Quick answer: For most open Ontario attics, blown cellulose or blown fiberglass is the practical choice: both are relatively low cost and easily reach R-50 or R-60 with enough depth. Spray foam costs more but also air seals, which makes it a strong fit for cathedral ceilings, flat roofs and leaky spots where depth is limited. Any of the three can qualify for the standalone attic rebate of up to $1,250 as long as the target R-value is met, and amounts can change.
Updated November 2026
On this page
- Key takeaways
- How do cellulose, fiberglass and spray foam compare?
- What is the math behind “inches needed”?
- What are the pros and cons of blown cellulose?
- What are the pros and cons of blown fiberglass?
- What are the pros and cons of spray foam?
- Why does air sealing matter as much as the material?
- Which should you choose for your Ontario attic?
- Does the material affect your rebate?
- How do you compare material quotes fairly?
- Frequently asked questions
- Get a free attic assessment
Key takeaways
- Cellulose gives about R-3.5 per inch, so R-50 takes about 14.3 inches.
- Blown fiberglass gives about R-2.5 to R-3 per inch, so R-50 takes about 17 to 20 inches.
- Closed-cell spray foam gives about R-6 to R-7 per inch and air seals, but costs the most.
- The rebate does not favour any material. It only cares that you reach the target R-value with a participating contractor. See current rebates.
- Many good jobs combine materials: spray foam or sealant for air leaks, loose fill for depth.
How do cellulose, fiberglass and spray foam compare?
R-values per inch are general industry figures. Actual values depend on the product and how it is installed.
| Factor | Blown cellulose | Blown fiberglass | Open-cell spray foam | Closed-cell spray foam |
|---|---|---|---|---|
| R-value per inch (approx.) | about R-3.5 | about R-2.5 to R-3 | about R-3.5 to R-3.7 | about R-6 to R-7 |
| Inches for R-50 | 50 ÷ 3.5 = about 14.3 | 50 ÷ 3 to 50 ÷ 2.5 = about 17 to 20 | 50 ÷ 3.7 to 50 ÷ 3.5 = about 13.5 to 14.3 | 50 ÷ 7 to 50 ÷ 6 = about 7.1 to 8.3 |
| Inches for R-60 | 60 ÷ 3.5 = about 17.1 | 60 ÷ 3 to 60 ÷ 2.5 = about 20 to 24 | 60 ÷ 3.7 to 60 ÷ 3.5 = about 16.2 to 17.1 | 60 ÷ 7 to 60 ÷ 6 = about 8.6 to 10 |
| Air sealing ability | Slows air a little; not an air seal | Does not stop air movement | Air seals well | Air seals well |
| Moisture behaviour | Absorbs moisture; can dry out, but stays wet if a leak continues | Does not absorb water, but wet or wind-washed fibres lose performance | Lets vapour pass; can hold water if soaked | Acts as a vapour retarder; resists water |
| Settling | Settles some; installers blow extra to reach settled depth | Settles less than cellulose | Does not settle | Does not settle |
| Best use | Open attic floors, topping up old insulation | Open attic floors, topping up old insulation | Roof decks, cathedral ceilings, sound control | Cathedral and flat roofs, rim joists, tight spaces |
| Relative cost | Low | Low | Medium | High |
| Rebate eligible? | Yes, if target R-value met | Yes, if target R-value met | Yes, if target R-value met | Yes, if target R-value met |
What is the math behind “inches needed”?
The rule is simple: inches needed = target R-value ÷ R-value per inch.
- Cellulose to R-50: 50 ÷ 3.5 = 14.29, so about 14.3 inches.
- Fiberglass to R-50: 50 ÷ 3 = 16.7 and 50 ÷ 2.5 = 20, so about 17 to 20 inches.
- Closed-cell to R-50: 50 ÷ 7 = 7.1 and 50 ÷ 6 = 8.3, so about 7.1 to 8.3 inches.
- Cellulose to R-60: 60 ÷ 3.5 = 17.1 inches.
- Fiberglass to R-60: 60 ÷ 3 = 20 and 60 ÷ 2.5 = 24, so about 20 to 24 inches.
If you already have insulation, subtract what is there. For example, if you have about R-14 now (4 inches of cellulose), you need about R-36 more to reach R-50. In cellulose that is 36 ÷ 3.5 = about 10.3 more inches.
To work out your starting point, use our guide on how to check your attic insulation R-value.
What are the pros and cons of blown cellulose?
Pros:
- Good R-value per inch for a loose fill, about R-3.5.
- Dense, so it fills around wires and framing well.
- Relatively low cost and quick to install.
- Works well as a top-up over existing dry insulation.
Cons:
- Heavier than fiberglass, which matters on older or weak ceilings.
- Settles over time, so the installer must blow enough to reach the settled depth.
- Absorbs water if there is a roof leak or ongoing condensation.
- It is not an air barrier. You still need air sealing.
What are the pros and cons of blown fiberglass?
Pros:
- Light, so it adds little weight to the ceiling.
- Does not absorb water the way cellulose does.
- Settles less than cellulose.
- Relatively low cost.
Cons:
- Lower R-value per inch, so you need more depth for the same result.
- Air can move through it more easily, which can reduce real-world performance in very cold, windy conditions.
- Needs proper baffles at the eaves to keep wind from blowing through it.
- It is not an air barrier either.
What are the pros and cons of spray foam?
Pros:
- Air seals as it insulates, which deals with leaks and insulation in one step.
- Closed-cell gives the most R-value per inch, about R-6 to R-7, useful where space is tight.
- Does not settle.
- Closed-cell resists moisture and acts as a vapour retarder.
Cons:
- Highest relative cost of the three.
- Hard to remove or change later.
- Needs a trained installer, proper mixing and ventilation during install.
- Hides wiring and framing, which can make future repairs harder.
- Rarely the most cost-effective way to add depth on a big, open attic floor.
Read more on our spray foam insulation service page.
Why does air sealing matter as much as the material?
Air leakage represents about 20–30% of heat lost from an older home (Natural Resources Canada). Loose fill slows heat moving through the ceiling, but it does very little to stop warm air leaking up through gaps around pot lights, top plates, plumbing stacks and the hatch.
One small air leak can let in 100 times more moisture than diffusion through the same area (Natural Resources Canada). That moisture can condense in a cold attic. So for most homes, the right answer is not just “which material” but “seal first, then insulate.”
A common approach is to seal leaks with foam or sealant, then add cellulose or fiberglass on top. That gives you the air sealing benefit of foam without the cost of foaming the whole attic.
Which should you choose for your Ontario attic?
Use this decision framework. Find the situation closest to yours.
| Your situation | Usually the best fit | Why |
|---|---|---|
| Open attic, plenty of headroom, dry existing insulation | Blown cellulose or blown fiberglass | Lowest cost path to R-50 or R-60 |
| Older home with a weak or sagging ceiling | Blown fiberglass | Lighter weight |
| Attic with a history of roof leaks (now repaired) | Blown fiberglass, after a moisture check | Does not hold water like cellulose |
| Many air leaks around lights and top plates | Air seal first, then cellulose or fiberglass | Fixes leaks without full foam cost |
| Cathedral or flat roof with little space | Closed-cell spray foam or dense-pack | Gets the most R-value per inch |
| Rim joists, knee walls, tight corners | Spray foam | Seals and insulates in awkward spaces |
| Old insulation is soiled, mouldy or damp | Remove first, then any material | Do not bury a problem |
| Suspected vermiculite | Stop, test first | Health Canada advises not disturbing it |
Quick rules:
- If depth is easy, go loose fill. Cellulose and fiberglass are the usual best value for open attics.
- If depth is limited, think spray foam. Cathedral ceilings and flat roofs are where closed-cell earns its cost. For R-28 on a cathedral roof, that is about 28 ÷ 7 to 28 ÷ 6 = 4 to 4.7 inches of closed-cell.
- Always plan air sealing. Whatever material you pick, ask for an air sealing scope.
- Pick the installer, not just the product. A well-installed loose fill job usually beats a rushed one in any material.
For ceilings without an open attic, see dense-pack wall and ceiling insulation.
Does the material affect your rebate?
No. Under the Home Renovation Savings Program standalone attic path, any insulation type can be used as long as the target R-value is reached and the work is done by a participating contractor. The rebate is 50% of your after-tax insulation cost, up to the tier maximum.
| Attic before upgrade | Upgrade to at least | Maximum rebate |
|---|---|---|
| R-12 or less | R-50 | up to $1,250 |
| Above R-12 to R-25 | R-50 | up to $1,000 |
| Above R-25 to R-35 | R-50 | up to $800 |
| Cathedral/flat roof, R-25 or less | R-28 | up to $750 |
| Cathedral/flat roof, R-12 or less | R-20 | up to $650 |
The program is currently scheduled to run until March 31, 2027, is first-come, first-served, and amounts can change. Because the rebate is capped, a more expensive material does not earn a bigger rebate once you pass the cap. Learn more on the Ontario attic insulation rebate page and in our Ontario attic rebate study.
How do you compare material quotes fairly?
Ask each contractor for the same things in writing: target R-value, material, installed depth, air sealing scope, baffles and hatch work. Our insulation quote comparison template lays this out side by side. Then run through the attic insulation checklist before install day.
Frequently asked questions
Is cellulose or fiberglass better for an Ontario attic?
Both work well in open attics. Cellulose gives more R-value per inch (about R-3.5 versus about R-2.5 to R-3), so you need less depth. Fiberglass is lighter and does not absorb water, which helps on weak ceilings or attics with a past leak.
Is spray foam worth the extra cost in an attic?
It depends on the space. On a large open attic floor, loose fill usually reaches R-50 for much less. Spray foam is often worth it for cathedral ceilings, flat roofs, rim joists and spots where you need air sealing and high R-value in little space.
How many inches of cellulose do I need for R-60?
About 17.1 inches, because 60 ÷ 3.5 = 17.1. The installer should blow enough that it still measures that depth after settling.
Can I mix insulation types in my attic?
Yes. Adding cellulose or fiberglass over existing dry insulation is common, and so is using spray foam for air sealing with loose fill on top. Add the R-value of each layer to find your total.
Do all insulation types qualify for the Ontario attic rebate?
Yes. The standalone attic path allows any insulation type as long as you reach the target R-value and use a participating contractor. Rebate amounts can change, so check our rebates page.
Get a free attic assessment
Not sure which material fits your attic? Pro Home Insulation will look at your depth, air leaks, ceiling and roof type, and explain the options honestly, including when the lowest-cost one is the right one. Call (647) 250-0606 or Book online.