Wondering how to choose the right building wrap for your walls and roof so moisture is properly managed?

Discover why foil sarking is to be avoided in modern airtight, well-insulated homes, and what vapour permeable membranes do differently.

Also, learn what the four classes of vapour control membrane mean, and what to specify for your climate zone.

Listen to the episode now.

Hello! This is Episode 422, and Way #22 of the 44 Ways to Create Your Sustainable Home series here on the podcast. We’re continuing through Section Five: The Building Envelope.

In the last two episodes, we covered thermal mass and window selection. Before that, in Way #19, we talked about what building tight actually means. This episode is about a layer that sits within the wall and roof assembly of your home: the building wrap.

Way #22 is: Review Building Wraps to Manage Moisture for Climate.

Historically, especially in Australia, we’ve seen foil sarking act as this layer. 

However, with learnings over time about how this has detrimentally affected the performance of the building envelope, and trapped moisture inside the wall, bringing issues such as mould and rot, better products are now available to do this job. 

Yet the industry isn’t up to speed, and in some cases, is still installing the wrong building membranes, and using the wrong detailing.

When we talk about ‘building wraps’, we’re referring to a layer that gets added within your wall or roof assembly to manage the movement of air and moisture, and the condensation that may form. And using the right building wrap is about the climate you’re in, and how your building is constructed.

A building wrap is a membrane installed in the wall and roof assembly to manage both liquid water and water vapour. 

• In walls, it typically sits on the outside face of the wall frame, between the frame and the cladding. 
• In roofs, it’s usually under the roofing material over the rafters. 

Building wraps ideally stop liquid water from reaching the timber frame, and manage the movement of water vapour through the assembly. And they can also act as an air barrier.

As DuPont’s Maria Spinu writes in the White Paper ‘Vapor Permeable or Impermeable Building Envelope Materials, Does It Matter?’: 

‘Building assemblies may periodically get wet, or start out wet, yet can have an acceptable performance and can provide a long, useful service life, if allowed to dry. Problems only occur when buildings get wet and stay wet long enough under adverse conditions for materials to deteriorate. It is therefore important to understand that proper moisture management in the building enclosure must consider the balance of wetting versus drying.’

For most of Australian residential construction history, the default building wrap was foil sarking. Reflective foil laminate, often called RFL or sarking, became standard practice from around the 1960s and 70s, installed under metal roofing and on the outside face of wall frames before cladding was applied. 

Foil sarking as a material is vapour impermeable: 

It does not allow water vapour to pass through it. And because it was tearing off frames when subjected to windy conditions, it was supplied with holes already punched in it, so once enclosed in a wall, that allowed uncontrolled movement of air and vapour in both directions through the building assembly.

Think about what happens when you put foil over warm food. The steam condenses on the underside of the foil and drips back down. That same process can happen in your wall when warm, moisture-laden air meets a foil wrap installed on the outside of the frame. Condensation forms on the inner face of the foil, wets the insulation and timber framing behind it, and in a wall with no pathway to dry outward, can accumulate season after season until the conditions for mould are met inside the cavity, where you can’t see it.

There is an assumption that underpins a lot of building wrap decisions: 

That the cladding is the waterproof layer, and the wrap behind it is just there as backup. This is incorrect. Most cladding or external wall products allow water to reach the building wrap under the right conditions.

A building wrap, when specified and installed correctly, provides a layer of weather security that supports the longevity of the building.

There are 4 Classes of Vapour Control Membrane, set to AS/NZS 4200.1. In a detailed conversation with Jesse Clarke from Pro Clima in Episode 292, Jesse recommended that the best approach is to specify a Class 4 membrane.

Also, if you’re renovating, or planning a flat roof for your future home, there’s specific things to consider with your building membrane, which I share in more detail in this podcast.

In this Episode, I cover:

• Where building wraps fit in the moisture story, and how they relate to airtightness, ventilation and the WUFI analysis from Way #17
• What a building wrap actually is, and the difference between vapour impermeable and vapour permeable membranes
• Why foil sarking made sense in leaky homes, and why it’s become a problem in modern, better-insulated airtight construction
• Why the assumption that your cladding is waterproof is incorrect (brick veneer, weatherboards, fibre cement)
• How climate zone shapes moisture drive, and why cool, hot-humid and mixed climates each need a different approach
• The 4 Classes of Vapour Control Membrane in Australia (AS/NZS 4200.1), and why Class 4 is generally the preferred choice
• The role of external vs internal membranes, and where each fits, including Passive House
• Why flat roofs (10 degrees or shallower) need particular attention with building membranes, detailing and ventilation
• What retrofitting a building wrap looks like in a renovation
• The questions to ask your designer and builder so your building wrap is appropriately selected and correctly installed for your climate

Plus a whole lot more.

LISTEN TO THE EPISODE NOW.

RESOURCES:

Episode 291 with Jesse Clarke, Pro Clima [NCC Series] >>> Building Science 101

Episode 292 with Jesse Clarke, Pro Clima [NCC Series] >>> Building Membranes + Condensation Management

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

Episode 419 >>> 44 Ways #19: Build Tight, Ventilate Right

Retrofitting a building membrane to an existing wall frame from the inside, when renovating >>> Cooee Architecture Instagram reel

Free download on airtightness and moisture management in Australian housing >>> Pro Clima Australia Study

ABCB’s guide to managing condensation in buildings >>> Condensation in Buildings Handbook

ABCB’s webinar on condensation management under NCC 2022 >>> NCC 2022 Condensation Management webinar

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