Fitting insulation batts between rafters sounds straightforward, but compressing them — even slightly — can dramatically reduce their thermal performance. The good news is that with the right preparation, correct sizing, and a few simple techniques, you can install rafter insulation that performs exactly as rated and helps your home meet the NZ Building Code requirements for thermal performance.
Why Compression Is Such a Problem
Insulation batts work by trapping tiny pockets of still air within their fibres. When you compress a batt, you crush those air pockets and reduce the material's thickness — and it's thickness that delivers the R-value (thermal resistance). A batt compressed to 75% of its intended thickness can lose 20–30% or more of its rated R-value. That means you're paying for performance you're not getting, and your home will be harder and more expensive to heat and cool.
BRANZ research consistently shows that installation quality has a major impact on real-world insulation performance, making correct fitting just as important as choosing the right product.
Choose the Right Batt Width for Your Rafter Spacing
Before you buy anything, measure your rafter spacing carefully — don't assume it's standard. Common spacings in NZ homes are 400 mm and 600 mm centre-to-centre, but older homes can vary. Measure the clear gap between the sides of adjacent rafters (not centre-to-centre), as this is the space the batt must fill.
Insulation batts are manufactured slightly wider than the nominal spacing so they friction-fit snugly without needing staples or adhesive. For example, a batt labelled for 600 mm spacing is typically around 580–600 mm wide, designed to compress just enough to grip the rafter sides — not to be squashed flat.
| Rafter spacing (c-to-c) | Typical batt width to use |
|---|---|
| 400 mm | ~360–400 mm batt |
| 600 mm | ~570–600 mm batt |
If your spacing is irregular, buy the next size up and cut to fit rather than forcing an undersized batt to stretch.
Check the Rafter Depth vs. Batt Thickness
This is where most DIYers go wrong. Your rafter depth must be at least equal to the batt thickness — ideally a few millimetres deeper — so the batt sits flush or just below the bottom edge of the rafter without being squashed. If your rafters are 90 mm deep and you're trying to fit a 100 mm batt, it will compress every time.
If your rafters are shallower than the batt you need for your target R-value, you have a few options:
- Use a thinner batt and accept a lower R-value (then consider adding a layer of rigid board insulation below the rafters)
- Fit timber battens to the underside of the rafters to increase the cavity depth before installing the batt
- Consult an insulation installer about alternative products such as spray foam or rigid board systems
For new builds or significant alterations, check the minimum R-values required under the NZ Building Code clause H1 (Energy Efficiency). Requirements vary by climate zone — check with your local council or visit MBIE Building Performance for current zone maps and R-value tables.
How to Cut Batts Accurately
For length, measure the rafter bay and add about 10–15 mm so the batt butts firmly against the top plate and ridge without a gap. Use a long straight edge (a piece of timber works well) and a sharp utility knife or purpose-made insulation knife. Always cut on a firm flat surface.
For width adjustments, compress the batt gently with the straight edge and cut — the batt will spring back to fill the space neatly.
Safety note: Always wear a long-sleeved shirt, gloves, safety glasses, and a P2 dust mask when handling glass wool or polyester batts. Work in a ventilated space. See WorkSafe NZ for guidance on working safely with insulation materials.
Installing the Batts
- Start from one end of the rafter bay and push the batt up into the cavity, working from below if it's an accessible ceiling space, or from above if you're working in an open roof.
- Friction-fit, don't force. The batt should slide in with light hand pressure and stay in place on its own. If you're forcing it, it's too wide — trim it.
- Maintain a ventilation gap if required. In many NZ roof constructions, particularly those with sarking or roofing underlay, a clear air gap of at least 25 mm must be maintained between the top of the insulation and the underside of the roof cladding or underlay. This prevents moisture build-up and is important for roof durability. Check the requirements for your specific roof type.
- Butt joints tightly. Where batts meet end-to-end, push them together firmly so there's no gap.
- Don't leave gaps at the edges. Small gaps around pipes, wires, or blocking can be filled with offcuts — just don't compress them.
Polyester batts (such as those available from Bunnings or Mitre 10) are a popular DIY-friendly option as they're non-itchy and easy to handle. Glass wool products from brands such as Pink Batts (available through PlaceMakers) offer excellent performance when installed correctly.
Vapour Control and Underlay
Depending on your roof construction and climate zone, you may need to consider a vapour control layer. In colder, wetter parts of NZ (such as the South Island and higher-altitude areas), moisture management is particularly important. Check with your local council or a qualified insulation installer about whether your roof assembly needs additional vapour control measures.
When to Call a Professional
While fitting insulation batts in an accessible roof space is generally not restricted work under the NZ Building Code, there are situations where you should bring in a qualified tradesperson:
- The roof space is confined, steep, or difficult to access safely
- You need to work near electrical wiring — never move or disturb wiring yourself; contact a registered electrician (see EWRB)
- You're unsure whether your roof assembly requires a building consent
- You want to achieve a specific R-value for a new build or alteration subject to Building Code compliance
A qualified insulation installer will also ensure the job meets Healthy Homes Standards if you're a landlord. Find a trusted local insulation installer on tradedirectory.co.nz.
