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Blown Cellulose vs Blown Fibreglass for an Ontario Attic: R per Inch, Settling, Fire, Mice, Cost per Square Foot and Which One I Would Put in Your Attic
Insulation & Air Sealing
Cellulose or Fibreglass in the Attic: Both Work, Neither Stops Air, and the Price Gap Is Twenty-Five Cents
Almost every Ontario attic that gets topped up gets one of two things blown into it: cellulose, which is ground-up newspaper treated with borate, or loose-fill fibreglass, which is the same glass as a batt chopped into fluff. Installed, the difference is about $0.25 a square foot, with fibreglass the dearer one. Either will take a 1980s attic from R-20 to R-60 for $2.60 to $2.85 a square foot before HST. The choice is not about which one is warmer. It is about settling, fire, mice, wind and what is already up there, and none of that is on the bag.
The numbers
$2.60cellulose top-up, R-20 to R-60, per sq ft
$2.85fibreglass top-up, same job, per sq ft
R-3.6cellulose per inch, settled
R-2.6blown fibreglass per inch, typical
| Blown cellulose | Blown fibreglass | |
|---|---|---|
| What it is | Shredded newsprint, 80 to 85% recycled, treated with borate salts | Spun glass fibre, loose, with no binder |
| R per inch | About 3.6 to 3.8 settled | About 2.2 to 2.8 for most loose-fill; some newer products higher |
| Inches for R-60 | About 16 to 17 in. settled | About 20 to 23 in. |
| Settling | Up to 20%; the bag chart gives the installed depth to allow for it | 2 to 4%; it stays about where it was blown |
| Installed, per sq ft, R-20 to R-60 top-up | $2.60 | $2.85 |
| Installed, bare joists to R-60 | $3.75 | $4.00 |
| Fire | Borate treated, smoulders rather than flames; passes the same CAN/ULC tests; keep it 3 in. off chimneys like anything else | Does not burn; the glass melts at a temperature your attic will not see |
| Mice | Borate is a mild deterrent, not a wall. They tunnel in both. | Mice love it. It is their favourite bedding in Ontario. |
| Wind washing at the eaves | Dense enough to sit still behind a baffle | Light enough to drift; needs the baffle and a dam at the plate |
| Roof leak | Holds water, packs down, stains the drywall sooner; dries slowly | Drains through, dries faster, does not pack |
| Dust during the blow | A lot. Mask the hatch, cover everything below it | Less, but the fibres itch |
The $0.065 per R-point rule for cellulose is what the attic insulation cost calculator uses; it adds the $0.25 when you pick fibreglass. Material is about 45% of either rate, the rest is two men, the truck and the hose.
Neither one is an air barrier, and that is the whole game
A blown product, either one, is a wool sweater. It slows heat down and does nothing at all about wind. Every top plate, plumbing stack, wire hole and pot light in your ceiling is still blowing warm wet house air into the attic through 17 in. of fluff, and that air carries more water into the attic in a winter than any roof leak. The frost on the underside of the sheathing in February did not come from outside.
So the attic gets air sealed first, then blown. That is a separate job at about $1.00 a square foot and it is on the attic air sealing guide. A quote that goes straight to blowing without a line for sealing is burying the leaks under a layer you can no longer walk on to find them. I have had people argue that cellulose is “dense enough to stop air”. It is denser than fibreglass, and it still does not stop air. Neither does a sweater, however thick the knit.
Settling, honestly
Cellulose settles. The bag chart says so, in print: an installer blowing for R-60 puts in roughly 20 in. so that it is 16.5 in. a year later, and the bag count per 1,000 sq ft is what proves he did. The bad version of this is the installer who blows to the settled depth on day one and lets it drop to R-48 by Christmas. The good version is the one who shows you the chart, counts the bags on the invoice and staples depth markers at the finished height, which is what the depth chart page is about.
Fibreglass barely settles, which is its honest advantage. What it does instead is lose R when it gets cold on top and warm underneath: air convects through the loose fibre, and a 20 in. blanket of it at minus 25 C performs a little worse than the label. Cellulose does not do that because the fibre is too fine for air to loop through. In a Simcoe County January the two end up closer than the bag numbers suggest. Neither effect is a reason to pick one over the other; both are reasons to blow a little more than the minimum.
Fire, mice and the roof leak you will eventually have
Fibreglass does not burn and that is the end of that sentence. Cellulose is paper, and it is treated with borate until it will not flame; it chars and smoulders, which is why both pass the flame-spread tests and why the fire departments stopped worrying about it thirty years ago. The one thing to actually check is the clearance: 3 in. off a masonry chimney, with a sheet-metal dam, for either product, and no insulation in contact with a non-IC pot light or knob-and-tube wiring, ever. The products are not the fire risk. The light fixture buried under them is. Pot lights and the hatch cover that.
Mice: the borate in cellulose makes it less pleasant for them, and they still nest in it. Fibreglass they simply prefer; it is warm, it tears into bedding, and a loose-fill attic with a gap at the soffit is a mouse hotel with no front desk. If the attic has a rodent history, the product matters less than the soffit screens. Removal of a contaminated attic is $1.00 a square foot more than clean removal plus a $300 fog, on the removal cost page.
A roof leak into cellulose shows on the ceiling faster because the wet paper packs down and holds the water against the drywall; into fibreglass it drains and spreads. Wet insulation of either kind does not get buried under new insulation; it comes out, the leak gets fixed, and the sheathing gets looked at. That rule is not softened for anybody.
Which one goes where
- Topping up over old cellulose: cellulose. Same product, same behaviour, no argument about which layer settled.
- Topping up over old fibreglass batts or loose fill: either. Cellulose on top of fibreglass is common and works; it adds a little weight on a flat ceiling, which an Ontario truss does not notice. Fibreglass on fibreglass is the lighter attic.
- A low-slope attic with wind at the eaves: cellulose, with baffles and a dam at the plate, because it stays put. Blown fibreglass at a windy eave ends up in a drift against the chimney.
- An attic with any vermiculite in it: nothing goes over it until a lab test says what it is. Test $100 to $200 a sample; if it is positive, licensed abatement before anyone blows anything.
- A house with a bathroom fan ending in the attic: fix the fan first. It will make either product wet within a winter.
What I blow in the attics of the ICF houses I build is cellulose, over a sealed 6 mil poly ceiling, the only vapour barrier in the house. The walls are foam and concrete and need nothing; the attic is the one place a blown product belongs. The R-60 figure and why it is the number are on the attic R-value page.
Questions people ask
Is cellulose or fibreglass better for an attic in Ontario?
Both reach R-60 and both work if the attic is air sealed first. Cellulose costs about $0.25 a square foot less, sits still in wind and needs about 17 in. settled; fibreglass needs 20 to 23 in., does not settle, does not burn and is what mice prefer. Match the product to what is up there and to the eaves.
How much does blown attic insulation cost per square foot?
A cellulose top-up runs $1.50 to $3.50 a square foot depending on how many R-points you add, about $0.065 per point: R-20 to R-60 is $2.60. Fibreglass adds $0.25. From bare joists to R-60 the default is $3.75 for cellulose, before HST, with a $900 minimum job.
Does blown cellulose settle and lose R-value?
Yes, up to 20%, and the bag chart is written to allow for it: the installer blows to an installed depth that settles to the rated depth. Ask for the bag count on the invoice and depth markers at the finished height. Fibreglass settles 2 to 4%.
Can I blow cellulose over existing fibreglass?
Yes, as long as the old insulation is dry, there is no vermiculite in it, and the attic has been air sealed and baffled first. Cellulose over fibreglass is one of the commonest top-ups in Ontario. Wet or mouldy old insulation comes out first, always.
Is cellulose insulation a fire hazard?
No. It is treated with borate and passes the same CAN/ULC flame-spread tests as fibreglass; it chars rather than flames. The fire risk in an attic is the non-IC pot light or the knob-and-tube wiring buried under any insulation, and the chimney without its 3 in. clearance.
Price it both ways
The attic insulation cost calculator takes your attic area, what is up there now and where you want to end up, and prices cellulose and fibreglass side by side with the air sealing, baffles and hatch counted. If you already have quotes that say “blown insulation to R-60” and nothing about the product, the depth or the bag count, the $99 review will tell you which one you are actually being sold.
Sources and further reading
- Keeping the Heat In, Section 5: Roofs and attics: NRCan on checking existing insulation, fixing moisture first, air sealing before insulating, baffles, hatches, chimneys and pot lights.
- Owens Corning Canada: fibreglass batts, blown fibreglass and rigid foam product data: R-value per inch, thickness and coverage.
- Canadian Construction Materials Centre: National Research Council: the product evaluations behind spray foam and other insulation products, and what an evaluation number means.
