Waterproof vs Water Resistant Membrane: Key Differences Explained for Construction

What Do Waterproof and Water Resistant Actually Mean?

Waterproof means the membrane does not allow liquid water to pass through under a defined hydrostatic pressure for the duration of the test. The industry standard used to measure this is EN ISO 811, which applies a rising water column height, measured in millimeters, against the membrane and records the pressure at which leakage occurs.

Water resistant means the membrane resists liquid water penetration but allows it to pass through after sustained exposure or sufficient pressure. A typical water resistant membrane may hold back rain for a limited period, but it is not designed to stop a continuous water column.

The Technical Difference in Performance

The measurable difference between waterproof and water resistant membranes is the hydrostatic head rating, the duration of water exposure, and the vapor permeability of the membrane.

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When a product achieves a 2000 mm water column result, it has proven it can resist a static head for hours. A 300 mm water resistant product stops rainfall but fails in a wall assembly where wind-driven rain creates hydrostatic pressure. In timber-frame construction, a roof underlayment with a waterproof rating is essential to keep the roof deck dry, while a water resistant product is not sufficient.

Why Waterproof Does Not Equal Breathable

Waterproof and breathable are independent properties. A membrane can be waterproof and breathable, waterproof and vapor-closed, or water resistant and vapor-open. Combining these properties correctly is the core of building envelope design.

Jiangsu Aotelong New Materials Co., Ltd. supplies both classes of membrane, and their technical data sheets clearly separate the hydrostatic head rating from the vapor permeability result, which helps specifiers avoid mixing up the two properties.

• Waterproof breathable membranes block rain and wind-driven water, and allow vapor diffusion
• Waterproof non-breathable membranes block water and vapor completely
• Water resistant membranes resist moderate moisture, but do not meet hydrostatic head requirements
• Vapor barriers control condensation, and often serve as the air-tightness layer

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Matching the Membrane to Your Building Envelope

Choose a waterproof membrane anywhere that liquid water can be pushed through a wall by wind or gravity. Choose a water resistant membrane where the primary goal is vapor management and only occasional splash exposure is expected.

The application also determines whether to specify a breathable membrane or a vapor barrier. Understanding how these systems work in a full assembly helps avoid condensation failures. For a deeper look at how these membrane classes fit into a complete enclosure strategy, this guide on different types of waterproofing in construction covers the full range of systems.

Selecting between a waterproof and a water resistant membrane comes down to three factors: the test results, the position within the building assembly, and the expected water exposure. The technical data sheet should always be the deciding document. If the hydrostatic head result is below 1000 mm, the membrane is water resistant. If it is 1000 mm or above, the membrane is waterproof. When in doubt, choosing a higher waterproof rating is safer than specifying too little protection.

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Frequently Asked Questions

Can a water resistant membrane be used as a waterproof membrane?

No. A water resistant membrane is only rated for moderate moisture conditions. It does not meet the minimum 1000 mm hydrostatic head requirement that defines a waterproof membrane. Using a water resistant product where waterproofing is required will lead to water damage over time.

What is the minimum hydrostatic head for a membrane to be called waterproof?

Under EN ISO 811, a membrane is classified as waterproof when it withstands a water column of at least 1000 mm pressure. Many high-performance building membranes are rated in the 1500 to 2000 mm range. The ATL breathable membrane series from Aotelong is designed around this classification.

Are waterproof breathable membranes better than non-breathable waterproof membranes?

It depends on the wall assembly. Waterproof breathable membranes are better for exterior cladding where the wall needs to release vapor. Non-breathable films are better for interior vapor barriers where vapor diffusion must be controlled. Selecting the right membrane for the envelope arrangement is more important than choosing one class over another.

What performance standards should I check when comparing membranes?

Check the hydrostatic head test result (EN ISO 811), the water vapor transmission rate (EN ISO 12572), and the UV resistance rating if the membrane will be exposed for more than 30 days. A reliable manufacturer publishes these values in the technical data sheet.