Do You Really Need Underfloor Insulation?

For most Kiwi homes with suspended timber floors, underfloor insulation is an effective way to improve warmth and reduce energy costs. However, not all suspended floors are uninsulated. To check whether yours needs it, look underneath your home—if you can see the bare timber floorboards from below, it likely means there’s no insulation in place.

Uninsulated floors can account for up to 15% of total heat loss in a typical home. Over time, this contributes to higher power bills and colder indoor temperatures, especially during winter.

Here's a quick self-assessment:

• Can you access under your house? Check what's there

• Feel cold drafts through floorboards? You need insulation

• High winter power bills? Uninsulated floors are often the culprit

• Musty smell in older homes? Missing insulation creates moisture problems

How to Choose the Right R-Value for Underfloor Insulation

All insulation products are measured by R-values, which indicate how well a material resists heat flow. The higher the R-value, the better your insulation performs—helping retain warmth in winter and keep heat out in summer.

For underfloor insulation, the minimum required R-value is R1.3 for rental homes under the Healthy Homes Standards. However, this is a baseline, and higher values are strongly recommended for better performance.

Recommended R-Values for Existing Homes:

• R1.8–R2.0 for typical retrofits

• R2.6–R3.2 for colder climates or energy-efficient upgrades

• Underfloor insulation should always be paired with ceiling insulation to maximise energy efficiency

New Builds and Renovations:
If you’re building or renovating, the New Zealand Building Code (H1 Clause) sets mandatory R-value requirements by climate zone. These apply to new floors, walls, and ceilings:

Minimum Wall R-Values by Climate Zone:

• Zone 1 (Auckland, Northland): R1.9 minimum

• Zone 2 (Wellington, Nelson, Taranaki): R1.9 minimum

• Zone 3 (Canterbury, Otago, Southland): R2.0 minimum

These requirements represent the minimum acceptable performance. Many homeowners choose to go beyond these standards for long-term energy savings, especially when considering future-proofing or high-performance building design.

To know more about how R-values work and what they mean in practical terms, you can check out our resource page.

Types of Underfloor Insulation

Like ceiling insulation the types of product and methods of installation are controlled. We install two types of underfloor insulation: polyester and polystyrene.

Polyester

Polyester insulation has developed into an extremely popular underfloor option. We use the Greenstuf and Mammoth brands which have been designed in NZ for new and older NZ homes. Installed between the joists hard up against the floor, the product is stapled into the sides of the joists so as not to compress it. With a 50 year durability rating the product will last the life of the home.

Polystyrene

We supply the Chillguard brand of polystyrene. Polsytyrene is a good trusted insulation used in many items such as chilly bins and coffee cups.

Produced in 1.2m long sheets the edges are concertinaed to allow them to be compressed between floor joists to sit hard up against your floor.

The difficulty with polystyrene is the cost to install. Whilst the material cost is similar to polyester the actual labor to install it is three times as much. Being a rigid material each piece needs to be cut to size. Generally it adds approx $750 to the cost of underfloor insulation into an average home over polyester.