Functions
In the opposite position to the vapour control layer, the breather membrane is attached to the external face of the sheathing material fixed to the timber studwork. It has several key functions:
Allow moisture vapour passing out through the inner leaf to escape into the ventilated cavity.
Protect the timber frame panel during construction until the external cladding can be installed.
Act as a defensive barrier against external moisture and the elements that may penetrate the cladding.
Installation
All breather membrane materials must conform to BS4016:1997.
Installation of the breather membrane is as follows:
The breather membrane should be fixed to the timber studwork by austenitic stainless steel straps at 300 mm centres.
The breather membrane should be lapped over studs to the following criteria
- Minimum 150 mm lap for horizontal
- Minimum 100 mm for vertical
When opes are required in the breather membrane for penetration of services it is critical to seal the ope after the services have been installed.
Similarly the breather membrane must be cut and dressed in door frames, heads, window sills, etc.
The bottom edge of the breather membrane should be left long enough to overlap with the DPC at the sole plate.
Full height stud positions should be indicated on the external surface of the breather membrane
As with the vapour control layer, any damage that is incurred within the breather membrane needs to be repaired to provide a proper seal or replaced.
Vertical cavity barriers and DPCs are required to be installed at jambs in the external walls.
Window frames should be installed into the timber-frame studwork of the external wall instead of the masonry leaf.
Location of cavity barriers around opes.
Vertical and horizontal cavity barrier locations at heads and jambs.
DPC position around the head.
Vertical DPC and cavity barrier connection at window at jambs.
Breather membrane connection with DPC at head.
Moisture regulation
If moisture penetrates the cavity it can reach the DPC, which redirects it to prevent it from affecting the framework. It is of critical importance that DPSs are installed correctly where joints occur such as at the head, where the breather membrane is lapped with the horizontal DPC at the head, which in turn is lapped over the vertical DPC.
Weepholes must be provided at 450 mm centres when the outer masonry leaf is unrendered. If a lintel angle is installed for the process of supporting the masonry leaf and the lintel angle does not completely close the barrier, a cavity barrier is then required to be installed to seal the cavity.
When a pre-cast lintel is used, the DPC should extend beneath the lintel. The purpose of this is to allow any moisture to enter the cavity, where it can be directed back out at the designed areas.
