Yes, you can add insulation to a concrete block wall yourself in many cases — but the method you choose, the location of the wall, and whether a building consent is required will all affect how far you can go as a DIYer. Read on for a practical NZ guide.
Understanding Monolithic Concrete Block Walls
Monolithic concrete block (also called concrete masonry or CMU) walls are common in New Zealand homes built from the 1950s through to the 1980s, as well as in garages, sleepouts, and commercial-to-residential conversions. Concrete block has very low thermal resistance (R-value), meaning heat escapes quickly in winter and the wall can feel cold and damp — a real issue in NZ's varied climate, from Southland winters to humid Auckland summers.
Adding insulation improves comfort, reduces energy bills, and helps meet the spirit of the Healthy Homes Standards if you're a landlord. It can also help your home perform better against the NZ Building Code's requirements around thermal performance (Clause H1).
Do You Need a Building Consent?
This is the first question to answer before you pick up a tool. Under Schedule 1 of the Building Act 2004, some insulation work is exempt from building consent — but the exemptions have conditions. Generally, adding insulation to existing walls without altering the structure or weathertightness is likely exempt, but you must confirm this with your local council before starting. Don't assume.
If you're adding a new internal framed wall (a common method — see below), that framing work may also be exempt under Schedule 1, but again, check with your council. You can find guidance on consents and exemptions at MBIE Building Performance.
Main Methods for Insulating a Concrete Block Wall
1. Internal Framed Wall (Most Common DIY Method)
The most practical approach for most homeowners is to build a timber stud wall (typically 70 mm or 90 mm framing) inside the concrete block wall, leaving a small gap or sitting against it, then filling the stud cavities with bulk insulation batts (glass wool or polyester). The wall is then lined with GIB plasterboard or another suitable lining.
Pros: Good R-values achievable, familiar materials, DIY-friendly framing and insulation installation. Cons: You lose some floor area; moisture management is important — a vapour control layer may be needed on the warm side of the insulation in colder regions. Check with BRANZ for guidance on vapour barriers in your climate zone.
2. Rigid Foam Board (Glued or Mechanically Fixed)
Rigid polystyrene (EPS/XPS) or polyisocyanurate (PIR) boards can be glued or fixed directly to the interior face of the block wall, then lined. Products like Sika adhesives or compatible construction adhesives are commonly used.
Pros: Slim profile, no framing needed, reasonable R-values. Cons: Lower R-values per dollar than a framed wall; the foam must be covered with a fire-rated lining (e.g., GIB plasterboard) — exposed foam is a fire hazard and not code-compliant as a finished surface.
3. Injected or Blown-In Insulation
If the block wall has hollow cores, a professional installer can inject loose-fill or foam insulation into the cores. This is generally not a DIY job — it requires specialist equipment and knowledge of the block type.
4. External Insulation (EIFS / Cladding Systems)
External insulation and finish systems (EIFS) are applied to the outside of the wall. This is a higher-cost, higher-complexity option that almost always requires a building consent and professional installation. It also affects the exterior appearance of your home.
Comparing Common DIY Methods
| Method | Typical R-value | DIY Friendly? | Consent Likely Needed? |
|---|---|---|---|
| Internal framed wall + batts | R1.8–R2.8+ | Yes | Check with council |
| Rigid foam board (glued) | R1.0–R2.0 | Yes | Check with council |
| Injected core fill | Varies | No | Check with council |
| External EIFS | R1.5–R3.0+ | No | Usually yes |
R-values are approximate and depend on product thickness and type. Always check product data sheets.
Moisture: The Critical Factor in NZ
Concrete block can wick moisture, and poorly detailed insulation can trap it, leading to mould — a serious health issue. Before insulating:
- Check for existing dampness or rising moisture in the wall.
- Ensure any subfloor ventilation and exterior drainage is adequate.
- Consider a dimple mat or drainage membrane between the block and insulation if moisture is present.
- In colder South Island climates especially, get advice on vapour control from BRANZ or a qualified insulation installer.
Safety Notes
- Wear gloves, safety glasses, and a dust mask (P2 rated) when handling glass wool batts.
- If your home was built before 1990, check for asbestos in any existing linings before disturbing them. If in doubt, get a sample tested by a licensed asbestos assessor. See WorkSafe NZ for guidance.
- Any electrical work (moving power points, lighting circuits) inside the new wall is restricted electrical work and must be done by a registered electrician. See EWRB.
When to Call a Professional
Call a qualified insulation installer or Licensed Building Practitioner if:
- You're unsure about moisture or dampness in the wall.
- You want injected core-fill insulation.
- The project involves structural changes or a building consent.
- You're a landlord needing to meet Healthy Homes compliance and want documented proof of installation.
- You're not confident with framing or lining work.
You can find experienced local insulation installers and builders on tradedirectory.co.nz.
