Insulation Coverage Chart: Bags and Rolls by Area
Fourteen wall and ceiling areas against six package coverages, and the cavity area that is actually left once the framing is taken out of the gross square footage.
The number on the bag is the square footage it covers, and the number you measured is the gross area of the wall. They are not the same thing, because between eight and ten percent of that wall is stud rather than cavity.
That gap is small enough to ignore on a closet and large enough to matter on a house. On 3,000 square feet of wall at 16 inches on center it is 300 square feet of framing you are not insulating, or three to four rolls of material you would otherwise have bought.
Bags or rolls by area and package coverage
| Wall or ceiling area | 40 sq ft | 50 sq ft | 60 sq ft | 75 sq ft | 88 sq ft | 107 sq ft |
|---|---|---|---|---|---|---|
| 100 sq ft | 3 | 2 | 2 | 2 | 2 | 1 |
| 200 sq ft | 5 | 4 | 3 | 3 | 3 | 2 |
| 300 sq ft | 7 | 6 | 5 | 4 | 4 | 3 |
| 400 sq ft | 9 | 8 | 6 | 5 | 5 | 4 |
| 500 sq ft | 12 | 9 | 8 | 6 | 6 | 5 |
| 600 sq ft | 14 | 11 | 9 | 8 | 7 | 6 |
| 800 sq ft | 18 | 15 | 12 | 10 | 9 | 7 |
| 1,000 sq ft | 23 | 18 | 15 | 12 | 11 | 9 |
| 1,200 sq ft | 27 | 22 | 18 | 15 | 13 | 11 |
| 1,500 sq ft | 34 | 27 | 23 | 18 | 16 | 13 |
| 1,800 sq ft | 41 | 33 | 27 | 22 | 19 | 16 |
| 2,000 sq ft | 45 | 36 | 30 | 24 | 21 | 17 |
| 2,500 sq ft | 57 | 45 | 38 | 30 | 26 | 22 |
| 3,000 sq ft | 68 | 54 | 45 | 36 | 31 | 26 |
Cavity area after framing, 16 inch against 24 inch centers
| Gross area | Cavity at 16 in oc | Cavity at 24 in oc | Rolls at 16 in | Rolls at 24 in |
|---|---|---|---|---|
| 100 sq ft | 90 | 93 | 2 | 2 |
| 200 sq ft | 180 | 186 | 3 | 3 |
| 300 sq ft | 270 | 279 | 4 | 4 |
| 400 sq ft | 360 | 372 | 5 | 5 |
| 500 sq ft | 450 | 465 | 6 | 6 |
| 600 sq ft | 540 | 558 | 7 | 7 |
| 800 sq ft | 720 | 744 | 9 | 9 |
| 1,000 sq ft | 900 | 930 | 11 | 11 |
| 1,200 sq ft | 1,080 | 1,116 | 13 | 13 |
| 1,500 sq ft | 1,350 | 1,395 | 16 | 16 |
| 1,800 sq ft | 1,620 | 1,674 | 19 | 20 |
| 2,000 sq ft | 1,800 | 1,860 | 21 | 22 |
| 2,500 sq ft | 2,250 | 2,325 | 26 | 27 |
| 3,000 sq ft | 2,700 | 2,790 | 31 | 32 |
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The insulation calculator takes your own measurements and returns the number for your job, plus the material list as a printable document. Every cell in the table above came out of that same calculator, run at build time.
Framing factor, and why the cheap estimate is short
A wall framed at 16 inches on center is about ten percent lumber once the studs, plates, headers, corners and channels are counted, so the insulated cavity is about ninety percent of the gross. At 24 inches on center it is closer to seven percent, so the cavity is about ninety three.
Estimating the gross area and buying exactly that much is the usual mistake in the other direction: it overbuys by roughly a roll in ten and leaves a partial bale in the garage. Which way you want to be wrong is a real decision, and running short on a Saturday is usually the worse one.
R value is set by the cavity, not by ambition
A 2 x 4 wall holds R-13 or R-15. A 2 x 6 wall holds R-19 or R-21. Compressing a thicker batt into a thinner cavity does not give the printed R value, it gives a lower one, because the loft is the insulation and squashing it out is the same as removing it.
Attics are different, because there is no cavity depth limit. Blown insulation there is specified by depth to a target R value, and the bag says how many square feet it covers at each depth, which is a different number on every line of the chart printed on the bag.
Fit it to the cavity or do not bother
A batt that is cut short, stuffed behind a wire, or left with a gap at the top of the cavity loses a disproportionate amount of its value, because air moving through the gap defeats the whole assembly rather than a proportional part of it. Split batts around wiring instead of compressing them behind it.
The same applies to the small pieces. Around windows, at corners and in narrow bays, cut to fit rather than folding, and that is where the extra percent of material in the order goes.
Air sealing first, insulation second
Insulation slows conduction. It does very little about air moving through a gap, and in most older houses the air leakage is the bigger number. Sealing top plates, penetrations, rim joists and can lights before insulating usually returns more than adding depth does.
This matters for estimating because the sealing materials are a separate line item that is easy to leave off a bid, and a crew that has to stop and buy foam has already lost the morning.
Questions people ask
How many rolls of insulation do I need for 1,000 square feet?
Eleven rolls at 88 square feet a roll for a wall framed at 16 inches on center, because 900 of the 1,000 square feet is cavity rather than framing and 88 does not divide into 900.
What is the framing factor for insulation?
About ten percent at 16 inches on center and about seven percent at 24 inches, once studs, plates, headers and corners are counted. That is the part of the wall you are not insulating.
How much does a bag of insulation cover?
It is printed on the bag and it changes with the R value. Common batt packages run from about 40 square feet for a high R value up to about 107 for an R-13. Blown insulation is quoted at several depths on the same bag.
Can I put R-19 in a 2 x 4 wall?
You can fit it in, but you will not get R-19 out of it. Compressing a batt reduces its value, so a 2 x 4 wall gets R-13 or R-15 and that is the ceiling for that cavity.
Should I buy extra insulation?
A few percent, yes. The cut pieces around windows, corners and narrow bays waste more than a flat area calculation shows, and an extra bale is cheaper than a second trip.