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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Enter your attic footprint and what's already up there. This works out the extra depth needed to reach your target R-value, how many bags or batts that takes, and what it should cost.
With these inputs:
| Result | Value |
|---|---|
| Additional depth to add | 20 in |
| Target R-value | R-60 |
| Existing R-value | R-10 |
| Bags required | 34 bags |
| Estimated material cost | $1,428 |
| Installed cost (pro) | $2,448 |
Existing R = existing depth × R-per-inch of what's up there · R needed = target R − existing R · Depth to add = R needed ÷ R-per-inch of new material · Bags = (R needed × area) ÷ bag coverage rating, then add waste
Yes, as long as the old material is dry and not mouldy. Never lay faced (paper or foil backed) batts over existing insulation — the facing traps moisture. Use unfaced batts or blown-in.
About 19–20 inches of blown fiberglass (at R-2.5 per inch), about 14 inches of cellulose, or 13 inches of high-density batts. Loose-fill bags list a coverage chart on the side — follow it.
Usually yes, and it is the cheapest upgrade available. Blown-in or unfaced batts go straight over existing material. The one thing not to do is lay faced batts on top of existing insulation with the facing down, because that puts a vapour retarder in the middle of the assembly where moisture can collect against it. Use unfaced material when topping up.
Divide your target R-value by the material's R per inch. At R-49 that is roughly 13.5–14 inches of cellulose or about 19.5 inches of blown fibreglass — the difference is why the same target produces very different depths. Mark the depth on a rafter before you start so you can see when you have reached it.
The hatch yes, with a rigid box or a purpose-made cover, because an uninsulated hatch is a large hole in the middle of the ceiling. Recessed lights need care: only IC-rated fixtures may be covered, and anything not rated for contact needs a clearance kept around it. Burying a non-IC fixture is a genuine fire risk, not a code technicality.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.