Why Opening a Window Isn’t a Ventilation Strategy (and What G4 Really Requires)

In New Zealand, it’s common to hear people say, “Just open a window for fresh air.” But when it comes to building code compliance and creating healthy indoor environments, that advice is outdated—and often ineffective. Clause G4 of the New Zealand Building Code is clear: ventilation must be appropriate for the intended use of the space, and it must deliver enough fresh air to maintain indoor air quality. Simply cracking a window won’t cut it, especially in today’s more airtight, better-insulated homes.

At BEO Buildingscience, we work with homeowners, builders, and developers to make sense of G4 ventilation requirements. We help ensure homes and buildings are not just compliant, but also comfortable, healthy, and future-ready. In this blog, we’ll explore what Clause G4 really means, why relying on natural ventilation alone can lead to problems, and how mechanical ventilation strategies can make all the difference.

What Is Clause G4 – Ventilation?

Clause G4 of the New Zealand Building Code requires that buildings are constructed to provide adequate ventilation to occupied spaces.

The Main Objectives of G4 Are:

• Provide sufficient fresh air to maintain healthy indoor conditions

• Remove moisture and contaminants generated by occupants and activities

• Avoid condensation and poor air quality

G4 Acceptable Solution G4/AS1:

• Allows for natural ventilation through windows and vents

• Requires mechanical extraction in wet rooms like kitchens, bathrooms, and laundries

While G4/AS1 gives basic guidance, real-world performance often fails to meet the intent of the clause—especially when natural ventilation is relied on too heavily.

The Problem With “Just Open a Window”

1. It Assumes Occupants Will Always Do It

People don’t open windows when it’s cold, windy, raining, or noisy outside. This is especially true in urban or high-density areas.

2. It Ignores Airtightness and Building Physics

In modern homes designed to meet energy efficiency standards (Clause H1), airtightness is much higher. That’s good for thermal performance, but it reduces unintended airflow through gaps and cracks—which older homes relied on for ventilation.

Without mechanical ventilation, these newer homes become sealed boxes that trap moisture, CO₂, and pollutants.

3. It’s Unpredictable

Natural ventilation relies on wind and temperature differences to move air. That means it works sometimes, but not always. There’s no consistency—so indoor air quality suffers.

4. It Doesn’t Remove Moisture Effectively

Bathrooms and kitchens generate large amounts of moisture. Opening a window may help in summer, but in winter, it’s not effective enough to prevent condensation and mould.

What Happens When Ventilation Fails?

Health Impacts

• Poor indoor air quality can lead to respiratory issues, fatigue, and allergies

• High humidity encourages mould growth, which is harmful for asthma and immune systems

Structural Damage

• Condensation inside walls and roofs can lead to rot, decay, and costly repairs

• Persistent dampness damages finishes and materials

Compliance Risk

• Even if a house has openable windows, if air quality is poor or moisture is building up, the intent of G4 is not being met

• This can lead to issues with council sign-off, tenant complaints, or future regulation changes

What G4 Really Requires: Fresh Air That Actually Works

Clause G4 isn’t about ticking a box—it’s about making sure people inside the building have access to sufficient, clean, dry air. That means ventilation systems need to be:

1. Effective

They must deliver the right airflows where they’re needed—bedrooms, living areas, and wet rooms.

2. Continuous or Demand-Controlled

Intermittent use (e.g. fan only when switched on) isn’t enough. Systems should run continuously at low levels or respond automatically to humidity and CO₂.

3. Quiet and User-Friendly

Noisy systems get turned off. Poor controls confuse users. Systems must be simple to use and easy to live with.

Mechanical Ventilation: The Smart Way to Meet G4

Mechanical ventilation ensures consistent airflow and air quality, regardless of weather or occupant behaviour.

1. Extract-Only Systems

These are common in bathrooms and kitchens. But they only remove air—they don’t bring in fresh air, so they rely on the house being leaky.

2. Supply & Extract Systems (Balanced Ventilation)

Balanced systems bring in filtered fresh air and extract stale air. When done right, they also recover heat.

3. MVHR (Mechanical Ventilation with Heat Recovery)

MVHR systems are the gold standard:

• They recover up to 90% of heat from outgoing air

• Provide consistent fresh air to all rooms

• Reduce moisture and CO₂ build-up

• Great for Passive House or high-performance homes

"We installed MVHR and the house feels fresher all the time—no more waking up stuffy. It even cut our heating bills."
— Homeowner, Queenstown

Our Process for G4 Ventilation Assessment and Design

At BEO Buildingscience, we don’t just check that windows open—we ensure your home or project delivers real ventilation performance.

Our Process Includes:

• Initial consultation to understand the property and concerns

• Energy Modelling to assess how ventilation interacts with heating, airtightness, and moisture

• On-site testing: blower door testing to understand air leakage

• Clause alignment: H1 (energy), G4 (ventilation), and E3 (moisture) reviewed holistically

• Custom report with photos, data, and prioritised upgrade paths

Our recommendations are actionable and practical—whether you need quick fixes or a phased retrofit.

Optional Add-Ons:

• Hygrothermal (WUFI) modelling to assess internal moisture transport

• Thermal bridge calculations to identify cold surfaces contributing to condensation

• Ventilation design and commissioning guidance to ensure installed systems meet performance goals

• Blower door testing after upgrades to verify improvements and demonstrate compliance

Why Builders and Architects Should Pay Attention to G4

G4 is often the most misunderstood part of the Building Code. Designing a healthy, airtight, energy-efficient home means treating ventilation as a critical component—not an afterthought.

Why This Matters:

• Prevents costly remedial work from mould or condensation

• Supports better H1 performance by recovering heat

• Improves occupant satisfaction and health outcomes

• Builds long-term durability into your projects

Book a Ventilation Assessment or Design Review Today

If your project relies on windows alone for ventilation, it’s time for a rethink. BEO Buildingscience can help ensure you’re:

• Meeting G4 in a way that actually works

• Designing for durability and occupant health

• Avoiding future complaints, damage, and rework

Book Your Assessment or consultation to ensure your building isn’t just compliant—but performing.

Conclusion: Open Windows Are Not a Strategy

In a modern, airtight building, relying on windows for ventilation is like relying on candles for lighting. It’s outdated and unreliable. Clause G4 demands more—and so should you.

BEO Buildingscience: Making ventilation real, not theoretical.