Timber frame wall build-up: the layers from interior to facade

A typical external timber frame wall has seven layers. Materials and names differ between systems, but the sequence stays the same.

• Interior board: plasterboard, gypsum fibreboard or timber lining, the surface that is finished and decorated.
• Service cavity: battens that leave a gap for cables, sockets and small pipes.
• Vapour control layer: film, membrane or taped boards that keep moist indoor air out of the wall.
• Studs with insulation: the frame carries the loads, and mineral wool or wood fibre insulation fills the space between the studs.
• Sheathing and wind barrier: boards or a membrane that brace the frame and keep wind out of the insulation while letting vapour escape.
• Ventilated gap: counter-battens that leave an air space behind the cladding.
• Cladding: timber boards, fibre cement or another facade finish that sheds rain.

The three inner layers make the wall airtight. The vapour control layer lies on the warm side of the insulation and must be continuous, with taped laps and sealed junctions at the floor, ceiling and windows. The service cavity in front of it holds the wiring, so sockets and switches never need holes in the airtight layer.

The interior board is fixed to the service cavity battens. Its choice depends on the finish and on what will hang on the wall: heavy cabinets, shelves or a wall-hung toilet need backing boards or extra battens, planned before the boards go on.

The studs carry the vertical loads and hold the other layers. Their size and spacing come from the structural design. The insulation is cut to fill the space between them completely, with no gaps at the studs, at the top and bottom plates or around windows.

How thick the insulation should be is not a rule of thumb. The designer sets it in the energy calculation for the whole building, which accounts for the wall area, windows, heating and the energy performance the building has to reach. Some designs add a second insulation layer across the studs, inside or outside, to reduce the heat lost through the timber itself.

On the cold side the wall must stop wind but let vapour out. Sheathing boards brace the frame, and the wind barrier, either the board itself or a separate membrane, keeps moving air out of the insulation. Joints and window junctions are taped or sealed here as carefully as on the inside.

Counter-battens then form a ventilated gap behind the cladding. Air enters at the bottom and leaves at the top, drying the outer layers and any rain that gets behind the boards. The openings need insect mesh, and the gap has to stay open around windows and at the eaves.

The principle is simple: the layers become more open to vapour from the inside out. Vapour that does get into the wall can then leave through the outer layers instead of being trapped. Reverse the order, for example with a vapour-tight board outside and an open layer inside, and moisture collects in the insulation every winter.

The same logic applies when the wall is altered later. A vapour-tight finish added outside, or new sockets cut through the inner layer, changes how the whole wall behaves, so any change should be checked against the original build-up.

Most problems in timber frame walls come from details, not from the choice of materials, and nearly all of them can be caught by checking each layer before the next one covers it. What Else builds timber frame walls as part of its turnkey and exterior projects.

• Gaps in the insulation at studs, corners and around windows, which become cold spots.
• A vapour control layer with untaped laps, or one not connected to the floor, ceiling and window frames.
• Sockets cut through the airtight layer instead of sitting in a service cavity.
• Open wind barrier joints, or a membrane torn while the cladding is fixed.
• A ventilated gap closed at the bottom or top, or blocked by insulation.
• Insulation thickness chosen out of habit rather than from the energy calculation.