So you want to build a healthy, sustainable and energy efficient home that ‘breathes’?

Discover why homes don’t need to breathe, but the people inside them do, and how to design and build air tightness and ventilation as a system.

Learn what it means to build tight and ventilate right, and where mechanical ventilation becomes essential in a healthy home.

Listen to the episode now.

Hello! This is Episode 419, and Way #19 of the 44 Ways to Create Your Sustainable Home series here on the podcast. It’s the first episode in Section Five: The Building Envelope, and it’s about a phrase that gets thrown around a lot: ‘Build Tight, Ventilate Right’.

Section Five is about creating a sustainable skin in your building envelope. 

When you understand how vapour, moisture and temperature move from the inside to the outside of your home, you’ll see it’s important to design and build a building envelope that suits your climate, mitigates the risk of mould growth, helps maintain comfortable indoor air temperatures, and that is durable and long-lasting.

Way #19 is: Build Tight, Ventilate Right.

I want to start by taking on one of the most persistent myths in residential construction: the idea that homes need to breathe. 

Our homes don’t need to breathe. The people inside them do.

Many homeowners say to me ‘I want my home to breathe’…

But our homes don’t need to breathe. The people inside them do. And they need to ideally breathe fresh, unpolluted air, whatever the weather or conditions outside their home.

The conclusion that many draw from this desire to have their home ‘breathe’ is that they want to be able to open windows, ensure there’s regular breezes, and that they’ll be living with direct and regular contact with the outside and the elements.

However, the reality is that, as we’ve discussed earlier in this series, a ‘breathing’ home is often confused with one that leaks uncontrollably, through gaps, cracks and openings in the building envelope. And it’s a conclusion that has justified decades of poorly performing, draughty, thermally uncomfortable homes.

Uncontrolled air leakage risks carrying moisture into the building fabric. We talked about what happens to that moisture in Way #17, Episode 417 when we covered the subject of WUFI analysis. 

Uncontrolled air leakage also creates draughts that make spaces uncomfortable. And it wastes energy, because every time air leaks out, it takes conditioned air you’ve paid to heat or cool with it. In a cold winter, every gap in your building envelope is a pathway for heat to escape. In a hot summer, every uncontrolled opening is a pathway for hot air to enter.

What homes actually need isn’t to breathe, but to enable their occupants to control how air moves through them. To be able to ventilate them in a sufficient and controlled manner. 

So, not experiencing random air movement through gaps, but a deliberate, designed exchange of air at a rate and through pathways that serve the people living in the home, and that’s designed in deliberately, and actively controlled to support the health and comfort of the home’s occupants.

What ‘building tight’ actually means…

Building tight means creating a continuous, unbroken air barrier around the building envelope. 

Jessica Allen from Climasure spoke to me in Episode 296 about identifying this at the drawing stage by drawing a red line to show the continuous barrier or membrane around the entire home. At the roof, it may occur at the roof line or ceiling line. And then it’s continuous through the external walls and the floor structure. And being continuous, it means sealing every penetration where a pipe, cable, duct or structural element passes through that barrier. It means installing windows and doors that close and seal properly.

This is something that’s not exclusive to Passive House. It can be done with conventional construction. However, just because it’s not complicated, doesn’t make it common.

Any time I talk about air tightness, this question inevitably comes up.

“But Amelia – I want to be able to open my windows, so this isn’t relevant to me, is it?”

Yes, opening windows works to ventilate a home. When you’re home, when the outdoor conditions are right, when it’s not raining, not too cold, not too hot, and when the outdoor air quality is acceptable. Which is a lot of conditions to align.

But…

• you won’t always be home (and you may not be able to leave them open if you’re not)
• you can’t always open your windows (on extreme temperature days, or when there’s noise or pollution)
• just because you open the windows doesn’t mean they’ll ventilate your home (they may ventilate a room, but not necessarily your whole home)
• your air conditioning does not manage fresh air in your home (and it’s not an air purifier)

The CSIRO published research in 2024 that found the average new Australian home was at approximately 15.4 air changes per hour at 50 pascals of pressure. The range of air tightness in Aussie homes tested went up to 38ACH, which is basically a glorified tent. 

The NatHERS energy rating system assumes 10ACH as part of assessing the energy efficiency of a home, so two thirds of the homes tested weren’t compliant with that assumption.

By contrast, a high-performing NON-Passive House home might target 3 to 5 air changes per hour. HOME Method members regularly achieve this or lower, purely through how they’ve navigated things with their team. Passive House itself has a certified standard of 0.6ACH, which is a very tight envelope where air movement is almost entirely controlled.

The general guidance is that at or below approximately 5ACH, relying on incidental leakage and intermittent window opening is no longer adequate to reliably maintain good indoor air quality. 

At that point, the recommendation is that your project should ideally include mechanical ventilation. 

A well-designed mechanical ventilation system actually gives you more control over the quality of the air your family breathes, not less.

I find the resistance to building in an airtight way really interesting. I hear it packaged up in lots of different ways. I want to reiterate that it can be done on conventional projects, it has so many benefits, and it doesn’t stop you from being able to open a window. AND, opening a window does waste or undo the investment in your home’s air tightness either. 

Because a building can never be too air tight, but it can be under-ventilated.

MVHR is Mechanical Ventilation with Heat Recovery. It supplies fresh, filtered air into the building and simultaneously extracts damp, stale or contaminated air, without the energy, comfort and noise issues that can arise with open windows. 

In well-designed systems, heat recovery efficiency is typically 80 to 90%, meaning most of the warmth or coolness in the outgoing air is transferred to the incoming air. 

In warm or mild climates, MVHR systems primarily provide continuous fresh air filtration, control indoor humidity, and can use summer bypass modes to flush out excess heat on cool nights without needing heavy air conditioning.

In humid climates, there’s a variant called Energy Recovery Ventilation (ERV) that also transfers moisture between the two airstreams.

In this Episode, I cover:

• Where the phrase ‘build tight, ventilate right’ came from, and why it isn’t owned by the Passive House movement
• Why the desire to have your home ‘breathe’ often gets confused with uncontrolled air leakage, and the problems that creates
• What ‘building tight’ actually means in practice, including the continuous air barrier and how it can be achieved with conventional construction
• Why relying on opening windows alone often isn’t enough as a ventilation strategy, and the common objections I hear about air tightness
• At what point ‘ventilating right’ may mean the inclusions of a mechanical ventilation system to maintain your indoor air quality.
• What Mechanical Ventilation with Heat Recovery (MVHR) and Energy Recovery Ventilation (ERV) are, and how they work in a residential context
• The difference between ducted and decentralised MVHR systems, and what to design in early
• The conversations to have with your designer, builder and HVAC consultant so air tightness and ventilation are designed in from the start

Plus a whole lot more.

LISTEN TO THE EPISODE NOW.

RESOURCES:

Episode 296 with Jessica Allen, Climasure [NCC Series] >>> Air Tightness and Blower Door Tests

Episode 297 with Jessica Allen, Climasure [NCC Series] >>> What’s a WUFI Analysis?

Episode 234 with James Cornell, Life Panels >>> Building Healthier Homes

Episode 235 with James Cornell, Life Panels >>> Moisture Control in Your Home’s Construction

Episode 236 with James Cornell, Life Panels >>> Home Insulation and Wood Fibre Products

Episode 291 with Jesse Clarke, Pro Clima >>> Building Science 101

Episode 292 with Jesse Clarke, Pro Clima >>> Building Membranes + Condensation Management

Renew’s guide to choosing an MVHR system >>> MVHR buyers guide

Episode 409 >>> 44 Ways #09: Improve Your Indoor Air Quality

Episode 417 >>> 44 Ways #17: Get a WUFI Analysis Done on Your Floors, Walls and Roof

A free guide to simplifying sustainability across research, planning, design, materials, and building, from Amelia Lee’s 30+ years in residential design and construction >>> 44 Ways to Create a Sustainable Home

Access the support and guidance you need to be confident and empowered when renovating and building your family home inside my signature online program >>> HOME Method