Insulation Calculator
Batts or rolls for a wall or ceiling, with both the openings and the framing deducted — studs are not insulated, and at sixteen-inch centres they are about a tenth of the wall.
Insulation needed
5bags
- Gross area
- 416 sq ft
- Cavity area
- 335 sq ft
- Framing loss
- 10%
- Bags or rolls
- 5
- Square metres
- 31.1
- Doors and windows are subtracted before the framing deduction.
- Studs at 16 inch centres take up about 10% of the surface and are not insulated, so that is deducted.
- Coverage per bag is printed on the packaging and varies by R-value — a higher R-value batt is thicker, so a bag covers less. Check yours rather than trusting the default.
- Buy batts sized to your stud spacing. A 16in batt stuffed into a 24in bay leaves gaps, and gaps cost more performance than the R-value difference.
About the insulation calculator
Insulation fills the cavities between studs, not the whole wall, and the framing takes up more of the surface than people expect. At sixteen-inch centres it is roughly ten per cent once you count plates, headers and the doubled studs at corners and openings. At twenty-four-inch centres it is nearer seven.
So the order is: gross area, minus doors and windows, minus the framing fraction. What is left is the cavity you are actually filling, and ordering on gross area is how people end up with spare bags.
Coverage per bag moves more than you might think, because a higher R-value batt is thicker and fewer fit in a bag. A bag of R-13 might cover a hundred square feet where the same size bag of R-21 covers fifty, so the figure on the packaging is the one to use.
What it works out
- Bags or rolls at your own coverage figure
- Gross area, cavity area and the framing deduction
- Walls or ceilings, at 16 or 24 inch centres
The formula
Bags = ((Area − Openings) × (1 − Framing)) ÷ Coverage per bag
Insulation goes in the cavities between the studs, not across the whole wall, and the studs themselves take up more of the surface than people expect. At sixteen-inch centres framing is roughly ten per cent of a wall once you count the plates, headers and the doubled studs at corners and openings. At twenty-four-inch centres it is nearer seven.
So the sequence is: work out the gross area, take off the doors and windows, then take off the framing fraction. What is left is the cavity area you are actually filling.
Coverage per bag is the other variable, and it moves more than you might think. A bag of R-13 covers considerably more wall than a bag of R-21, because the higher R-value batt is thicker and fewer fit in the bag. The figure is on the packaging.
One thing the arithmetic will not tell you: buy batts sized for your stud spacing. A 16-inch batt pushed into a 24-inch bay leaves gaps down both sides, and gaps cost far more performance than the difference between two adjacent R-values.
- Area
- Wall area is perimeter times height; a ceiling is length times width.
- Openings
- Doors and windows, subtracted before the framing deduction.
- Framing
- The share of the surface taken up by studs and plates — about 10% at 16in centres, 7% at 24in.
- Coverage
- Square feet per bag, from the packaging. Falls as R-value rises.
A worked example
The walls of a fourteen by twelve room, eight feet high, studs at sixteen inches, one door and two windows.
That works out to 5 bags.
- Gross area
- 416 sq ft
- Cavity area
- 335 sq ft
- Framing loss
- 10%
- Bags or rolls
- 5
- Square metres
- 31.1
Questions
Should I subtract the studs when calculating insulation?
Yes. Studs, plates and headers are not insulated, and at sixteen-inch centres they account for around ten per cent of a wall. Ordering on gross area leaves you with spare bags.
How much insulation do I need for a 12x14 room?
The walls come to about 416 square feet gross with eight-foot ceilings. Less a door, two windows and the framing, you are insulating roughly 335 square feet — about five bags at 75 square feet a bag.
What R-value do I need?
It depends on your climate and which part of the building you are insulating, and recommendations differ by region. Check the current guidance for your area rather than a general figure, since walls, floors and attics all differ.
Why does coverage per bag change so much?
Because a higher R-value batt is physically thicker, so fewer fit in a bag. A bag of R-13 might cover 100 square feet where the same size bag of R-21 covers 50. Always read the packaging.
Do I need 16 or 24 inch batts?
Whichever matches your framing. Batts are made in both widths, and using the wrong one leaves gaps along the studs. Gaps and compression are the two things that most reduce real-world performance.
Does this cover blown-in insulation?
No. Blown-in loose fill is sold by the bag to a target depth over an area, which is a different calculation — the coverage figure on the bag is given per depth, so work from that instead.