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Very narrow strips of polystyrene insulation material being cut from a larger sheet for use in smaller gaps


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.


1Prepare your joists for installing the polystyrene insulation

Check to see if there is any plumbing or electrical cable installed on the sides of your joists. If you find electrical cable, cover it with wireguard waxed paper. If you find plumbing, cut a gap in your polystyrene panel that leaves 100mm of clearance to allow for future maintenance.
Several sheets of polystyrene insulation material, along with gloves, a hammer, measuring tape, a knife and other protective equipment

2Trim polystyrene insulation panels to size

Polystyrene insulation panels come in different sizes to match the standard widths between floor joists. When you find a smaller width, trim the panels to fit the size required. Each panel has accordion edges, so you can easily trim them on either side with a standard trimming knife
A polystyrene insulation panel being trimmed to size with a trimming knife

3Fit support brackets for the polystyrene insulation

Place a bracket once every 600mm on both sides of every joist, approximately 60mm below the top of the joist. This will hold the insulation panels in place just below the top of the joist. Nail the brackets in lightly. Then use a panel offcut to confirm your brackets are in the right position before nailing them in fully. Now put your panels in place.
Support brackets being fitted to the house framework to hold polystyrene insulation material in place

4Trim special shapes in the polystyrene insulation

Polystyrene insulation panels are easy to trim. For very thin gaps between joists, it is possible to trim the accordion edges on both sides. When the floor joists have an odd shape or overlap one another, you can simply cut a notch out where you need to.

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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. You can also use a simple test kit from Bunnings to indicate the presence of lead-based paint.