Quick answerMost Ontario attics should be insulated to R-50, which is about 14 inches of settled blown cellulose or 18–20 inches of blown fiberglass. Many older GTA homes have R-12 to R-20 today.
Attic insulation depth chart
| Depth you measure | Approx. R-value (cellulose) | Approx. R-value (fiberglass) | What it means |
|---|---|---|---|
| 3–4 in | R-11 to R-15 | R-8 to R-11 | Badly under-insulated |
| 6 in | ≈ R-22 | ≈ R-16 | Common in older GTA homes |
| 10 in | ≈ R-36 | ≈ R-27 | Worth topping up |
| 14 in | ≈ R-50 | ≈ R-38 | Target for cellulose |
| 18–20 in | R-60+ | ≈ R-50 | Target for fiberglass |
Approximate values using R-3.6/in for settled cellulose and R-2.7/in for blown fiberglass. Bag coverage charts govern the actual install.
How to measure your attic yourself
- Open the hatch with a flashlight and a ruler or tape measure. Don't step off the joists.
- Push the ruler straight down to the drywall in two or three spots you can reach.
- Compare to the chart. If you can see the tops of the joists, you're under about R-20.
- Look for dark staining on the roof boards above, which is a sign of a moisture problem to fix before insulating.
Why insulation level alone isn't the answer
R-value only works if air isn't leaking past it and moisture can escape. That's why every job we do pairs the top-up with air sealing and baffles.
Guide FAQ
How many inches of insulation should be in my attic?
For R-50: about 14 inches of blown cellulose or 18–20 inches of blown fiberglass, measured after settling.
How do I check my attic insulation depth?
From the hatch, push a ruler straight down through the insulation to the ceiling drywall in a few spots away from the hatch. If you can see the tops of the joists, you have less than about R-20.
Is R-60 better than R-50?
R-60 adds some benefit but with diminishing returns. For most existing GTA attics, R-50 with good air sealing and ventilation is the best value.