What are “in-ceiling walls” and bulkheads on my quote?

They're the spots where your ceiling line isn't actually closed. An 'in-ceiling wall' is an internal wall that stops at the ceiling rather than carrying on to the roof, so the top of that wall cavity opens straight into the hot roof space. A bulkhead is a dropped section of ceiling, usually over a kitchen cupboard, a wardrobe or a bathroom, and the void inside it also connects to the roof. Both are open holes in the blanket your ceiling insulation is meant to be. If we leave them, heat and noise pour down through them and bypass everything we've laid across the rest of the ceiling. So we fill them. Usually with polyester batts pushed into the wall top and the bulkhead void, so the ceiling insulation seals the whole house instead of stopping short at every cupboard and dropped ceiling.

Because the top of that wall is open to the roof. An internal wall that stops at the ceiling line leaves a tall, hollow cavity that's capped by nothing but the roof space above it, which in a SE-QLD summer sits well above the temperature in your rooms. Hot air and roof noise travel straight down that open cavity and come out at the skirting, the power points and the cornice, completely sidestepping the insulation we've laid flat across the ceiling. The Australian Government's yourhome guide makes the underlying point plainly: even a small gap can greatly reduce the insulating value, and thermal bridges (uninsulated paths through the roof, walls or ceiling) need to be identified and insulated. An open wall top is exactly that kind of path. Pack it with polyester and the heat and noise stop using it as a shortcut.

A bulkhead is the dropped, boxed-in section of ceiling you've probably never thought about, the lowered run above the kitchen overhead cupboards, the drop over a built-in robe, or the squared-off ceiling over a bathroom or hallway. Behind that plasterboard is a hollow void, and in most homes that void opens into the roof space rather than being sealed off. So unless it's filled, it's a chunk of your ceiling with no insulation in it at all, a warm bridge straight into the room below. It matters most over the rooms you most want comfortable and quiet: bedrooms with robes, and bathrooms. We fill the bulkhead void, usually with polyester batts, when we do the ceiling. Leaving it out is exactly the kind of missed spot that quietly drags down the effective R-value of an otherwise good job.

Because they're vertical and boxed-in, not a flat ceiling. Our pump-in cellulose is brilliant across an open horizontal ceiling, where it flows into one seamless blanket. But the top of an internal wall and the inside of a bulkhead are upright, closed cavities; polyester batts cut to fit and pushed firmly into place are the cleaner, more reliable way to pack those. So a normal quote often mixes the two: cellulose pumped across the main ceiling, polyester batts into the wall tops and bulkhead voids. It's the same thinking we use everywhere, the right product for the spot, not a one-size-fits-all sell. The whole point is a continuous insulated layer with no open holes, whichever material seals each part of it best.