Expanding foam can be used to seal certain gaps around drain pipes, but whether it's the right product depends on where the gap is, what it's next to, and what you're trying to achieve. In some situations it works well; in others it can cause problems or fail to meet NZ Building Code requirements. Read on to understand the options and when you should call a licensed drainlayer.
Why Sealing Gaps Around Drain Pipes Matters
Gaps around drain pipes where they pass through walls, floors, or foundations aren't just a cosmetic issue. They can allow:
- Moisture and water ingress, leading to rot, mould, and structural damage
- Vermin and insects to enter the building cavity
- Draughts and heat loss, which matters under the Healthy Homes standards for rental properties
- Fire and smoke to spread between building elements if the pipe passes through a fire-rated assembly
The NZ Building Code — particularly clauses E2 (External Moisture), E3 (Internal Moisture), and C (Fire Safety) — sets out requirements that affect how pipe penetrations must be treated.
When Expanding Foam Is Suitable
Polyurethane expanding foam (the kind sold at Bunnings or Mitre 10) is a practical gap-filler in certain situations:
- Sealing around pipes passing through internal timber-framed walls (non-fire-rated) to block draughts and pests
- Filling gaps around pipes in subfloor areas to reduce cold air infiltration — useful for meeting Healthy Homes draught-stopping requirements
- Backing for a finishing sealant, where foam fills the bulk of a large gap and a compatible flexible sealant is applied over the top
For these uses, a low-expansion foam is generally preferred around pipes, as high-expansion foam can exert pressure on lightweight pipes or fittings.
When Expanding Foam Is NOT Suitable
There are several situations where expanding foam is the wrong choice:
Fire-Rated Penetrations
If a drain pipe passes through a fire-rated wall or floor (common between a garage and living space, or in multi-unit buildings), standard expanding foam provides no fire resistance. You must use a fire-rated intumescent sealant or collar that is tested and certified for that purpose. Using standard foam here is a Building Code breach. Check with your local council or a Licensed Building Practitioner if you're unsure whether a wall or floor is fire-rated.
Wet Areas and Waterproofing
In bathrooms, laundries, and kitchens, gaps around drain pipes at floor level need to be properly waterproofed in line with Building Code clause E3. Expanding foam is not a waterproofing membrane — it absorbs moisture over time and will degrade. A flexible, waterproof sealant (such as a sanitary silicone or a product from Sika or Ardex) is the correct solution here, applied as part of a proper wet-area waterproofing system.
Underground or Buried Pipes
Expanding foam is not rated for continuous ground contact or submersion. For gaps where drain pipes exit a foundation wall below ground, you need a hydraulic cement, purpose-made pipe penetration seal, or flexible boot seal designed for that environment.
Flexible or Push-Fit Pipe Joints
Foam can lock pipes in place and prevent the slight movement that PVC drain pipes need for thermal expansion. Avoid foaming directly around push-fit joints or flexible couplings.
What Product Should You Use Instead?
Here's a quick guide to matching the right product to the situation:
| Situation | Recommended Sealant |
|---|---|
| Internal wall/floor, non-fire-rated | Low-expansion foam or flexible sealant |
| Wet area (bathroom, laundry) | Waterproof flexible sealant / silicone |
| Fire-rated wall or floor | Intumescent fire collar or rated sealant |
| External wall penetration | Flexible weatherproof sealant + backing rod |
| Below-ground foundation | Hydraulic cement or purpose-made boot seal |
Always check the manufacturer's datasheet to confirm a product is suitable for your specific application. BRANZ publishes guidance on weathertight detailing that can help you understand what's required around penetrations.
DIY vs. Restricted Work
Sealing a gap around a drain pipe with foam or sealant is generally not restricted work under the Plumbers, Gasfitters and Drainlayers Act 2006 — a homeowner can do it themselves in most cases.
However, any work on the drain pipe itself — cutting, joining, relocating, or repairing drain pipework — is restricted drainlaying work and must be carried out by a licensed drainlayer. This includes:
- Replacing or repositioning a drain pipe
- Cutting into existing drain lines
- Installing new waste or drain connections
If you're not sure where the line is between sealing a gap and working on the pipe, err on the side of caution and get a professional involved.
Safety note: When working in subfloor or confined spaces, ensure adequate ventilation. Expanding foam fumes can be irritating. Wear gloves and eye protection — cured foam is very difficult to remove from skin and clothing. Check WorkSafe NZ guidance on working safely in confined spaces.
When to Call a Professional
Call a licensed drainlayer if:
- The gap has appeared because a pipe has moved, cracked, or failed
- You need to cut or modify the pipe to seal the penetration properly
- The pipe passes through a fire-rated assembly and you're unsure of the correct product
- Water is actively leaking around the pipe penetration
- The gap is in a wet area that requires a compliant waterproofing system
A qualified drainlayer can assess the situation, carry out any restricted work legally, and ensure the result meets the NZ Building Code. You can find licensed local drainlayers on tradedirectory.co.nz.