I want to store things in my roof. What’s the best insulation?

There's no insulation you can stack boxes straight on top of without a cost, so the answer isn't a product; it's a bit of setup. Every loose-fill and every batt gets compressed where you walk or stack on it, and compressed insulation loses R-value just in that spot, because squashing the fibres squeezes out the trapped air that does the insulating. The Australian Government's yourhome guide lists compression of bulk insulation as one of the things that reduces a ceiling's total R-value. So rather than flatten good insulation, we'd set up a small boarded storage platform on raised battens above the insulation, so your boxes sit clear of it, or run batts through the small storage zone and pump cellulose everywhere else. We'd rather build it right than have you crush what you paid for.

Only where you crush it, and only while it's crushed. Insulation works by holding still air in millions of tiny pockets; the depth and fluffiness is the R-value. Walk a path across it or stack boxes on it and you squash those pockets flat, so that compressed patch insulates at a lower R-value than the rest of the ceiling. The yourhome guide names compression of bulk insulation among the factors that reduce total R-value. It's not the whole ceiling that suffers, just the squashed footprint, but a stack of boxes can be a fair-sized cold patch right over a bedroom. Loose-fill cellulose springs back better than a flattened batt does, but no insulation gives you its full rating once it's been packed down under weight.

We raise the storage floor above the insulation instead of resting it on top. The trick is battens, timber rails laid across the ceiling joists to lift the storage boards up so the full depth of insulation can sit underneath, uncompressed, the way it's meant to. Then we board a small platform over those battens for your boxes. The insulation keeps its depth and its R-value, and you get a tidy, walkable spot to store things. We work the size and position out with you on the day around your manhole and what you actually need to store, usually a modest platform near the access hatch, not the whole ceiling. It's a small bit of carpentry that protects the insulation you've paid for, instead of slowly flattening it every time you climb up.

That's the other option, and sometimes it's the simpler one. We can run batts through the small storage zone and pump cellulose across the rest of the ceiling, so you still get the seamless, gap-free cellulose blanket over the bulk of the house, with batts in the bit you climb on. Batts handle the odd footstep without scattering, where loose-fill would just need raking back. It's not as good thermally as keeping the boxes up off the insulation entirely, since anything you stack on the batts still compresses them where it sits, but it's a sensible middle road if you don't want a boarded platform. We'll talk you through which way suits your roof rather than just selling you the dearer option.