Insulation Calculator
Work out the insulation for walls or an attic: batts per stud bay and packages, or how deep to blow cellulose or fibreglass to reach a target R-value over what is already there, and how many bags.
How to Use
- Choose Walls for batts between studs, or Attic to bring an attic floor up to a target R-value.
- For walls, enter the total wall length and height, the openings, the stud spacing, and the batt R-value; enter the square feet per package from the label.
- For an attic, enter the floor area, the depth and type of insulation already there, and the R-value you want.
- Choose blown cellulose, blown fibreglass or batts, and the bag weight from the bag.
- Read the depth to add, the bags or batts, and check the coverage chart printed on the bag before you buy.
Worked Example
An attic of 1,000 ft² with 3.5 in of old fibreglass batts, brought up to R-49 with blown cellulose. The old batts give about 3.5 × 3.2 = R-11.2. Still needed: 49 − 11.2 = R-37.8. Cellulose gives about R-3.6 per inch, so 37.8 ÷ 3.6 = 10.5 in more. Volume: 1,000 × 10.5 ÷ 12 = 875 ft³. At about 1.6 lb/ft³ settled that is 1,400 lb, so 56 bags of 25 lb.
Walls: 120 ft of 8 ft wall at 16 in centres, less 120 ft² of doors and windows. The net wall is 960 − 120 = 840 ft². A 15 × 93 in batt covers 9.69 ft², so 840 ÷ 9.69 = 86.7; add 5% for cutting around openings and that is 92 batts.
The common mistake: counting the old insulation as nothing, or as everything. Ignoring the 3.5 in of old batts orders 3 in more cellulose than needed (about 16 extra bags); counting it at a modern R-value when it is flattened or damp leaves the attic short. Measure its depth, and check it is dry and fluffy.
Show Work
Formulas
What R-Value Measures
R-value is resistance to heat flow: the higher it is, the slower heat leaks through. Layers add up, so R-11 of old batts plus R-38 of new cellulose make R-49. US R-values are in ft²·°F·h/BTU; the metric RSI, used in Canada, Europe and Australia, is in m²·K/W and is 0.176 times as large, so R-38 is about RSI 6.7.
Most insulation works by trapping still air in fine fibres or particles; it is the air, not the fibre, that insulates. That is why compressing a batt, letting it get wet, or leaving gaps for air to move through cuts its performance so much. Cellulose is mostly recycled newspaper treated with borate fire retardant; fibreglass is spun from molten glass, much of it recycled.
About This Calculator
For attics, this calculator credits the insulation already there, works out how deep to add blown cellulose, blown fibreglass or batts to reach your target R-value, and estimates the bags from the area, depth, settled density and bag weight. For walls, it counts batts by stud bay and packages by the coverage on the label, and warns when a batt is too thick for the studs.
Everything runs in your browser; nothing is sent anywhere. Bag counts are estimates; the coverage chart printed on the bag is the figure to buy by, because densities differ between brands.
Related calculators: BTU Calculator, Framing Calculator, and Drywall Calculator.
Frequently Asked Questions
What R-value do I need in my attic?
In the US, ENERGY STAR recommends R-30 to R-49 in the warmest zones and R-49 to R-60 in most of the country, which is roughly 14 to 24 inches of blown insulation, depending on the material. Building codes set the minimum for new homes; topping up an older attic to these levels is one of the cheapest energy upgrades.
How deep does insulation need to be for R-38 or R-49?
Divide the R-value by the R per inch of the material. Cellulose is about R-3.6 per inch, so R-38 is about 10.6 in and R-49 about 13.6 in. Loose-fill fibreglass is about R-2.5 per inch: R-38 is about 15 in. Fibreglass batts are about R-3.2 per inch, so an R-30 batt is about 9.5 in thick.
How many bags of blown-in insulation do I need?
Multiply the area by the depth to get the volume, then by the settled density (about 1.6 lb per cubic foot for cellulose, 0.6 for loose fibreglass) and divide by the bag weight. 1,000 ft² of cellulose to R-38 is about 56 bags of 25 lb. In the US, the FTC R-value rule requires every bag to carry a coverage chart; buy by that chart for your brand.
Can I put new insulation on top of old?
Usually, yes, if the old insulation is dry and not mouldy or rodent-damaged. Add unfaced batts or loose fill on top; do not add another vapour retarder, and lay new batts across the joists to cover the tops of the framing. Seal air leaks around lights, pipes and the attic hatch first.
What happens if a batt is too thick for the wall?
It gets compressed, and compressed fibreglass loses R-value: an R-19 batt (6¼ in) squeezed into a 3½ in 2×4 wall gives only about R-13 to R-14. Use R-13 or high-density R-15 batts in 2×4 walls, and R-19 or R-21 in 2×6 walls.
How do I use the Insulation Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Is it free? Does it work without internet?
Yes to both. It is free with no sign-up, and once the page has loaded it keeps working even with no internet.
Where does my data go?
Nowhere — every calculation runs on your own device. Nothing you enter is uploaded, logged, or stored.
Common Use Cases
Attic top-up
1,000 ft² with 3.5 in of old batts (R-11) brought to R-49: 10.5 in of cellulose, about 56 bags of 25 lb.
Bare attic
1,200 ft² from nothing to R-38 in loose fibreglass: 15.2 in deep, about 33 bags of 28 lb.
Exterior walls
120 ft of 8 ft wall at 16 in centres, less 120 ft² of windows and doors, is 840 ft²: about 92 R-13 batts (15 × 93 in).
2×6 walls
The same walls framed 24 in apart with 23 in wide R-21 batts: about 60 batts.
Metric loft
A 50 m² loft with 100 mm of old wool topped to RSI 6.7 (R-38): about 180 mm more of cellulose.
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