Expanding foam is a tempting, easy-to-use product for sealing gaps around pipes through walls — and in some situations it works perfectly well. However, in New Zealand there are important building code requirements, particularly around fire rating and weathertightness, that mean expanding foam isn't always the right or legal choice. Here's what you need to know before you reach for that can.
Why Sealing Pipe Penetrations Matters
Any time a pipe passes through a wall, floor, or ceiling, it creates a gap that can allow:
- Draughts and heat loss, affecting your home's energy efficiency
- Moisture and pests entering the building envelope
- Fire and smoke spreading between rooms or storeys — a serious life-safety issue
- Non-compliance with the NZ Building Code, which could affect your insurance or future sale
The NZ Building Code sets out requirements for fire separation (Clause C), weathertightness (Clause E2), and moisture control (Clause E3) that directly affect how pipe penetrations must be treated.
When Expanding Foam Is Acceptable
Standard polyurethane expanding foam (the kind you buy at Bunnings or Mitre 10) is generally suitable for:
- Non-fire-rated walls — for example, sealing a pipe through an exterior timber-framed wall where there is no fire-separation requirement
- Weathertightness and draught sealing — filling gaps around pipes penetrating the building envelope to keep out wind, rain, and insects
- Subfloor and roof space penetrations — where fire rating is not required
In these situations, expanding foam provides a good airtight and water-resistant seal once cured. Choose a product labelled as suitable for plumbing penetrations, and check the manufacturer's data sheet to confirm compatibility with the pipe material (some foams can be aggressive on certain plastics over time).
When Expanding Foam Is NOT Acceptable
This is the critical part. Standard expanding foam has no fire-resistance rating and must not be used where the Building Code requires fire separation. This includes:
- Pipes passing through fire walls or fire-rated floors between separate occupancies (e.g., a house and an attached garage, or a multi-unit dwelling)
- Penetrations in fire-rated assemblies required under NZ Building Code Clause C
- Any wall or floor that forms part of a specified fire-resistance rating (FRR)
In these locations you must use a fire-rated penetration seal — sometimes called a firestop or intumescent collar/wrap. These products are specifically tested and certified to maintain the fire rating of the wall or floor assembly. Brands such as Sika offer intumescent pipe collars and fire-rated sealants suitable for NZ conditions. Always check that the product is tested to the relevant standard and is appropriate for the pipe material (plastic pipes require intumescent collars that expand to close the pipe as it melts in a fire).
Weathertightness and Exterior Walls
For pipes exiting through exterior cladding — such as a garden tap or waste pipe through a James Hardie fibre cement wall — expanding foam alone is generally not sufficient for long-term weathertightness. You should also use:
- A pipe flashing or escutcheon plate to cover the gap at the cladding face
- A flexible weatherproof sealant (such as a polyurethane or silicone product) around the pipe at the exterior face
- Foam behind the cladding to fill the cavity gap if needed
Refer to James Hardie installation guides for specific details on sealing penetrations in their cladding systems, as incorrect sealing can void warranties and breach E2 requirements.
Healthy Homes and Rental Properties
If you're a landlord, note that the Healthy Homes Standards require rental properties to be draught-stopped, including sealing gaps around pipes. Expanding foam is a practical solution for this purpose in non-fire-rated locations, helping improve warmth and energy efficiency. Energywise also has useful guidance on draught-sealing your home.
Practical Tips for Using Expanding Foam
- Wear gloves and eye protection — cured foam is very difficult to remove from skin and clothing
- Apply foam in thin layers; it expands significantly and can distort lightweight wall linings if over-applied
- Allow full cure time before trimming (check the product label — typically several hours)
- Use low-expansion foam around pipes where you don't want excessive pressure on the pipe or surrounding framing
- Trim flush with a sharp knife once cured, then paint or seal over if needed for aesthetics
Building Consent Considerations
Most routine pipe penetration sealing is maintenance work and does not require a building consent. However, if you are doing new plumbing work that involves penetrating fire-rated walls or floors, the plumbing work itself may require consent and must be carried out by a licensed tradesperson. Always check with your local council if you're unsure — consent requirements vary. You can find more guidance at MBIE Building Performance.
When to Call a Professional
Call a licensed plumber (registered with the Plumbers, Gasfitters and Drainlayers Board) if:
- The pipe penetration involves sanitary plumbing, gas, or drainlaying — this is restricted work under the Plumbers, Gasfitters, and Drainlayers Act and must be done by a licensed tradesperson
- You are unsure whether the wall or floor is fire-rated
- The penetration is in a multi-unit building or commercial property
- You need a Code Compliance Certificate for the work
A qualified plumber will know the correct firestopping products and installation methods required. You can find licensed local plumbers on tradedirectory.co.nz.
Master Plumbers
