Building wrap and vapour barriers might look similar, but they are designed to do very different jobs — and using the wrong one under your floor insulation can lead to moisture problems, rot, and a failed building inspection. The short answer is: no, standard building wrap is not suitable as a ground vapour barrier under floor insulation, and the NZ Building Code is clear about what is required.
What Is the Difference Between Building Wrap and a Vapour Barrier?
Building wrap (also called sarking or breather membrane) is designed to be water-resistant but vapour-permeable. It lets water vapour pass through while blocking liquid water and wind. This breathability is exactly what you want on walls and roofs — but it is the opposite of what you need under a suspended timber floor.
A ground vapour barrier (sometimes called a ground moisture barrier) is designed to be vapour-impermeable — it stops moisture vapour rising from the soil into your subfloor space. Products used for this purpose are typically heavy-duty polyethylene film (polythene sheeting), usually at least 0.25 mm (250 microns) thick, or purpose-made ground vapour barrier membranes.
What Does the NZ Building Code Require?
Under NZ Building Code Clause E3 (Internal Moisture), buildings must be constructed to prevent moisture from causing damage or creating unhealthy conditions. For suspended timber floors over a subfloor crawl space, controlling ground moisture is a key requirement.
NZS 3604:2011 (Timber-framed buildings) provides the accepted solution for most residential construction. It specifies that where a ground vapour barrier is required, it should be a polyethylene film of at least 0.25 mm thickness, lapped at joints and turned up at edges. The barrier should cover the entire ground area of the subfloor space.
Whether a ground vapour barrier is required at all depends on factors like your subfloor ventilation, ground clearance, and local conditions — so always check with your local council or a qualified professional if you are unsure.
Why Using Building Wrap Is a Problem
Because building wrap is designed to let vapour through, laying it on the ground under your floor insulation will do very little to stop moisture rising from the soil. Over time, this can lead to:
- Condensation building up on the underside of your floor structure
- Timber rot in floor joists and bearers
- Mould growth in the subfloor space and potentially inside the home
- Insulation losing effectiveness as it absorbs moisture
- Failing a building inspection if consent is required for your work
For rental properties, landlords also have obligations under the Healthy Homes Standards to ensure underfloor insulation is installed correctly, which includes appropriate ground moisture control.
What Products Should You Use?
For a ground vapour barrier under floor insulation, look for:
- Heavy-duty polyethylene (polythene) sheeting — minimum 0.25 mm (250 microns), though many installers use 0.30 mm or thicker for durability during installation
- Purpose-made ground vapour barrier membranes — some products combine vapour resistance with additional durability or reflective properties
You can find suitable polythene sheeting and purpose-made vapour barrier products at Bunnings, Mitre 10, and PlaceMakers. Always check the product specifications confirm it is suitable for use as a ground vapour barrier — look for the thickness rating and any relevant compliance information on the packaging.
How to Install a Ground Vapour Barrier Correctly
If you are installing the barrier yourself as part of a DIY underfloor insulation project, follow these general guidelines (always defer to manufacturer instructions and NZS 3604 requirements):
- Cover the entire ground area — do not leave gaps or bare soil exposed
- Lap joins by at least 150–200 mm and tape them with a compatible tape to maintain continuity
- Turn the membrane up at the edges against foundation walls or piles, typically 150 mm or more
- Avoid puncturing the membrane with sharp objects or foot traffic during installation — work carefully
- Ensure adequate subfloor ventilation is also in place, as per NZS 3604 requirements
Do You Need a Building Consent?
In many cases, installing underfloor insulation in an existing home falls under Schedule 1 of the Building Act as exempt building work, meaning no consent is required. However, this depends on the specifics of your project. Always confirm with your local council before starting work — requirements can vary and the exemptions have conditions. You can find general guidance at MBIE Building Performance.
For research on moisture control and insulation performance in NZ conditions, BRANZ is an excellent resource with technical guidance tailored to our climate.
When to Call a Professional
While laying a ground vapour barrier and fitting underfloor insulation batts is generally DIY-friendly work (no restricted trades are involved), there are good reasons to hire a professional insulation installer:
- Subfloor spaces can be confined, awkward, and physically demanding to work in
- A professional will ensure the correct products are used and installed to meet the Building Code
- If your subfloor has existing moisture damage, rot, or mould, a tradesperson can assess the extent of the problem before insulation goes in
- For rental properties, professional installation provides documentation to help meet Healthy Homes compliance
You can find qualified local insulation installers on tradedirectory.co.nz.
