How Much Insulation for a 1,500 Square Foot Attic?
Bags or rolls
14 bags
A 1,500 square foot attic floor, joists at 24 inches on center, with 106.8 square foot bags.
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How that number is worked out
These are the figures this page was built with and every line the calculator produces from them. Change anything above and all of it moves.
- Wall or ceiling area1500 sq ft
- Stud or joist spacing24 in
- Coverage per bag or roll106.8 sq ft
Bags or rolls14 bags
| Gross area | 1,500 sq ft |
|---|---|
| Framing factor at 24 in oc | 93% |
| Cavity area to insulate | 1,395 sq ft |
| Bags at 106.8 sq ft | 14 |
| Coverage bought | 1,495 sq ft |
The material list this produces
| Insulation batts, bags | 14 |
|---|---|
| Vapor retarder, sq ft (if required) | 1,500 |
Coverage per bag compared
Bag coverage is the number that decides this, and it is printed on the bag. Here is the same area at five common coverages.
| Coverage per bag | Bags or rolls | Coverage bought |
|---|---|---|
| 40 sq ft | 35 bags | 1,400 sq ft |
| 50 sq ft | 28 bags | 1,400 sq ft |
| 75 sq ft | 19 bags | 1,425 sq ft |
| 90 sq ft | 16 bags | 1,440 sq ft |
| 106.8 sq ft | 14 bags | 1,495 sq ft |
What else matters here
Fifteen hundred square feet at 24 inch centers is 1,395 square feet of cavity and fourteen bags. That is the whole ceiling of a modest single story house.
The mistake that costs the most here is pushing insulation out over the top plate and into the eaves. It feels like the thorough thing to do and it blocks the soffit vents, which stops the airflow that keeps the roof deck dry and cold. A blocked soffit in a cold climate is how ice dams form, and in any climate it is how a roof deck rots.
Install a baffle in every rafter bay at the eave before the insulation goes in. It holds a clear air channel from the soffit up to the ridge and lets you bring the insulation right to the top plate without sealing the vent. The pair of them, baffle plus full depth insulation at the edge, is worth more than a higher R value in the middle of the attic.
What this calculator assumes
- Framing takes up roughly 10 percent of a wall at 16 inch on center and 7 percent at 24. Cavity area is reduced accordingly, so you do not buy insulation for the studs.
- Bag coverage varies a lot by R value and width. The default is a common R-13 kraft faced batt bag; read your own bag and change it.
Questions people ask
How many bags of insulation for a 1,500 square foot attic?
Fourteen bags at 24 inch joist spacing, which leaves 1,395 square feet of cavity after the framing factor.
Can insulation block soffit vents?
Easily, and it is a common mistake. Blocked soffits stop the airflow that keeps the roof deck dry, which causes ice dams in cold climates and rot anywhere.
What is an attic baffle for?
It holds an open air channel from the soffit vent up past the insulation, so you can insulate fully at the eave without closing off the ventilation.
The insulation calculator takes any dimensions you like and hands back the same list.