Installing Wall Underlays for Maximum Watertightness

 

 

Making a home weathertight is as much about what’s inside the walls as the walls themselves. Using the right wall underlay, whether it’s building paper or something synthetic, is vital to allow the escape of water vapour from behind the walls.

Of course, not all wall underlays are equal. They can be absorbent or non-absorbent, air permeable and non-air permeable, rigid or flexible. It’s important to choose the right wall underlay for the cladding you’re using. BRANZ recommends using non-absorbent wall underlay directly behind absorbent claddings such as fibre-cement, timber weatherboards, brick or stucco. For direct-fix applications with no cavity, semi-absorbent (paper) underlay is recommended. Rigid sheathing is mandatory in very high wind zones.

Whether you use building paper or synthetic underlays, there are some key rules of thumb to help ensure the whole system is working effectively to stop moisture from becoming a problem.

• Only one wall underlay product should be used on a building, as all types perform differently.
• Install underlay to the outer face of the framing. The edges of higher layers should be lapped over the lower layers, running to 10–15 mm below the bottom plate.
• Tape each join horizontally, as loosely installed underlays with insufficient laps may allow water to get in. Ensure the tape is well fixed to the underlay and is wrinkle and fold free without too many layers.
• Trim openings by cutting at 45º away from the corners and folding the wrap around the framing.
• Underlay must be prevented from bulging into the drained wall cavity. Where stud spacing is greater than 450mm, a Stud Strap run horizontally should be used. Then plastic tape should be installed vertically from top to bottom plate at mid stud, to hold back the underlay.

The order you install your build components is also really important. Install the underlay first, before any windows, doors or objects such as pipes that penetrate the wall, then seal any gaps between them and the underlay with waterproof flashing tape. Tape up any tears or cuts in the underlay, or use a second layer to cover any large cut areas.

The cladding goes on last. Once this and the window joinery are on, you’re not going to be able to access your underlay again, so it needs to be as weathertight as possible before the closing-in stage.  

Finally, always follow manufacturer guidelines and beware of using products for purposes they weren’t intended for. Some wall underlays aren’t guaranteed when exposed to wetting (including before the cladding is installed). Once installed, you’ll also need to make sure the underlay isn’t left exposed for longer than the manufacturer specifies. Never use flexible underlay as temporary weather protection for your new build, as it has not been designed for this purpose.

Note that flexible products such as Thermakraft 220 underlay must also be installed by a licensed building practitioner. For more information on selection and installation of wall underlays, visit the BRANZ website.