Understanding the Crucial Differences: Negative Side Waterproofing Membranes vs. Moisture Barriers

In the world of waterproofing, it's essential to recognise that a membrane suitable for negative side waterproofing is not necessarily the same as a moisture barrier. Let's delve into this critical distinction: 

Moisture Barriers: Often referred to as vapour barriers, these are typically epoxy-based. They prevent both liquid water and water vapour from penetrating, ensuring comprehensive protection against moisture ingress. 

Waterproofing Membranes for Negative Side Applications: Many of these membranes are cement or mineral-based. While they effectively block liquid water, water vapour can still pass through, and the transmission rate can vary significantly depending on the product design. 

The differences of water vapour transmission between the types of membrane can be quite significant. Based on test reports reviewed, the water vapour transmission rate of a moisture barrier can be as little as 0.0006 g/m2/24hours, while the water vapour transmission rate of a cementitious negative side membrane can be between 1 & 2 g/m2/24 hours. 

To put this into perspective, if a negative side membrane with a water vapour transmission rate of 1.5g/m2/24 hours is applied to 100m2, 150g (0.15 litres) of water vapour is transmitted through the membrane, every day. That’s just over a litre a week. It doesn’t sound like much, but is certainly adds up. 

Key Considerations for Your Project: 

• Design Impact: The choice between a moisture barrier and a cementitious membrane will affect your overall design. If natural or mechanical ventilation is feasible, a cementitious product might suffice. However, if moisture vapour buildup or condensation is a concern, incorporating a moisture barrier is crucial 

• Rising Damp: Negative side membranes should not replace moisture barriers in scenarios where rising damp could impact flooring installations, such as vinyl or hybrid flooring. 

Real-World Example:

Here is a vinyl plank installation where a cementitious negative side waterproofing membrane was used on a concrete slab with rising damp. Shortly after installation, homeowners noticed spongy planks. Upon inspection, water was found beneath the planks due to water vapour migrating through the membrane and condensing. This issue could have been avoided with a proper moisture barrier. 

Stay informed and choose the right products to ensure the longevity and effectiveness of your waterproofing projects.  

Contributed by: Frank Moebus

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