How to install underfloor insulation batts

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.

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.

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.

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.