How Much Insulation for a 16x20 Room?
Bags or rolls
5 bags
The four walls of a 16 by 20 room at 8 foot ceiling height, 72 linear feet of wall, framed at 16 inches on center.
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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 area576 sq ft
- Stud or joist spacing16 in
- Coverage per bag or roll106.8 sq ft
Bags or rolls5 bags
| Gross area | 576 sq ft |
|---|---|
| Framing factor at 16 in oc | 90% |
| Cavity area to insulate | 518 sq ft |
| Bags at 106.8 sq ft | 5 |
| Coverage bought | 534 sq ft |
The material list this produces
| Insulation batts, bags | 5 |
|---|---|
| Vapor retarder, sq ft (if required) | 576 |
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 | 13 bags | 520 sq ft |
| 50 sq ft | 11 bags | 550 sq ft |
| 75 sq ft | 7 bags | 525 sq ft |
| 90 sq ft | 6 bags | 540 sq ft |
| 106.8 sq ft | 5 bags | 534 sq ft |
What else matters here
Seventy two feet of wall at eight feet is 576 square feet gross, 518 of cavity, five bags.
If this room is a finished basement, the assembly matters more than the count. A concrete foundation wall is damp most of the year, and a fiberglass batt pressed against it holds that moisture against wood framing in the dark. That is the classic mold sandwich, and it is why so many finished basements smell.
The accepted fix is to put a layer of rigid foam directly against the concrete first, taped at the seams, then frame in front of it and insulate the cavity if more R value is wanted. The foam keeps the framing above the dew point, which is the whole point, and it means no poly vapor barrier on the inside face where it would trap moisture the other way.
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 16x20 room?
Five bags for all four walls at 8 feet high, based on 576 square feet of wall and 518 of cavity.
Can I put fiberglass batts against a basement wall?
Not directly. Concrete is damp and a batt against it holds moisture on the framing. Put rigid foam against the concrete first, then frame in front of it.
Why is the cavity area less than the wall area?
Because studs and plates take up part of the wall and they are not insulated. Framing runs about 10 percent of a wall at 16 inch on center and about 7 percent at 24 inch, so the calculator takes that off before dividing.
The insulation calculator takes any dimensions you like and hands back the same list.