Is fibreglass insulation safe near my drinking-water tank?

In a normally built and insulated home, your ceiling insulation sits inside the roof cavity, above the plasterboard and below the roof sheets, not in the gutters or the path your rainwater takes into the tank. So a properly installed roof keeps the insulation out of your tank water. The risk people are really worried about is loose fibres or dust washing off an exposed or disturbed roof into the gutters, which is a job-quality issue, not a property of the material sitting quietly in your ceiling. To be straight with you: glasswool batts are still made of fine glass fibres, and the manufacturers' own safety data sheets tell installers to wear a P2 dust mask while handling them. Cellulose, by contrast, is recycled paper treated with borax, and borax is a mineral salt about as toxic to you as table salt.

Not in a roof that's built and installed properly. A rainwater tank collects what runs off your roof sheets into the gutters and down the pipe, and that water never travels through the ceiling cavity where your insulation lives. The insulation lies on top of the plasterboard ceiling, under the roof, completely separated from the gutter-to-tank path. Where you can get debris in the tank is from an open or part-finished roof, from leaf litter and roof dust generally, or from a sloppy install that leaves bits of insulation lying in the gutters instead of in the ceiling. That's why the job is done by a Comfort Zone franchise owner-operator working to one system: drop sheets down, work kept in the cavity where it belongs, site cleaned up, and every job photographed with the photos checked before you're invoiced. Most people never see inside their own roof, so we show them. A first-flush diverter on the tank downpipe is still cheap, sensible insurance against ordinary roof grime, worth fitting regardless of which insulation you choose.

Cellulose is recycled waste paper treated with borax (and boric acid). Borax is a naturally occurring mineral salt, about as toxic to you as common table salt. It's the bit that makes cellulose a place insects won't live in and has been shown to slow fire spread, and it's locked into the paper fibre sitting in your ceiling, not floating around your gutters. I won't tell you to eat the stuff; you shouldn't eat any insulation, the same way you wouldn't eat the dust off your roof. But I will say this: plenty of us ate a few pages of a book as kids and lived to tell the tale, and borax is in roughly that league of nasty. As with fibreglass, in a properly built roof the cellulose stays in the cavity and out of the water path, so it isn't sitting in your drinking water in the first place. If you're worried about anything entering the tank, the answer for any roof is a first-flush diverter and a screened inlet, not picking your insulation around it.

After installing insulation since 1986, the pump-in cellulose fibre is the only one I'd use in my own home, tank water or not, and the reasoning holds up here. It's recycled paper and borax rather than fine glass fibre, it pumps in as one seamless blanket with no gaps, it deters the rats and insects whose droppings are the real thing you don't want washing off a roof, and it gives you a far cleaner roof cavity over the decades. If you want batts instead, I'd quote you the polyester, locally made, no glass fibre, over the fibreglass. Whichever you choose, the things that actually protect a drinking-water tank are the same: a roof installed cleanly with the insulation kept in the cavity, gutters left clear, a first-flush diverter, a leaf screen, and a tank you have looked at now and then. Get the install right and the insulation question takes care of itself.

A standard 5-micron sediment filter catches most insulation fibres, because most are 5 microns across or wider, but glass-wool fibres run as fine as 1 micron and below, so to be confident of catching the fine ones you want finer filtration: a 1-micron cartridge, a 0.5–1 micron carbon block, a ceramic filter, or ultrafiltration or reverse osmosis. A coarse 20–50 micron pre-filter on its own won't do it, and because fibres are long and thin nobody should promise to remove 100% of them at any rating. NSW Health's position is that filters are a backstop and a well-maintained catchment is the main safeguard, so the real fix is keeping loose fibre off the roof. With cellulose there's no glass to filter out in the first place, and the World Health Organization has found no convincing evidence that swallowing the odd fibre is harmful anyway, so this is about clean water, not a poisoning scare.

Anticon is a foil-faced fibreglass blanket rolled out under the roof sheets. Unlike ceiling batts it sits right under the roofing, but the rainwater that fills your tank runs across the outside of the roof sheets into the gutter, so in a sound roof the blanket isn't in that water path. A damaged or disturbed blanket shedding fibres into the gutters is the one real fibre worry, and it's handled like any roof debris, gutter mesh, a first-flush diverter and a fine sediment filter. The other thing to know is that anticon is only a thin condensation and thermal-break blanket; it's not a substitute for proper ceiling insulation and doesn't give you the R-value your ceiling needs in our climate, so even with anticon you still want cellulose pumped into the ceiling. With cellulose in the ceiling there's no glass blanket in the picture at all.