Yes, you can use rigid foam board insulation under a suspended timber floor, and it can be an excellent choice for many New Zealand homes. Rigid foam boards offer good thermal resistance in a relatively slim profile, resist moisture better than bulk fibre products, and stay in place without sagging over time. That said, there are important considerations around product type, R-value requirements, fixing methods, and compliance with the NZ Building Code and Healthy Homes Standards before you get started.
Why Choose Rigid Foam Board for Underfloor Insulation?
Suspended timber floors — common in older NZ homes built on piles — are notoriously draughty and cold. Insulating underneath them can make a significant difference to comfort and energy use. Rigid foam board has several advantages over traditional bulk insulation (such as polyester or glasswool batts) in this application:
- Moisture resistance: Many foam boards have low water absorption, which matters in damp subfloor environments.
- Dimensional stability: Boards don't sag or fall out the way batts sometimes can if not properly retained.
- Slim profile: Useful where joist depth is limited.
- Vermin resistance: Rigid boards are harder for rodents to nest in than soft batts.
Types of Rigid Foam Board Available in NZ
Three main types are commonly available at NZ suppliers such as Bunnings and Mitre 10:
| Type | Common Name | Typical R-value per 25 mm | Moisture Resistance |
|---|---|---|---|
| Expanded Polystyrene (EPS) | Polystyrene / Styrofoam | ~0.7 | Good |
| Extruded Polystyrene (XPS) | Foamboard / Styrofoam X | ~0.9 | Excellent |
| Polyisocyanurate (PIR/PUR) | Polyiso / Thermofoil | ~1.1–1.3 | Moderate |
For underfloor use in a damp subfloor space, XPS is often the preferred choice due to its closed-cell structure and superior moisture resistance. EPS is also widely used and more affordable. PIR boards offer the highest R-value per millimetre but can be more expensive and may be less tolerant of prolonged moisture exposure.
What R-Value Do You Need?
Under the Healthy Homes Standards, rental properties must meet minimum underfloor insulation R-values that vary by NZ climate zone (Zone 1, 2, or 3). For owner-occupied homes undergoing renovation, the NZ Building Code clause H1 sets energy efficiency requirements. As a general guide:
- Zone 1 (Northland, Auckland, coastal areas): R1.3 minimum (Healthy Homes)
- Zone 2 (most of the North Island, Nelson, coastal South Island): R1.3 minimum
- Zone 3 (inland and southern South Island, central plateau): R1.5 minimum
Always check the current requirements with MBIE Building Performance or your local council, as these figures can be updated. For best comfort and long-term energy savings, exceeding the minimum is worthwhile — many installers recommend R2.0 or higher where joist depth allows.
Installation: What's Involved?
Preparation
Before installing, inspect the subfloor space for:
- Moisture and drainage issues — fix any water ingress first
- Existing vapour barriers — a polythene ground sheet (minimum 0.25 mm) on the soil is recommended under NZ Building Code clause E3 to reduce ground moisture
- Pest or rot damage to the floor structure
Cutting and Fitting the Boards
Rigid foam boards are cut with a sharp utility knife and straight edge, or a fine-toothed saw. Cut boards to fit snugly between joists. Gaps around the edges allow cold air to bypass the insulation, so a tight fit is important. You can use low-expansion polyurethane foam (available from hardware stores) to seal small gaps.
Fixing Methods
Boards must be retained securely so they don't drop out over time. Common methods include:
- Timber battens or noggings nailed or screwed across the bottom of the joists
- Wire mesh or netting stapled to the underside of the joists
- Proprietary insulation support clips designed for this purpose
- Adhesive (check compatibility with the foam type — some solvents attack polystyrene)
Avoid leaving boards unsupported; a board that falls onto the ground provides no insulation value and can trap moisture.
Vapour Control
In most NZ subfloor situations, the primary vapour control measure is the ground-level polythene sheet, not a vapour barrier on the underside of the floor. Check BRANZ guidance for your specific situation, as getting vapour control wrong can lead to condensation and timber decay.
Do You Need a Building Consent?
In most cases, installing underfloor insulation in an existing home is exempt from building consent under Schedule 1 of the Building Act. However, exemptions have conditions and your local council has the final say — always confirm with them before starting work. See MBIE Building Performance for Schedule 1 guidance.
Safety Considerations
- Work in a well-ventilated subfloor space and wear a dust mask, gloves, and eye protection.
- Be aware of asbestos in older homes (pre-1990s) — if you find suspect materials, stop work and contact a licensed asbestos assessor. See WorkSafe NZ for guidance.
- Never cover electrical cables or plumbing without checking clearance requirements.
When to Call a Professional
While underfloor insulation is generally a DIY-friendly job, consider hiring a qualified insulation installer if:
- Your subfloor space is very confined or difficult to access safely
- You have moisture, rot, or structural concerns that need assessment
- You want a Healthy Homes compliance certificate for a rental property
- You're unsure about vapour control or the correct product for your situation
A professional installer will also be familiar with local conditions and can recommend the best product for your climate zone. You can find qualified local insulation installers on tradedirectory.co.nz.
