How Much Insulation for a 30x40 Shop?
Bags or rolls
10 bags
The four walls of a 30 by 40 shop at 8 foot plate height, 140 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 area1120 sq ft
- Stud or joist spacing16 in
- Coverage per bag or roll106.8 sq ft
Bags or rolls10 bags
| Gross area | 1,120 sq ft |
|---|---|
| Framing factor at 16 in oc | 90% |
| Cavity area to insulate | 1,008 sq ft |
| Bags at 106.8 sq ft | 10 |
| Coverage bought | 1,068 sq ft |
The material list this produces
| Insulation batts, bags | 10 |
|---|---|
| Vapor retarder, sq ft (if required) | 1,120 |
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 | 26 bags | 1,040 sq ft |
| 50 sq ft | 21 bags | 1,050 sq ft |
| 75 sq ft | 14 bags | 1,050 sq ft |
| 90 sq ft | 12 bags | 1,080 sq ft |
| 106.8 sq ft | 10 bags | 1,068 sq ft |
What else matters here
A hundred and forty feet of wall at eight feet is 1,120 square feet gross, 1,008 of cavity, ten bags. Most shops this size have taller walls, so measure the plate height rather than taking the eight.
If the building is metal rather than wood framed, the problem changes from heat to water. A steel panel goes below the dew point on a clear night and condenses whatever moisture the air inside is holding, which then rains off the underside of the roof onto everything below it.
The fix is a continuous vapor barrier on the warm side, with insulation in contact with the panel so there is no air gap for moist air to reach cold steel. Faced fiberglass rolled out under the purlins before the sheeting goes on is the standard method, and a torn or taped-open facing in one place is where the drip will start. A vented ridge and eave, and an under slab vapor barrier if the slab is not yet poured, both matter as much as the R value.
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 30x40 shop?
Ten bags for the four walls at 8 foot plates, based on 1,120 square feet of wall and 1,008 of cavity.
Why does my metal building drip?
Condensation. The steel drops below the dew point and the moisture in the air inside condenses on it. It needs a continuous vapor barrier against the panel, not more heat.
Which side does the vapor barrier go on?
The warm side, in contact with the insulation, with no air gap between the barrier and the cold panel for moist air to travel through.
The insulation calculator takes any dimensions you like and hands back the same list.