What Is the Insulation Calculator?
The Insulation Calculator works out how much insulation is needed to bring an attic, wall or floor up to a target R-value. You enter the area, the R-value already in place, the R-value you want to reach and the type of insulation you plan to use. It returns the depth to add in inches and the total volume of material.
Insulation is one of the few home improvements that pays for itself through lower energy bills. The attic is usually the easiest place to add it and gives the largest return. Knowing the depth you need before you start lets you buy the right quantity and check the finished job with a ruler.
How the Calculator Works
R-value measures resistance to heat flow. It is additive, so new insulation laid over old simply adds to the total. The calculator subtracts the existing R-value from the target to find how much R-value must be added.
Each insulation material provides a certain R-value per inch of thickness. Blown cellulose gives about R-3.5 per inch. Loose-fill fibreglass gives about R-2.5. The calculator divides the R-value to be added by the material’s R-value per inch to get the depth.
It multiplies the area by that depth to get the volume in cubic feet. For blown products, the volume helps you check the bag count against the coverage chart printed on the bag.
The Insulation Formula
R-value to add = Target R-value − Existing R-value
Depth (inches) = R-value to add ÷ R-value per inch
Volume (cubic feet) = Area (sq ft) × Depth (in) ÷ 12
Typical R-values per inch
- Blown fibreglass: 2.2 to 2.7
- Fibreglass batts: 3.1 to 3.4
- Blown cellulose: 3.2 to 3.8
- Mineral wool batts: 3.7 to 4.2
- Open-cell spray foam: 3.5 to 3.8
- Closed-cell spray foam: 6.0 to 7.0
- Rigid polyiso board: 5.6 to 6.5
Step-by-Step Explanation
- Measure the length and width of the area and multiply for square feet.
- Measure the depth of existing insulation and multiply by its R-value per inch to estimate the existing R-value.
- Choose a target R-value for your climate.
- Subtract existing from target.
- Divide by the R-value per inch of the new material.
- Multiply the area by the depth in inches and divide by 12 for cubic feet.
Worked Example
An attic measures 40 feet by 25 feet. It has about 3.5 inches of old fibreglass, roughly R-11. The target is R-49, using blown cellulose.
- Area: 40 × 25 = 1,000 square feet.
- R-value to add: 49 − 11 = 38.
- Depth: 38 ÷ 3.5 = 10.9 inches.
- Volume: 1,000 × 10.9 ÷ 12 = 905 cubic feet.
You would blow in about 11 inches of cellulose over the existing batts, for a total depth of around 14.5 inches. The bag’s coverage chart will tell you how many bags give R-38 over 1,000 square feet, typically in the range of 35 to 45 bags depending on the product.
How to Use the Calculator
- Enter the length and width of the space to be insulated.
- Enter the existing R-value, or zero if there is none.
- Enter the target R-value.
- Choose the insulation material.
- Press Calculate.
- Read the depth to add and the volume.
To find the area of an irregular attic, use the Square Footage Calculator. Better insulation lowers heating and cooling loads, so recheck equipment sizes with the Furnace Size Calculator and the Air Conditioner BTU Calculator. Insulated walls are closed in with board estimated by the Drywall Calculator, and if the roof is being replaced at the same time, the Roofing Calculator gives the area.
Practical Tips
- Seal air leaks before adding insulation. Gaps around pipes, wires, light fittings and the attic hatch let warm air bypass the insulation entirely.
- Keep soffit vents clear. Install baffles at the eaves so insulation does not block airflow.
- Mark the target depth on several rafters or use depth rulers so you can blow to a consistent level.
- Do not compress batts. Squeezing a batt into a shallower space reduces its R-value.
- Lay a second layer of batts perpendicular to the joists to cover the framing.
- Keep insulation away from recessed lights that are not rated for insulation contact, and from flues.
- Wear a respirator, goggles, gloves and long sleeves.
- Insulate and weatherstrip the attic hatch. It is often the largest gap in the ceiling.
Common Mistakes
Skipping air sealing. Insulation slows heat conduction but does not stop air movement.
Blocking ventilation. Covering soffit vents leads to moisture problems and ice dams.
Using faced batts over existing insulation. A second vapour retarder can trap moisture. Use unfaced batts for top-ups.
Ignoring settling. Blown cellulose settles by up to 20 percent. Bags state both installed and settled thickness.
Measuring only the open areas. Include the space above every room, including closets and hallways.
Assuming more is always better. Beyond the recommended level for your climate, each added inch saves less.
Recommended R-Values
Guidance varies by climate zone. In warm southern climates, attics are typically insulated to R-30 to R-49. In mixed climates, R-38 to R-60. In cold northern climates, R-49 to R-60. Walls with 2×4 framing are usually R-13 to R-15, and 2×6 walls R-19 to R-21. Floors over unheated spaces range from R-13 to R-30. Check your local energy code for the required minimum.
Choosing a Material
Blown cellulose is made from recycled paper treated for fire resistance. It fills gaps well and is a good choice for topping up attics. Blown fibreglass is lighter and does not settle as much. Batts suit open stud bays and joist spaces with standard spacing. Mineral wool resists fire and moisture and is easy to cut accurately. Spray foam insulates and air-seals in one step, costs more and needs professional installation. Rigid foam boards are used on basement walls and under siding.
Estimating Batts
Batts are sold in packages that state their coverage in square feet. Divide the area by the package coverage and round up. For walls, coverage is based on stud spacing, so buy 15 inch wide batts for studs at 16 inches on centre and 23 inch batts for studs at 24 inches. Framing takes up about 10 to 15 percent of wall area, which offsets cutting waste.
Moisture and Vapour
In cold climates, a vapour retarder belongs on the warm side of the insulation, usually facing the living space. In hot, humid climates the rules differ. Faced batts have a kraft paper retarder built in. If you are unsure, check local code or ask a building professional, because a retarder in the wrong place can trap moisture inside the wall.
Finding Your Existing R-Value
Push a ruler down through the insulation to the ceiling drywall and read the depth. Multiply by the R-value per inch for the material you find. Grey, papery loose fill is cellulose at about 3.5 per inch. Fluffy white, pink or yellow loose fill is fibreglass at about 2.5. Batts usually have the R-value printed on the facing. Old, compressed or damp insulation performs below its nominal rating, so it is reasonable to mark it down by a fifth.
Insulating Walls and Floors
For walls, the depth is fixed by the framing, so the question becomes which product fits the cavity. A 2×4 wall has a 3.5 inch cavity and takes R-13 or R-15 batts. A 2×6 wall has 5.5 inches and takes R-19 to R-23. Enter the wall area in the calculator to find the volume, or simply divide the area by the package coverage. Floors over crawl spaces are insulated between joists with batts held up by wire supports or netting, with any facing turned up toward the heated room.
Estimating Savings
The return on insulation depends on your climate, fuel cost and starting point. Going from almost nothing to the recommended level gives the largest saving, and it often pays back within a few heating seasons.