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How to install underfloor insulation batts

Insulation batts installed between floor joists.

Overview

Installing underfloor insulation is a great way to help maintain a comfortable temperature inside your home, especially during the cooler months. This step-by-step guide will show you how to safely and effectively install underfloor insulation batts in homes with suspended timber floors.

Safety tip: Always wear the appropriate safety equipment (safety glasses, gloves, ear muffs and a mask, for example) and always follow the instructions for the product or equipment.

Steps

  1. 1

    Check the space and measure between joists

    Before you get started, make sure the underfloor area is accessible, dry and safe to work in. This project is best suited to homes with suspended timber floors. If you have a concrete slab or limited crawl space, other insulation options may be more suitable. It’s also important to check for signs of moisture or dampness, which should be addressed before underfloor insulation installation begins.

    Once you’ve confirmed the space is ready, measure the distance between your floor joists using a measuring tape. This will help you select insulation batts that fit snugly between the joists without needing to be compressed. A proper fit is essential, as insulation works best when it maintains its full loft.

    A person is measuring the gap between floor joists using a tape measure on a partially constructed wooden floor frame.   
  2. 2

    Choose insulation with the right R-value

    When selecting your insulation, look for underfloor-rated batts with a minimum R-value of 2.0. For better energy efficiency or if you're located in a cooler climate, consider going for higher-rated options like R2.5 or R3.0. A higher R-value means better resistance to heat flow, helping your home stay warmer in winter and cooler in summer.

    The image shows a person pointing to the R-2.5 rating on a pack of underfloor insulation.
  3. 3

    Install the strapping to support insulation

    Before laying underfloor insulation, install support strapping beneath the joists to hold the insulation batts in place. You can run the strapping in a zigzag pattern or straight across horizontally. Secure it with a staple gun to the inside edges of the joists.

    Each insulation batt will need at least three straps beneath it, with no part of the insulation more than 200mm from the nearest strap. Keep the strapping consistent and evenly spaced throughout the area. This step is important as it ensures the insulation won’t sag or fall over time.

    The image shows a person in protective gear is installing a support strapping beneath the joists to hold the insulation batts in place. 
  4. 4

    Lay the insulation batts

    With your strapping in place, begin laying the insulation batts snugly between the joists. If the batts are the right width, they should sit firmly in position without needing to be forced. Work your way across the underfloor area, placing each insulation batt securely on top of the strapping. Make sure to avoid compressing the insulation, as it works best when it retains its full loft.
    A person wearing protective gear is laying insulation batts between floor joists in a room under construction. 
  5. 5

    Trim insulation around pipes and small gaps

    As you install batts, you’ll likely come across areas where you need to cut the insulation to fit, especially around pipes, wiring or narrow spaces. Use a sharp utility knife and a straight edge to make clean, accurate cuts. Carefully shape each piece to suit its location, ensuring there are no gaps around obstructions. A tight fit in these trickier spots will help maintain the insulation’s performance throughout your home.

    Draughts often creep in around the perimeter, so make sure batts are fitted snugly in these areas too. Filling all gaps will help maximise the performance of your new underfloor insulation.

    A person wearing gloves is fitting insulation around a pipe by cutting a hole in the material to ensure a snug fit. 
  6. 6

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Health & Safety

Asbestos, lead-based paints and copper chromium arsenic (CCA) treated timber are health hazards you need to look out for when renovating older homes. These substances can easily be disturbed when renovating and exposure to them can cause a range of life-threatening diseases and conditions including cancer. For information on the dangers of asbestos, lead-based paint and CCA treated timber and tips for dealing with these materials contact your local council's Environmental Health Officer or visit our Health & Safety page.

When following our advice in our D.I.Y. videos, make sure you use all equipment, including PPE, safely by following the manufacturer’s instructions. Check that the equipment is suitable for the task and that PPE fits properly. If you are unsure, hire an expert to do the job or talk to a Bunnings Team Member.