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Cellulose Insulation in Ontario: What It Is, the Borate Fire Treatment, How Much It Settles, Loose Fill vs Dense Pack, Where It Belongs and Where It Does Not, and What It Costs Blown In
Insulation & Air Sealing
Cellulose: Yesterday’s Newspaper, Treated So It Will Not Burn, Blown to R-60 for Less Than Anything Else on the House, and Useless the Day It Gets Wet
Cellulose is shredded paper, mostly old newsprint, ground fine and mixed with about 15 to 20% borate minerals so that it will not burn, rot or feed mice. Blown loose over an attic floor it is the cheapest R-value you can buy in Ontario: a 1,200 sq ft attic from R-20 to R-60 is about $3,120 in October 2026. Packed dense into the empty walls of a 1950s house it is the only practical way to insulate them without stripping the plaster. It settles, it hates water, and it makes a cloud of dust that gets into everything, and none of that changes the fact that it is the right product on a flat attic floor nine times out of ten.
The numbers
$2.60per sq ft, attic top-up R-20 to R-60
$3.75per sq ft, bare joists to R-60
$3.25per sq ft of wall, dense pack drill-and-fill
R-3.5per inch, settled, about
| Job | Rate | Notes |
|---|---|---|
| Attic top-up over dry existing insulation | $0.065 per sq ft per R-point added; $1.50 to $3.50; minimum $1.50 | R-20 to R-60 is 40 points, $2.60. R-30 to R-50 is $1.30, so the $1.50 minimum applies. Blown fibreglass adds $0.25. |
| Full attic from bare joists | $3.00 to $4.50, default $3.75 to R-60; $3.25 to R-50 | New build, or after a removal. |
| Dense pack into a closed wall, drill and fill | $2.50 to $4.00 per sq ft of wall, default $3.25 | Plus patching the holes in the siding or the plaster. |
| Dense pack in a cathedral or flat roof | $3.50 to $5.00 per sq ft of roof, default $4.25 | The alternative to closed cell against the sheathing. |
| Material share | about 45% of the installed rate | The rest is the truck, the hose and two men for a morning. |
| Minimum attic job | $900 |
The attic rates with the air sealing, baffles, hatch and removal lines that go with them are on the attic insulation cost page; the attic cost calculator runs all of them together. How much R an Ontario attic should have, and why R-60 is the number, is on the attic R-value page.
What is in the bag
Cellulose is about 80 to 85% recycled paper fibre by weight and 15 to 20% fire retardant. The retardant is what matters. The good products use borates, boric acid and sodium borate, which make the paper char rather than flame, kill mould, discourage insects and mice, and do not corrode wiring or nails. Some cheaper products have used ammonium sulphate instead, which does the fire job but smells of ammonia when damp and corrodes metal over time. Ask for an all-borate product; it is printed on the bag.
The product is tested to CAN/ULC S703, the Canadian standard for cellulose fibre insulation. The R per inch is about R-3.5 settled, a little better than blown fibreglass and a little worse than a mineral wool batt, and it fills every gap around a wire, a pipe or a joist in a way a batt never will. That is the whole of why loose fill beats batts on an attic floor.
Settling: the 20% nobody measures
Loose-fill cellulose settles. Blown to 20 in. on a Tuesday, it will be about 17 in. a year later, and the R-value is the settled R, not the fresh one. Every bag has a chart with two columns, installed thickness and settled thickness, and the installer is meant to blow to the installed number so the settled number meets the target. R-60 wants about 17 in. settled, so it goes in at 20 to 21 in. and the depth markers on the trusses should be read that way. An installer who blows to 17 in. and writes R-60 on the card has given you R-50 by next winter and nobody will ever know, because nobody goes up to look.
The other way to count it is by weight. The bag says how many square feet it covers at a given R, and the empty bags in the driveway are the receipt. Count them. Dense pack does not settle, which is the point of it; packed at about 3.5 lb per cubic foot into a closed wall it is held by its own friction and stays put.
Loose fill, dense pack and where each belongs
Loose fill is the attic product: blown through a 3 in. hose at about 1.5 lb per cubic foot over the ceiling after the air sealing is done, the baffles are in at every rafter bay and the hatch has its box. It goes over dry existing fibreglass or cellulose without any trouble. It does not go over wet or mouldy insulation, which comes out first, and nobody blows anything over vermiculite until the lab test is back; the removal rules, the vacuum truck and the contamination lines are on the attic hub.
Dense pack is the wall product: blown through a 1 to 2 in. tube fed into the bay through a hole, until the machine stalls, at about 3.5 lb per cubic foot. It is how the empty 2×4 walls of a pre-1960 house get insulated: a 2 in. hole per bay, from outside under the siding or inside through the plaster, at $3.25 a square foot of wall plus the patching. A 1,200 sq ft net wall is $3,900 plus holes, and the wall goes from about R-4 to R-11 or so effective. That is the best money on an old house after the attic. Dense pack is also what goes into a double-stud wall on a new house, 12 in. of it for about R-40 effective, and into a vented cathedral where the owner does not want foam.
Where it does not go: against concrete, below grade, in a crawlspace, anywhere it can get wet, anywhere it is left exposed to a room, or in an attic with a bathroom fan venting into it. Cellulose holds water like the paper it is, dries slowly, and the borate does not stop a wet pile from sagging to half its height and staying there. One roof leak over a cellulose attic is a removal job.
A 1,200 sq ft attic, priced
Existing R-20 of old fibreglass, dry, taken to R-60. The top-up is 40 R-points at $0.065, $2.60 a square foot, 1,200 sq ft, $3,120. Air sealing the attic plane first, top plates, stacks, wires, duct boots and pot light covers, 1,200 at $1.00, $1,200. Soffit baffles $300. The hatch insulated and weatherstripped $200. Subtotal $4,820, HST $627, $5,447.
If the house qualifies for the standalone attic stream of the Home Renovation Savings program, existing insulation above R-12 to R-25 taken to R-50 or more, 100% of the roof assembly done, the house occupied more than six months, no previous attic rebate, and the work done by a contractor on the program’s participating list, up to $1,000 comes back afterward: 50% of the after-tax cost is $2,723, capped at $1,000. Contractor enrolment is closed, so an installer who is not on the list cannot get you the rebate however good he is, and funding has been reported as running through November 30, 2026 with no published end date. Check the program site before you sign anything; the mechanics are on the program page.
Rule of thumb: air seal first, then blow. Cellulose is not an air barrier, however deep it is. Blown over an unsealed ceiling it is a very thick filter with warm house air going through it to the roof deck, which is how attics grow frost and roofs grow ice dams.
What a good installer does
- Walks the attic first and comes down with a list: wet spots, vermiculite, knob-and-tube, non-IC pot lights, the bathroom fan that ends in the insulation, the chimney with no clearance. Any of those stops the job until it is dealt with.
- Seals before blowing. A separate line on the quote, $0.75 to $1.50 a square foot; if it is missing, ask why.
- Baffles at every rafter bay and a dam around the hatch, so the soffit vents stay open.
- Depth markers every 300 sq ft, visible from the hatch, marked at the installed depth from the bag chart, and blown evenly over the plates to the baffles, not in a pile under the hatch.
- Leaves the bags and a card at the hatch with the product, the date, the installed depth, the settled R and the bag count.
Questions people ask
Is cellulose insulation a fire hazard?
Treated cellulose chars and self-extinguishes rather than flaming; it is tested to CAN/ULC S703 and carries a flame-spread rating under 25. It still must keep the 3 in. (75 mm) clearance from a chimney and must not bury a non-IC pot light or knob-and-tube wiring, the same as any insulation.
How much does cellulose settle?
Loose fill in an attic settles about 20% in the first year or two. R-60 wants about 17 in. settled, so it is blown to 20 to 21 in. Dense pack in a wall, at about 3.5 lb per cubic foot, does not settle.
Is cellulose better than blown fibreglass?
A little more R per inch, better at filling gaps, better at slowing air movement through it, and about $0.25 a square foot cheaper. Fibreglass is lighter and does not hold water. On a dry attic floor over a sealed ceiling, cellulose; over a ceiling that may get damp from below, fibreglass.
Can cellulose be blown into existing walls?
Yes, dense packed through a 2 in. hole per bay from inside or out, at $2.50 to $4.00 a square foot of wall plus patching. It is the usual way to insulate the empty walls of a pre-1960 house without removing the plaster.
What does cellulose attic insulation cost in Ontario?
About $0.065 a square foot for each R-point added as a top-up, so $2.60 a square foot from R-20 to R-60, or $3.75 a square foot from bare joists to R-60. A 1,200 sq ft attic with air sealing, baffles and the hatch is about $4,820 before HST.
Price the attic, and the spot where cellulose will not go
Where the roof has no attic, a cathedral or a flat section, cellulose dense pack and closed cell foam against the sheathing are the two choices, and the spray foam cost calculator prices the foam side at its real thickness so the $4.25 a square foot for dense pack has something to stand against. If you have three attic quotes with three different depths for “R-60”, the $99 review will read the bag charts for you.
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.
- Home Renovation Savings: attic insulation without an assessment: the standalone attic rebate: starting and target R-values, caps, the participating-contractor rule and who is excluded.
- Ontario’s Building Code: the official page with the current regulation; vapour barriers are 9.25, attic ventilation 9.19, thermal barriers over foam 9.10.17, radon 9.13.
