Yes, it is possible to insulate internal walls without removing the plasterboard, and many NZ homeowners do exactly this. The most practical methods involve drilling small holes and either blowing in loose-fill insulation or injecting foam — no full wall strip-out required. That said, there are real limitations, safety considerations, and situations where a professional is the smarter call.
Why Insulate Internal Walls?
Most people focus on ceiling and underfloor insulation first, and rightly so — those areas offer the biggest thermal gains. However, internal wall insulation can meaningfully reduce noise transfer between rooms (think bedrooms next to living areas or bathrooms) and help maintain comfortable temperatures in individual rooms. It can also contribute to meeting the Healthy Homes Standards if you're a landlord.
Methods That Don't Require Removing Plasterboard
1. Blown-In Loose-Fill Insulation
This is the most common retrofit method. Small holes (typically 50–65 mm diameter) are drilled into the wall cavity — either through the plasterboard from inside, or through the exterior cladding from outside. Loose-fill insulation (usually polyester fibre or glasswool) is then blown in using a hose and machine. The holes are patched and repainted afterwards.
Key points:
- Works best in stud-framed walls with clear, unobstructed cavities
- Requires a blowing machine, which can be hired from some tool hire outlets
- Patching and repainting the holes adds extra steps
- Results can be uneven if there are noggins, blocking, or other obstructions inside the cavity
2. Injection Foam Insulation
Two-part injection foam (such as products from Sika or similar manufacturers) can be injected through small drilled holes. The foam expands to fill the cavity. This method is often used by professional installers and can be trickier for DIYers because overfilling can bow or crack the plasterboard.
Key points:
- Better at filling irregular cavities than loose-fill
- Risk of damage if over-injected — professional application is strongly recommended
- Some products have off-gassing considerations; always follow manufacturer safety data sheets
3. What About Spray Foam from the Outside?
If you have accessible wall cavities from the outside (e.g., removing a section of weatherboard or cavity batten), a contractor can spray polyurethane foam directly. This is generally not a DIY job and may affect your cladding warranty.
Can a Homeowner Do This Themselves?
For blown-in loose-fill insulation, a capable DIYer can tackle this job, provided:
- You are confident identifying wall framing, noggins, and any services (pipes, wiring) inside the wall
- You take care not to drill into electrical cables or plumbing — this is a serious safety risk
- You use appropriate PPE (dust mask rated for fibres, safety glasses, gloves)
- You patch and make good the holes properly afterwards
Before drilling, use a stud finder and a cable/pipe detector to check for hidden services. WorkSafe NZ has guidance on working safely around hidden hazards in walls.
Electrical and Plumbing — Restricted Work Warning
This is critical: do not drill into or disturb electrical wiring or plumbing. In New Zealand, any work on electrical wiring is restricted electrical work and must be carried out by a registered electrician licensed under the Electrical Workers Registration Board (EWRB). Similarly, plumbing work is restricted under the Plumbers, Gasfitters and Drainlayers Board (PGDB). If you accidentally damage wiring or pipes, stop work and call a licensed tradesperson immediately.
Does This Require a Building Consent?
In most cases, adding insulation to existing internal walls is exempt from building consent under Schedule 1 of the Building Act — but this depends on the scope of work and your specific situation. You should confirm with your local council before starting, particularly if the work involves any structural changes or affects fire-rated walls (e.g., between a garage and living space). MBIE Building Performance has useful guidance on what work requires consent.
Choosing the Right Insulation Product
For blown-in applications, look for products with a suitable R-value for your climate zone. NZ is divided into climate zones, and the BRANZ website has helpful guidance on recommended R-values. Common NZ products include glasswool and polyester loose-fill options available from suppliers like Bunnings or Mitre 10.
| Method | DIY Friendly? | Disruption | Best For |
|---|---|---|---|
| Blown-in loose-fill | Yes, with care | Low | Standard stud walls |
| Injection foam | Not recommended | Low | Irregular cavities |
| Strip and batt | Yes | High | Full renovations |
Limitations to Be Aware Of
- Obstructions: Noggins, blocking, and fire stops can prevent insulation from filling the full cavity. You may end up with gaps.
- Existing services: Pipes and cables take up space and create risk.
- Plasterboard condition: Older or damaged GIB board may not patch up neatly. GIB has patching guides on their website.
- Moisture: Internal walls generally don't need a vapour barrier, but check if the wall has any moisture issues before sealing it up.
When to Call a Professional
Hire a qualified insulation installer if:
- You're unsure about what's inside your walls (services, fire-rated construction)
- The walls have complex framing or many obstructions
- You want injection foam — over-injection by an inexperienced person can cause real damage
- The property is a rental and you need to meet Healthy Homes compliance
- You want a warranty on the work
A professional installer will have the right equipment, experience reading wall cavities, and can patch and finish the holes properly. You can find qualified local insulation installers on tradedirectory.co.nz.
