Glasswool, gutters and the rainwater tank
Is there glass in your drinking water?
I'm Peter Johnson, a Queensland insulation contractor since 1986. Out on the rural blocks the house drinks whatever the roof catches. The glasswool blanket that goes under a metal roof runs out to the fascia and finishes on the gutter line. The bushfire rules are part of why it's out there. The cartridge on a normal tank filter is rated coarser than the fine end of that fibre. Then there's the part that stopped me when I read it. Not one study exists, in people or in animals, on what swallowing glass fibre does.
0 studies
on swallowing glass fibre, humans or animals, any endpoint
Where the blanket finishes
The glasswool under a metal roof stops at the fascia, which is where your gutter hangs.
Rolled out during construction, the blanket goes in sandwiched directly between the underside of the roof sheet and the roof structure. Bradford Anticon blanket datasheet (2014 issue) — “The insulation is sandwiched directly between the underside of the metal roof sheet and the buildings roof structure” ↗ In a sound, sealed roof the weather never touches it, which is the manufacturer's whole case and a fair one. The bit people don't picture is where the roll ends. The same manufacturer's technical datasheet says the blanket is ideally suited to sealing ember entry points at ridges, valleys and fascias. Anticon FC blanket technical datasheet, Doc Ref B0238 08/17, Applications — “ideally suited to sealing ember entry points at ridges, valleys and fascia's” ↗ The fascia is the timber your gutter is screwed to. So the cut edge of a glass-fibre blanket sits at the exact point where every drop of water leaves your roof.
What I see, from forty years on roofs
On re-roofs I've picked loose fibreglass out of gutters by hand more times than I can count. That's my own observation off my own jobs. It isn't a measurement of what reaches a tank. I'm not dressing it up as one.
What the manufacturer says, printed here as well
CSR Bradford's own safety document states that once installed the product does not release dust or fibres. The same document says binder-coated insulation wool is hydrophobic with no adverse environmental effects expected if it's accidentally released into water or soil. CSR Bradford SUIS for FBS-1 Glasswool Insulation, §7 Handling and Storage p.3 and §12 Ecological Information p.6 ↗
What has actually been measured
Nothing. I went looking for a water-authority test or a published study measuring glass fibre in gutter runoff or in tank water. There isn't one, in either direction. The national guidance on rainwater tanks talks about bird droppings, leaves, roof dust and lead flashings, never insulation. Australian Government yourhome guide — Rainwater: catchment contaminants and tank maintenance ↗ So the pathway is physical and the material is at the water's edge. The measurement doesn't exist.
Bushfire-rated blocks
The bushfire standard puts something in that gap on purpose. It doesn't have to be glass.
This is where the wording matters, so here it is off the standard rather than off a sales sheet. From BAL-12.5 upwards, AS 3959:2018 requires a sheet-metal roof to be fully sarked and to have any gaps sealed with mesh or perforated sheet used as a screen with a maximum 2 mm aperture in corrosion-resistant steel, bronze or aluminium, or mineral wool, or other non-combustible material. AS 3959-2018 (Amdt No. 1) construction requirements, Roof → Sheet, BAL-12.5 column — “Have any gaps sealed with — Mesh or perforated sheet … maximum 2 mm aperture … or Mineral wool, or Other non-combustible material” ↗
Read that again. The standard requires the gap to be closed. Glasswool blanket is one of the accepted ways to close it, not the only one. Builders reach for the blanket because it's quick and it does two jobs at once, so homeowners end up believing the law names it. The law names an outcome. A bushfire consultant's own reference notes describe how it goes in: the cladding pinches the blanket down onto the top edge of the fascia to close off the corrugated profile openings. Bushfire Design Consultants, BAL-12.5 reference page (commercial consultancy, quoted for the physical detail only) — “The cladding will pinch the Anticon down onto the fascia top edge and close off the corrugated profile openings” ↗
Two things follow. If you're on a BAL-LOW block, which covers most suburban lots, the roof has no requirement at all, so none of this is your problem. If you're building or re-roofing on a rated block at BAL-12.5 or above and your family drinks the rainwater, you can ask for the 2 mm corrosion-resistant mesh instead. Same clause, same compliance, no glass fibre packed into the gutter junction.
The three compliant ways to seal that gap
- Mesh or perforated sheet, maximum 2 mm aperture, in corrosion-resistant steel, bronze or aluminium
- Mineral wool (the glasswool blanket most builders use)
- Other non-combustible material
All three are in the same row of the standard. Only one of them puts glass fibre at the point your roof drains into your drinking water.
Ask your builder or your bushfire consultant which option is on your plans before the roof goes on. Once the sheets are screwed down it's an expensive conversation.
The cartridge on the tank
A normal rainwater filter is rated coarser than the fine end of glass fibre.
Australia's national standard for synthetic mineral fibres defines a respirable fibre as one under 3 microns in diameter, more than 5 microns long, with a length-to-width ratio over 3 to 1. The same document puts glasswool at a nominal fibre diameter of 5 to 8 microns, with the typical proportion under 3 microns in the order of 10 to 20%. NOHSC:1004(1990) / NOHSC:2006(1990), definitions §4.1 p.12 and Glasswool schedule item 7 p.42 — “The typical proportion of fibres less than 3 µm contained in glasswool materials is in the order of 10 to 20 per cent” ↗
Now put that against the housing on the side of your tank. Puretec's replacement sediment range runs from 0.35 micron all the way to 250 micron. The cartridges that ship as standard in a domestic rainwater housing cluster at 1, 5, 10 and 20 micron. Puretec replacement sediment cartridge range — ratings of 0.35, 0.5, 1, 5, 10, 20, 25, 30, 50, 60, 100, 150 and 250 micron ↗ A 5 micron cartridge is rated to stop particles 5 microns across. The fine end of glasswool is under 3 microns across. That fraction goes through.
Two details that stop this being a slogan
A micron rating describes the fibre's diameter, not its length. A glass fibre can be under 3 microns across while it's tens of microns long, so “passes a 5 micron filter” is a true statement about the cross-section rather than about the whole fibre. Fibres being long and thin also means one can thread a rated pore end-on instead of being caught side-on.
And nominal is not absolute. A filter manufacturer's own spec sheet defines its micron ratings as 85% or greater removal of a given particle size. Pentair/Pentek CBC series carbon block cartridge spec sheet, notes — “Micron ratings based on 85% or greater removal of given particle size” ↗ So a nominal 1 micron cartridge lets some fraction straight through by definition rather than by fault. Nobody can promise 100% removal of glass fibre at any single rating. Anyone who does is selling you something.
What does step up to the job
NSW Health's rainwater treatment guide sets out the chain. Micro and ultrafiltration membranes at 0.1 to 0.01 micron remove sediment and bacteria. Reverse osmosis at 0.001 micron is the most sophisticated of them. The guide also puts a 20 micron pre-filter with a 1 micron stage before any UV unit. NSW Health, Rainwater Treatment Guide — “Micro/Ultra filtration membrane filters (0.1 - 0.01 micron) … Reverse osmosis filters (0.001 micron) are the most sophisticated” ↗ The same authority makes the point that matters more than any cartridge: water filters should not be necessary to maintain the quality of rainwater if catchments and tanks are well maintained. NSW Health, Rainwater Treatment Fact Sheet — “Water filters should not be necessary … if catchments and tanks are well maintained” ↗ The filter is the backstop. The roof is the fix.
| What's on the tank | Rating | Sub-3-micron glass fibre |
|---|---|---|
| Standard sediment cartridge, the common default | 5 – 20 micron nominal | Passes |
| Fine sediment cartridge | 1 micron nominal (85% removal at rating) | Mostly caught |
| Absolute-rated fine cartridge | 1 micron absolute | Caught |
| Ultrafiltration membrane | 0.1 – 0.01 micron | Caught |
| Reverse osmosis | 0.001 micron | Caught |
Ratings from the Puretec cartridge range and the NSW Health treatment guide cited above; the nominal-versus-absolute distinction from the Pentek spec sheet. Read “caught” as what the rating is built to do rather than as a guarantee for a long thin fibre.
The part that stopped me
Nobody has ever tested what drinking fine glass fibre does.
Everything the world knows about glass fibre and health comes from breathing it. That research is deep. Lungs, eyes, skin, decades of it. Then you go looking for the swallowing side and the shelf is bare. The US Agency for Toxic Substances and Disease Registry publishes a Toxicological Profile for Synthetic Vitreous Fibers, which is the family that covers glass wool by name alongside rock wool and slag wool. Section 3.2.2, oral exposure, reads the same line over and over.
“No studies were located regarding death in humans or animals after oral exposure to synthetic vitreous fibers.”
“No studies were located regarding respiratory, cardiovascular, gastrointestinal, hematological, musculoskeletal, hepatic, renal, endocrine, dermal, ocular, body weight, or metabolic effects in humans or animals after oral exposure to synthetic vitreous fibers.”
“No studies were located examining the possible absorption of ingested synthetic vitreous fibers in humans or animals.”
Immunological effects, neurological effects, reproductive effects, developmental effects, cancer. Every one of those subheadings returns the same sentence. No studies located, in humans or in animals, at any dose. Nobody has even checked whether a swallowed glass fibre is absorbed, where it goes in the body or how it leaves.
What that does and doesn't mean
No study shows harm from drinking glass fibre. No study shows safety either. That's the whole of it. I'd rather hand you that than a scare.
ATSDR's own explanation for the empty shelf is worth printing beside it. The agency writes that “the oral route of exposure is not of public health concern for synthetic vitreous fibers; therefore, no data need is identified,” and reinforces that by pointing at lifetime rat-feeding studies on asbestos, a far more aggressive fibre, which found no convincing evidence of non-malignant disease. ATSDR tp161, §3.5 Discussion — “The oral route of exposure is not of public health concern” ↗ The World Health Organization landed in the same place on asbestos in drinking water. It found no consistent, convincing evidence that ingested fibres are hazardous. It set no guideline value for them. WHO, Asbestos in Drinking-water, background document for the WHO Guidelines for Drinking-water Quality ↗
So the regulators aren't worried, on the strength of a different fibre. What nobody has is a single measurement on this one. My customers on tank water can weigh that for themselves, which is the reason the quotes and the links are on the page instead of my opinion of them.
The other product at the same gutter line
Stir a plant fibre and a pinch of salt into a glass of water.
That's about what our product is. Shredded plant fibre with a borate treatment of boric acid and borax, made at our own factory at Tiaro, which is why I can state the recipe rather than read you an importer's safety data sheet. The CSIRO describes the category as recycled paper combined with fire-retardant chemicals. CSIRO / Australian Government, Thermal Insulation guide — cellulose fibre insulation described as recycled paper combined with fire-retardant chemicals ↗ No glass. Nothing at the fascia that stays sharp for the life of the house.
On acute oral toxicity a borate sits in the same band as the salt in your kitchen. Table salt's oral rat LD50 is 3,000 mg per kilogram of body weight. Mallinckrodt Baker MSDS, Sodium Chloride ↗ Fisher Scientific MSDS, Sodium Chloride ↗ Boric acid's is 2,660 to 5,140 mg per kilogram and borax's is 3,493 to 6,080 mg per kilogram, on the EPA-cited figures published by the US National Pesticide Information Center. NPIC (Oregon State University / EPA), Boric Acid Technical Fact Sheet — “2660-5140 mg boric acid/kg body weight”; “3493 to 6080 mg borax/kg body weight” ↗ Australia's drinking water guideline for boron is 4 mg per litre, which is the number a health department would measure a tank against. NHMRC, Australian Drinking Water Guidelines — Boron: guideline value 4 mg/L ↗
Where that comparison stops
A single-dose LD50 describes one accidental swallow. It says nothing about a small dose repeated for years. Borates do have high-dose reproductive and developmental endpoints that table salt doesn't. Nobody should be eating insulation of any kind, mine included. The point is narrower than that. At the gutter line one product is a plant fibre with a mineral salt on it. The other is a glass sliver that nobody has ever fed to anything to find out.
Cellulose Fibre Insulation pumped in as one seamless blanket across the ceiling joists under a metal roof. Plant fibre with a borate treatment, sitting in the cavity rather than out at the fascia.
Practical, in order of what it costs you
What I'd do if the blanket has to stay up there.
Plenty of people reading this have a bushfire rating on the block, a blanket already under the iron and no intention of re-roofing a house to fix a maybe. Fair enough. Here's the order I'd work in.
- Clear the gutters. Clear them again after any roof work, which is when loose material turns up in them.
- Fit gutter mesh and a first-flush diverter so the dirtiest run of water never reaches the tank. This is the cheapest thing on the list and it works on leaves, dust and droppings at the same time.
- After a re-roof, a solar install or any trade on the roof, get up and look in the gutter yourself rather than taking somebody's word for it.
- If your household drinks off a roof with a glasswool blanket finishing at the fascia, fit a 1-micron absolute cartridge at minimum. An ultrafiltration or reverse-osmosis stage on the drinking tap does better again. The 5 to 20 micron cartridge that came with the housing is not that filter.
- Building new or re-roofing on a BAL-rated block? Ask for the 2 mm corrosion-resistant mesh option instead of the blanket. Same clause of the standard, no glass fibre at the gutter junction.
None of that is insulation advice dressed up as plumbing. It's what the health departments already recommend for any tank on any roof, with one extra line for households sitting under a glass-fibre blanket. NSW Health, Rainwater Treatment Fact Sheet — catchment maintenance first, filters as the backstop ↗ Australian Government yourhome — Rainwater ↗
The questions I get on tank-water blocks
Glass, gutters and rainwater tanks.
Can glass fibre from my roof insulation get into my rainwater tank?+
The glasswool blanket that goes under a metal roof runs out to the fascia, which is the board your gutter hangs off, so its cut edge finishes right where the water leaves the roof. CSR Bradford's own datasheet says the blanket is ideally suited to sealing ember entry points at ridges, valleys and fascias, so being out at that edge is what it's for. On re-roofs I've pulled loose glasswool out of gutters with my own hands. What I can't point you at is a published study measuring glass fibre in gutter runoff or in tank water. I went looking for one and there isn't any. So treat this as a physical pathway plus a tradesman's observation rather than a measured contamination figure. The manufacturer's position is worth printing too: their safety document says that once installed the product does not release dust or fibres.
Does the law force me to have a glasswool blanket under my roof?+
Not the blanket specifically. AS 3959:2018 requires the gaps under the corrugations of a sheet-metal roof to be sealed from BAL-12.5 upwards. The standard lists three ways to do it. Mesh or perforated sheet with a maximum 2 mm aperture in corrosion-resistant steel, bronze or aluminium; mineral wool; or other non-combustible material. Glasswool blanket is the option most builders reach for, so people assume it's mandated. It isn't. If you're building on a bushfire-rated block and you drink your rainwater, the steel mesh option seals the same gap with no glass fibre at the gutter line. Worth knowing as well, a BAL-LOW block has no roof requirement at all, so none of this applies to most suburban lots.
Will the filter on my rainwater tank take glass fibres out?+
Not the cartridge most houses have. Australia's national standard puts glasswool at a nominal fibre diameter of 5 to 8 microns with 10 to 20% of the fibres finer than 3 microns. A filter's micron rating describes the diameter of what it stops. A standard domestic rainwater housing runs a 5 to 20 micron cartridge, which is bigger than that fine fraction, so the fine fraction passes. Stepping down to 1 micron catches far more, though nominal ratings are defined by a filter manufacturer's own spec sheet as 85% or greater removal at the rated size, so some fraction goes through by definition. Nobody can promise 100% removal at any single rating. If you want real confidence you're into a 1-micron absolute cartridge, ultrafiltration at 0.1 to 0.01 micron or reverse osmosis at 0.001 micron.
Is it dangerous to swallow glass fibres in drinking water?+
Nobody knows. That's the finding rather than a dodge. The US Agency for Toxic Substances and Disease Registry's Toxicological Profile for Synthetic Vitreous Fibers, which covers glass wool by name, reports no studies located regarding death, systemic effects, immunological effects, neurological effects, reproductive effects, developmental effects or cancer in humans or animals after oral exposure. It also reports no studies on whether a swallowed fibre is even absorbed. So there is no study showing harm from drinking it. There is no study showing safety either. ATSDR's stated reason for not funding any is that the oral route isn't considered a public-health concern for these fibres, backed by lifetime rat-feeding studies on asbestos that found no convincing evidence of disease. The World Health Organization reached the same place on asbestos in drinking water and set no guideline value for it.
What is Cellulose Fibre Insulation actually made of?+
It's shredded plant fibre with a borate fire-and-pest treatment of boric acid and borax, made at our own factory at Tiaro, so I can tell you what's in it rather than read you somebody else's safety data sheet. Nobody should be drinking insulation of any kind, mine included. What I'll say is that on acute oral toxicity a borate sits in the same band as the salt in your kitchen: table salt's rat LD50 is 3,000 mg per kilogram of body weight, boric acid's is 2,660 to 5,140 and borax's is 3,493 to 6,080. A single-dose LD50 says nothing about a daily dose taken for years, so read it as the band it sits in rather than a licence to eat it. The point is what the product is at the gutter line, which is a plant fibre and a mineral salt instead of a glass sliver nobody has ever tested for swallowing.
I'm stuck with a glasswool blanket under my roof. What should I actually do?+
Get the catchment right first. That's what the health departments push and it costs the least. Keep the gutters clear, fit gutter mesh and a first-flush diverter, then after any roof work go up and check the gutters yourself for loose material rather than taking a word for it. NSW Health's own guidance says filters shouldn't be necessary if the catchment and tank are well maintained. Then, if your family drinks off a roof with a glasswool blanket finishing at the fascia, fit a filter fine enough to matter. A 1-micron absolute cartridge is the minimum worth bothering with. An ultrafiltration or reverse-osmosis stage on the drinking tap does better again. The standard 5 to 20 micron sediment cartridge that came with the housing is not that filter.
Figures on this page are read from primary government, standards-body and manufacturer documents, cited beside each claim, current as of August 2026. Safety data sheets and standards get revised, so check the current version before relying on one. This page is general information rather than medical, water-quality or bushfire-compliance advice. For a BAL rating on your own block, use a bushfire consultant.
If your ceiling is full of fibreglass batts as well as a blanket under the iron, we can quote to take the batts out or to pump Cellulose Fibre Insulation over the top.
Sorted out your roof and your tank with us? Worth a mention.
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On tank water and wondering what's up in your roof?
Send us your address and roof type for a fixed-price quote within 48 hours for most houses. Taking out old fibreglass, pumping Cellulose Fibre Insulation or laying polyester if you'd rather have batts. Servicing Brisbane, SE QLD and Northern NSW.
Peter Johnson
Owner / installer · Comfort Zone Insulation Team® · Since 1986