Moisture barriers, damp proof membranes (DPMs) and waterproofing membranes are terms that are often used interchangeably.

Strictly speaking, however, the only true barriers that completely stop the transmission of moisture are metal (generally aluminium or copper) and glass.

Nonetheless, AS 4858-2004 (Wet area membranes) defines a membrane as 'waterproof' if it achieves a specific performance level under test conditions.

This level is called the 'moisture vapour transmission rate'.

The types of moisture barriers that meet this definition include:

• Pre-formed sheet membranes that are either laid on top of the subfloor surface or bonded to the surface.
  
  
• Fibre reinforced liquid-applied membranes, which either contain chopped strands of fibre in the liquid, or have a separate sheet of mesh that's embedded into the liquid while it's being applied to the floor.
  
  
• Liquid-applied membranes that contain no reinforcement.

Pre-formed sheet membranes are commonly found in the modular fibreglass units installed in bathrooms, such as shower trays and other fixtures.

Liquid-applied membranes are used by floor tilers and vinyl floor layers working in wet areas.

These areas include bathrooms, laundries, commercial kitchens and hospital rooms where water may be splashed around.

They can also be used when there is a moisture problem coming from an external source.

In these cases, their purpose is to resist any moisture that travels through the slab from getting to the surface and affecting the floor coverings or walls.

In addition to these 'waterproofing' membranes, flooring installers sometimes use moisture suppressants if they want to begin an installation before green (freshly poured) concrete has dried to an acceptable level, or if there is low-level capillary movement of moisture through the slab.

Types of liquid-applied membranes

There are three main categories of liquid-applied membranes:

• Acrylic membranes are water-based and can be one or two part. One-part products are premixed, so they are applied direct from the container. Two-part systems normally contain a liquid polymer and a cement-based powder.
  
  
• Polyurethane membranes are 'reaction polymers' - that is, the molecules react with other chemicals to form strong cross-linked membranes.
  
  
• Epoxy membranes are all two-part systems that contain an epoxy resin and a reactive hardener. Some are designed to be flexible when cured, others are non-flexible.

Joints and bond breakers

The flexibility of a membrane is a major factor in its performance and the way it is installed.

AS 4858 defines three classes of moisture membranes, based on their 'extensibility', or ability to stretch when there is movement in a crack or join.

Class 1 membranes have low extensibility, so they don't 'extend' very well when there is movement in a crack or join in the floor, or between the floor and a wall.

This means that if there is regular movement, the membrane might fatigue over time and eventually rupture.

One solution is to put a foam backing rod where the floor and wall meet and cover it with a piece of bond breaker tape.

When the moisture membrane is installed on top, the tape 'breaks the bond' over the joint and allows the membrane to flex and then return to normal.

Note that if you're carrying out a resilient flooring installation, the backing rod should not protrude beyond a 90 degree angle.

Class 2 membranes have medium extensibility.

Because they're able to stretch more than Class 1 membranes, the width of the bond breaker tape can be proportionately less.

Class 3 membranes have high extensibility. They are very flexible, so the bond breaker can be as little as a 12 mm bead of silicone.

The different types of bond breakers used in residential construction are described in AS 3740-2010 (Waterproofing of domestic wet areas).

Expansion joints are a different matter. These openings must be carried right through to the finished floor and fitted with an approved jointing system.

We'll discuss this further in the next lesson.