From TRADA Wood Information Sheet 1-35.
All timber frame walls are designed to ‘breathe’. The term ‘breathe’ refers to the ability of a wall to allow water vapour to diffuse through the structure from the inside of the building to the outside. To ensure that water cannot condense within the wall structure (interstitial condensation), there is a rule of thumb which indicates that the vapour resistance of the layers on the warm side of the insulation should be at least five times that of those on the cold side. This controls the flow of water vapour by ensuring that it escapes more quickly through the outer layers of the wall than it can enter through the inner layers.
The conventional timber frame wall design comprises (from the inside):
plasterboard with metallised polymer backing or separate polyethylene vapour control layer
solid timber studs with mineral wool or other type of insulation between
oriented strand board (OSB) sheathing
breather membrane
drained and vented cavity
external cladding
This provides a vapour resistance ratio from inside to out of between 12:1 and 50:1, depending upon the materials used.
Methods for calculating condensation risk and recommendations for condensation control are given in BS 5250 Code of practice for control of condensation in buildings. Computer software programmes are readily available to enable calculations to be carried out.
A breather membrane restricts the passage of liquid water, whilst allowing water vapour to pass through. The breather membrane performs a number of functions in a timber frame wall:
It protects the fabric of the building from rainwater penetration during construction before external claddings are completed.
It provides a second line of defence against water penetration during the life of the building as most claddings act as rainscreens, rather than as complete barriers.
It allows water vapour to escape from the construction.
It can also contribute to air sealing the wall and reducing ventilation heat losses. This aspect is likely to be of increasing importance as air leakage becomes more significant in thermal performance requirements under Building Regulations.
A breather membrane is required to protect timber frame walls where the outer sheathing of a conventional timber frame wall (see Figure 1) is oriented strand board, plywood or certain types of medium board.
Where the sheathing is on the inner face of the wall, so-called reverse wall construction (see Figure 2), a breather membrane may be used on the outer face.
A breather membrane is not required where the outer sheathing of a timber frame wall is inherently moisture resistant and sufficiently vapour permeable, eg bitumen impregnated softboard, provided that all the external wall, including the intermediate floor zone and sole plate, is protected and all the joints between the boards are weather-proofed. However, breather membranes may still be specified in very exposed locations.
Breather membranes need to combine a high degree of wet strength and water resistance with very low vapour resistance. A maximum vapour resistance value of 0.6 MNs/g is required but commonly available membranes are well below this. In summary breather membranes need to be:
There are three types of breather membrane which may be used in timber frame construction. Methods for determining the levels of performance are laid down in BS 4016: 1997 Specification for flexible building membranes (breather type). The minimum performance requirements are shown in Table 1.
Table 1 Sunmmary of performance requirements
The difference between types of membranes is significant. Under test conditions Types 1 and 2 membranes do not allow any water penetration whereas Type 3 membranes allow 5% by volume of water to penetrate through the membrane during a test period of 24 hours. Type 2 membranes have reduced tear resistance requirements when dry and may have none at all when wet.
Suitability and application
For all wall designs where breather membranes are required TRADA Technology recommends that Type 1 membranes are specified due to their superior resistance to water penetration and tearing.
Rainwater can penetrate outer claddings. Certain locations in the UK are within ‘very severe’ ‘exposure zones’. Type 1 breather membranes are essential in these areas. These locations are defined in the BRE Report Thermal Insulation – avoiding risks, - figure 30.
Breather membranes may be applied to wall panels on site or in the factory. Type 1 and Type 3 membranes are suitable for both site and factory fixing. Type 2 membranes are not suitable for use on prefabricated assemblies, which will be exposed during transport and erection as they have insufficient tear strength. They are only suitable for site fixing where the specifier is confident that the external cladding will be applied soon after the wall panels have been erected and the breather membrane applied.
Another reason for installing cladding soon after the breather membrane is applied, in all cases is that sunlight can degrade breather membranes over a relatively short period of time. The manufacturer’s technical information and recommendations should be checked, reviewed and implemented.
Breather membranes should be applied from 25mm below the lowest timber member, usually the sole plate, (see Figure 3).
Where the sheathing is on the inner face of the wall (as shown in Figure 2), unless rigid boards are to be installed to the outer face of the studs care should be taken to ensure that the insulation is no thicker than the depth of the studs. This will avoid undue pressure being applied to the breather membrane and its fixings. Horizontal laps where possible should be fixed to solid timber to aid air sealing. An alternative is to seal the lap with an approved double-sided adhesive tape.
Breather membranes should also cover the intermediate floor zone. Where membranes are applied to panels in the factory, they should be fixed as listed in Table 2 and at the sides, head and base of each panel. The membrane should extend beyond the sides and base of panels to ensure that the lap requirements shown in Figures 3 and 4 are met. The membrane at the base of upper storey panels should extend sufficiently to cover the intermediate floor zone and provide 100mm lap over the lower panel. The lap sections should be temporarily fixed back to the panels for transport.
For additional moisture protection around window and door openings the breather membrane should be folded over and inwards to cover all timbers. Corners may be sealed with self-adhesive tape (see Figure 7).
Breather membranes should be fixed with austenitic stainless steel staples or nails, as detailed in Table 2.
Fixings at stud positions should be made through pvc banding tape. This provides additional tear resistance and marks the stud position for fixing wall ties on site. It is essential to use tape in the very severe exposure locations where Type 1 membranes are required. If another system of marking is utilized this should be clear, indelible, and fade and tear resistant.
As increased thermal performance of buildings is required air infiltration becomes significant. Air infiltration through gaps in the building fabric can accelerate the rate of heat loss due to convection and so reduce thermal performance. The breather membrane can contribute to reducing heat loss, when this is required laps can be sealed with adhesive tape. Breather membrane manufacturers should be consulted.
