat loss to improve on the default. This detail has a psi-value of 0.062 W/m.K, which is 22% better than the default value of 0.08 W/m.K.
The temperature factor is above the critical value of 0.75, and so there is no risk of condensation or mould growth.
SAP Appendix K Reference | E14 |
---|---|
psi-value | 0.062 W/m.K |
temperature factor | fRsi = 0.95 |
approved value | N/A |
default value | 0.08 W/m.K |