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

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

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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More D.I.Y. Advice

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.