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How to install polystyrene insulation

Polystyrene insulation

Overview

Installing polystyrene insulation beneath a suspended timber floor is a straightforward D.I.Y. job that can make a big difference to your home’s energy efficiency and comfort. Rigid polystyrene or foil-faced insulation panels are ideal for underfloor use - they’re lightweight, easy to cut, and fit snugly between floor joists. Follow this simple guide to get the job done safely and effectively. 

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

    Choose the right insulation

    Before getting started, make sure you’ve selected a polystyrene or foil-faced insulation board that’s suitable for use under suspended timber floors. Foil-faced board is a common choice thanks to its durability and excellent thermal performance. Some types are also rated for use in walls and ceiling insulation, so it’s important to check the product specifications to make sure it’s the right fit for your project.
    Choose the right insulation
  2. 2

    Measure the area accurately

    Take time to measure the space between your floor joists or bearers. Getting these measurements right will help you determine how much insulation you need and how to cut it to size. Use a tape measure and jot down the length and width of each section, especially if your floor joists aren’t evenly spaced. Remember the golden rule: measure twice, cut once. This is particularly important when you’re working in tight or irregular spaces under the floor.
    Measure the area accurately with a tape measure
  3. 3

    Cut the insulation boards to fit

    Using a utility knife and a straight edge, carefully cut your insulation panels to the sizes you measured. Aim for a snug fit inside each cavity between the joists - tight-fitting boards perform better by minimising air gaps that could let heat escape or moisture seep through. Take your time with this step and use offcuts to fill in any smaller spaces as needed.
    Cut the insulation boards to fit
  4. 4

    Install saddles between the joists

    If your floor has double joists like many older homes, you’ll need to cut the saddles in half to suit the narrower cavities. Once prepared, staple the saddles into place along the inner sides of the joists at intervals of roughly 600mm. These saddles will help hold the insulation panels in place and prevent them from sagging or slipping over time.
    Install saddles between the joists
  5. 5

    Position the insulation panels

    With the saddles installed, carefully place your cut insulation panels into position from above. Slide them between the joists so they sit neatly on the saddles and fit tightly against the joist edges. Take care to ensure each insulation panel sits flush and doesn’t bow or leave gaps along the sides.
    Position the insulation panels
  6. 6

    Seal all gaps with tape

    To finish the installation and improve the thermal performance of your underfloor insulation, apply foil tape along the seams and edges where panels meet. This step helps lock in the insulation, keeps out moisture, and adds an extra layer of durability to your work. Take the time to press down the tape firmly so it sticks securely to the polystyrene insulation surface.
    Seal all gaps with tape
  7. 7

    Now that you’ve learned how to install polystyrene insulation

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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.