Installing a vapour barrier under foil underfloor insulation is a straightforward DIY job that makes a real difference to your home's warmth and long-term health. By stopping ground moisture from rising into your subfloor space, you protect your timber framing from rot and mould, and help your insulation perform as it should. Here's how to do it properly to meet NZ Building Code requirements.
Why a Vapour Barrier Matters
New Zealand's climate — especially in the South Island and wetter North Island regions — means subfloor spaces can be persistently damp. Ground moisture rises as vapour and condenses on cold timber, leading to rot, mould, and reduced insulation effectiveness. The NZ Building Code clause E3 (Internal Moisture) and clause B2 (Durability) both require that subfloor spaces be protected from moisture. A polythene vapour barrier laid on the ground is the standard solution, and it's required under most foil underfloor insulation installations to comply with these clauses.
BRANZ research consistently shows that homes without a ground vapour barrier have significantly higher subfloor moisture levels, which shortens the life of both the structure and the insulation.
What You'll Need
- Polythene film — minimum 0.25 mm (250 micron) thick; heavier (0.3–0.4 mm) is better for durability underfoot
- Foil underfloor insulation batts or segments (sized to your joist spacing)
- Insulation support wires or rods (to hold foil batts between joists)
- Tape rated for polythene joins (ask your supplier for a compatible product)
- Utility knife or scissors
- Tape measure and marker
- Knee pads and a head torch
- Dust mask (P2 rated) and gloves
Polythene film and foil insulation are widely available at Bunnings, Mitre 10, and PlaceMakers.
Step-by-Step Installation
1. Check Your Subfloor Access and Conditions
Before you start, confirm you have safe access under the house — typically via an access hatch at least 500 mm × 500 mm. Check that the subfloor height is workable (minimum 400 mm clearance is required under NZS 3604 for timber-framed houses). Look for any existing moisture problems, standing water, or signs of rot. If you find serious moisture issues or structural damage, stop and get a professional assessment before proceeding.
2. Clear the Ground
Remove any debris, rocks, or sharp objects from the ground surface. These can puncture the polythene over time. If the soil is very uneven, rake it roughly flat.
3. Lay the Polythene Vapour Barrier
Start at the far end of the subfloor and work towards the access point so you're not crawling back over laid film.
- Unroll the polythene across the ground, running it up the inside face of the foundation walls or piles by at least 150 mm on all sides.
- Overlap joins by a minimum of 200 mm (300 mm is better). Tape all joins with polythene-compatible tape to create a continuous seal.
- Cut around piles, pipes, and other penetrations as neatly as possible, and tape the polythene snugly around them.
- The barrier should cover the entire ground area with no gaps.
4. Install the Foil Underfloor Insulation
Once the vapour barrier is down, install your foil insulation batts between the floor joists:
- Cut batts to fit snugly — they should be slightly wider than the joist spacing so they grip without gaps at the edges.
- The foil face should face upward (towards the underside of the floor), where it reflects radiant heat back into the living space.
- Secure batts with support wires or rods clipped or tensioned between joists. Space these roughly every 500–600 mm along the batt to prevent sagging.
- Ensure batts butt tightly together end-to-end with no gaps.
5. Check Subfloor Ventilation
A vapour barrier alone isn't enough — your subfloor also needs adequate cross-ventilation to carry away any residual moisture. The NZ Building Code requires ventilation openings in foundation walls. Check that existing vents are unblocked. If ventilation is inadequate, consult your local council or a professional before sealing everything up.
Does This Work Need a Building Consent?
In most cases, installing underfloor insulation and a vapour barrier is exempt from building consent under Schedule 1 of the Building Act 2004, as it is maintenance and repair work that doesn't affect the structure. However, rules can vary, so always confirm with your local council before starting. You can find guidance at MBIE Building Performance.
Safety Notes
- Working in a confined subfloor space carries real risks. Never work alone — have someone nearby who knows where you are.
- Wear a P2 dust mask, gloves, and eye protection. Old subfloor spaces can harbour dust, allergens, and rodent droppings.
- Check for any asbestos-containing materials (common in homes built before the mid-1980s) before disturbing anything. If in doubt, contact WorkSafe NZ for guidance on asbestos identification and safe handling.
- Do not cover or disturb any electrical wiring, gas pipes, or plumbing without professional advice.
When to Call a Professional
Call a qualified insulation installer or builder if:
- You find rot, structural damage, or persistent standing water in the subfloor.
- The subfloor is too confined or difficult to access safely.
- You're unsure about asbestos, electrical wiring, or plumbing in the subfloor space.
- Your home is a rental — landlords must meet Healthy Homes Standards for underfloor insulation, and professional installation provides a compliance record.
A qualified local insulation installer can be found right here on tradedirectory.co.nz.
