Wondering if the walls, roof and floors of your future home will actually manage moisture safely long-term?

Discover what a WUFI analysis tells you about condensation and mould risk inside your building envelope, before construction.

Learn the questions to ask your designer, architect or builder to make sure your airtight, well-insulated home doesn’t create hidden moisture problems down the track.

Listen to the episode now.

Hello! This is Episode 417, and Way #17 of the ‘44 Ways to Create Your Sustainable Home’ series here on the podcast. We’re continuing through Section Four: Modelling and Measuring Your Sustainability.

Building a more airtight, better insulated home is the direction you want to go for performance and comfort. 

And it is. 

But there can be a negative consequence of building this way that needs to be managed. When you seal a building up tightly, and you insulate it well, moisture behaves differently than it does in a leaky home.

Way #17 is: Get a WUFI Analysis Done on Your Floors, Walls and Roof.

If you’ve never heard the word ‘WUFI’ before, this episode will introduce you to a tool that most of the residential building industry isn’t routinely using or talking about, but that is increasingly important from a risk-management perspective as we build more airtight and better insulated homes.

For most of the history of residential construction in Australia and in many other parts of the world, buildings were fairly leaky. Air moved in and out through gaps around windows, through joins in the building fabric, through vents in the wall, eaves and soffits. 

That air movement wasn’t great for energy efficiency, but it acted as an accidental safety valve for moisture. Any moisture that got into a wall or roof could usually find its way out again. 

As we’ve improved airtightness and insulation, that accidental safety valve has been reduced. And materials like foil sarking which has been used for decades in Australia, are impermeable to vapour, which can trap moisture inside modern wall assemblies.

When warm, moisture-laden air meets a cold surface inside your wall assembly, condensation can form, due to where the ‘dew point’ occurs. 

When that dew point occurs inside the layers of your building envelope, where it’s hidden from view, and condensation forms within the layers of your building envelope, it’s called interstitial condensation. 

If that moisture can’t dry out, it can wet insulation, timber framing, and the back of plasterboard, and grow mould over time, causing serious degradation of your building envelope, and issues for the indoor air of your home and your health.

As we have become more aware of the issues created by moisture in our building envelopes through this type of construction, many locations now legally require vapour permeable membranes to be used instead of foil sarking. These are membranes that keep your home weather resistant from the outside, but allow moisture vapour to pass through them from inside to outside. 

When coupled with a ventilated cavity on the outside between the membrane and cladding, any moisture moving from inside to outside has somewhere to dry out. And any moisture that makes it through your cladding can dry out in the cavity, whilst the vapour permeable membrane protects it from entering your home.

But whether your specific wall, roof and floor assemblies actually handle moisture safely for your climate is not something the building code alone can confirm. That’s what a WUFI analysis is for.

WUFI is hygrothermal analysis software, developed in Germany, that models the behaviour of both heat and moisture within your building envelope assemblies: specifically, your walls, your roof, and your floors. 

A WUFI analysis models your specific assembly using real climate data for your location, and simulates what happens to moisture inside those layers over a ten-year period. It also takes into account driving rain and solar radiation specific to the climate and weather systems of your location.

WUFI assesses mould growth risk by modelling the layers in a construction build-up and assessing where moisture may get trapped. WUFI ultimately helps you know whether your assembly passes before you build it, not after.

A WUFI analysis can be done entirely remotely. The consultant needs the detail of your floor, wall and roof construction, the drawings of your home, and the location of the project. They then model each layer in the WUFI software, and produce a report that tells you what the mould index is in the critical layer of your assembly. If the report produces problematic results, the consultant can help you understand required changes to the specification to improve its performance.

In this Episode, I cover:

• Why moisture has become a bigger issue in residential construction as homes have become more airtight and better insulated
• The change from foil sarking to vapour permeable membranes, and why ventilated cavities matter
• What ‘the dew point’ is, and why interstitial condensation is such a risk inside your home’s building floor, walls and roof assemblies
• What WUFI (hygrothermal analysis software) is, and how it models heat and moisture through your walls, roof and floors over a 10 year period
• How the mould index works, and what a ‘pass’ vs ‘fail’ result actually looks like
• What the WUFI analysis process involves in practice, and how to get one done for your project
• The questions to ask your designer, architect or builder about how they specify assemblies to manage moisture

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?

Software used for hygrothermal analysis >>> WUFI®

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