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How to install polystyrene insulation

Polystyrene insulation

Overview

Insulating your floor with polystyrene panels is a cost-effective way to help reduce your heating and cooling bills. We show you how to trim the panels to size and install them between your floor joists. You will also see how to use brackets to support the panels in place.

Steps

1Choose 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

2Measure 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

3Cut 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 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

5Position 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

6Seal 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

7Now that you’ve learned how to install polystyrene insulation

Health & Safety

Please make sure you use all equipment appropriately and safely when following the advice in these D.I.Y. videos. You need to be familiar with how to use equipment safely and follow the instructions that came with the equipment. If you are unsure, you may feel it is safest to consult an expert, such as the manufacturer or an expert Bunnings Team Member.

Grave health hazards are linked to asbestos, which may be in homes built up to 1990. Health hazards may result from exposure to lead-based paints in older materials and copper chromium arsenic (CCA) treated timber. 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.