Windows in Waterproof Walls

Addressing AS 3740 shortfalls

NCC 2022 Volume Two

Area of NCC Requirements:

• H4P1 – Wet areas
• H4D2 – Wet areas (DTS pathway reference) 
  

The Challenge

Windows are often introduced into bathrooms to bring in natural light and ventilation, but they can become a high-risk junction when located within a shower wall. In this project, an aluminium-framed sliding window was proposed within the primary wet zone of an ensuite shower, meaning the window and its surrounding wall junction would be exposed to direct water spray during normal use.

This arrangement does not sit neatly within typical Deemed-to-Satisfy (DTS) wet area detailing expectations, because standard wet area construction is generally framed around continuous waterproof surfaces without openings in the direct spray zone. As a result, a performance-based pathway was required to demonstrate that the bathroom would still achieve the moisture control outcomes intended by H4P1, namely preventing water from penetrating behind linings and into concealed spaces.

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What This Really Means

Wet area compliance is outcome-based. The NCC is ultimately concerned with whether the construction prevents moisture from entering concealed spaces and damaging building elements over time, not whether a bathroom feature is common or uncommon.

Where a detail increases moisture exposure—such as a window in a shower wall—the design needs to show that the risk is controlled and that the overall system still performs to the required level.

The Solution

A performance-based assessment was prepared to confirm that the proposed wet area arrangement achieves the intent of NCC 2022 Volume Two, Amendment 1 – H4P1.

The assessment considered:

• How the window location and wet area geometry influence direct spray exposure and moisture load 
• Whether the waterproofing strategy provides continuous protection at critical junctions, including around openings and terminations 
• How sealing and drainage principles are applied so incidental water does not migrate behind finishes or into concealed spaces 
• Whether verification steps and in-service maintenance expectations support durable long-term performance 

The solution focused on achieving an outcome that is equivalent to, or better than, the protection intended by DTS wet area provisions without relying on a 'one-size-fits-all' detail.

Why This Matters 

The report concludes that the proposed shower window arrangement can satisfy the performance intent of H4P1, provided the installation is carried out in accordance with the assessed design intent and verified at key construction stages.

This is a common real-world example of where performance solutions unlock practical design outcomes: you can retain natural light and ventilation benefits, while still delivering a robust wet area system that protects the building fabric and occupant amenity.

DTS restricting your design creativity? 

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