YepCalc

Insulation Calculator

Find how much insulation you need for an attic floor, walls, or floors. In the UK, insulation on the attic floor is called loft insulation. For roof insulation between the rafters, measure the sloped area. Enter the area, your target R-value, and the R-value of any insulation already there to see the R-value to add, how many bags of blown-in insulation to buy, or how many packages of batts or rolls.

Section 1

Add more areas, or subtract areas you won't insulate, such as doors and windows in a wall.

Insulation type

Choose the insulation type

R-value

The total R-value you want, including what's already there.

The R-value already in place. Leave it blank if there's none.

RSI is the R-value in metric units (m²·K/W). R-1 is about RSI 0.176.

Waste, bags, and cost

%

Recommended for insulation: 10%

From the chart on your bag, enter the coverage per bag for the R-value you need to add.

per

Result

Enter the dimensions of an attic floor or a wall to see the insulation you need.

How much insulation you need

The calculator adds up the area to insulate and adds the waste allowance:

Total area = net area × (1 + waste % ÷ 100)

Layers of insulation add up, so the R-value you need to add is what's left after the insulation already there:

R-value to add = target R-value − existing R-value

For blown-in insulation, the calculator finds the row of the bag's coverage chart for the R-value to add, and divides by the coverage of one bag on that row. For batts or rolls, it divides by the area printed on the package:

Bags = total area ÷ coverage per bag, rounded up

Packages = total area ÷ coverage per package, rounded up

When the R-value to add falls between two rows of a chart, the calculator uses the higher row. The label R-value is only promised at the coverage and depth on a row, so it doesn't average between rows. Rounding up gives a few more bags, not too few. Below the lowest row, it uses the lowest row. Above the highest row, the chart doesn't say how far a bag goes, so that bag is left out of the results.

Measure an attic floor's length and width, and walls and floors with the studs or joists included. For a sloped ceiling, measure along the slope. The area of each shape is calculated as follows:

Shape Area
Rectangle length × width
Circle π × diameter² ÷ 4 (or π × radius²)
Triangle (base and height) base × height ÷ 2
Triangle (three sides) Heron's formula: √(s(s − a)(s − b)(s − c)), where s = (a + b + c) ÷ 2
Trapezoid (parallel side A + parallel side B) ÷ 2 × height
Ring π × (outer diameter² − inner diameter²) ÷ 4
Rectangular border outer length × outer width − (outer length − 2 × border width) × (outer width − 2 × border width)
Known area the area you enter
Room walls 2 × (length + width) × height

Blown-in insulation coverage charts

Every bag of blown-in insulation sold in the US has a coverage chart on its label, with the most area one bag may cover and the minimum depth for each R-value. In the tables below, these are the Coverage and Depth columns. The calculator uses these charts from the makers' product data. The coverage is the net coverage, with no adjustment for framing, and the depth is the minimum depth right after blowing, which you can check with a ruler standing in the attic. When you add to existing insulation, the depth is for the new layer.

Fiberglass, 27.5 lb bags (Owens Corning AttiCat chart)

R-value Coverage Depth
R-13 169.8 ft² 5 in
R-19 110.7 ft² 7 in
R-22 94.8 ft² 8 in
R-26 79.3 ft² 9.5 in
R-30 68.5 ft² 10.75 in
R-38 52.6 ft² 13.5 in
R-44 44.6 ft² 15.5 in
R-49 39.9 ft² 17 in
R-60 31.8 ft² 20.5 in

Source: Owens Corning, AttiCat PINK Blown-In Insulation Product Data Sheet (Pub. No. 10011273-L, May 2026).

Cellulose, 25 lb bags (Greenfiber Sanctuary chart)

R-value Coverage Depth
R-13 65.8 ft² 3.9 in
R-19 44.2 ft² 5.7 in
R-22 38.0 ft² 6.6 in
R-30 25.9 ft² 8.9 in
R-38 19.0 ft² 11.2 in
R-44 15.7 ft² 12.9 in
R-49 14.1 ft² 14.3 in
R-60 11.6 ft² 17.3 in

Source: Greenfiber, SANCTUARY Attic Card (PM-6.3330-SDBZ Rev A 04/26), net coverage.

Fiberglass, 30 lb bags (Owens Corning AttiCat (Canada) chart)

R-value Coverage Depth
RSI 2.1 15.05 m² 114 mm
RSI 2.8 11.29 m² 151 mm
RSI 3.5 9.03 m² 189 mm
RSI 4.2 7.53 m² 227 mm
RSI 4.9 6.45 m² 265 mm
RSI 5.6 5.63 m² 302 mm
RSI 6.3 4.94 m² 337 mm
RSI 7.0 4.40 m² 372 mm
RSI 7.7 3.96 m² 407 mm
RSI 8.4 3.60 m² 442 mm
RSI 8.75 3.46 m² 458 mm
RSI 9.1 3.30 m² 477 mm
RSI 9.8 3.05 m² 511 mm
RSI 10.5 2.83 m² 546 mm
RSI 11.2 2.64 m² 581 mm
RSI 11.9 2.48 m² 615 mm
RSI 12.25 2.41 m² 632 mm
RSI 12.6 2.33 m² 650 mm
RSI 13.3 2.21 m² 684 mm
RSI 14.1 2.07 m² 724 mm

Source: Owens Corning Canada, AttiCat Expanding Blown-In Insulation Product Data Sheet (Pub. No. 501140D, June 2025).

Makers revise these charts. For example, Owens Corning's 2010 AttiCat chart was for a 28.5 lb bag that covered 38.8 ft² at R-49, and recent charts are for a 27.5 lb bag that covers 39.9 ft². Check the chart on your bag, and if it differs, enter its coverage for the R-value you need to add as a custom bag coverage. Do the same for blown-in bags in the UK, Australia, and elsewhere.

These charts are for blowing onto an open attic floor. Blowing into closed walls (dense packing) needs a different chart and machine, and Owens Corning says the AttiCat machine isn't meant for walls.

R-value, RSI, and existing insulation

R-value measures how well insulation resists heat flow. In the US it's written as R-30, in ft²·°F·h/Btu. In metric units it's called RSI, in m²·K/W. R-1 is about RSI 0.176, and RSI 1 is about R-5.68. In Australia and New Zealand, R-values such as R4.0 are already in metric units, so R4.0 there is RSI 4.0.

For existing insulation, use the R-value on its label. Without a label, you can estimate it from the depth. The US Department of Energy's Insulation Fact Sheet suggests multiplying the thickness in inches by 3.2 for fiberglass batts under a floor, and Greenfiber's attic instructions say about 6 in of fiberglass batts is R-19.

New insulation on top can squash the old layer a little. The same DOE fact sheet suggests adding about 1 extra inch over old fiberglass, or about ½ in over rock wool or cellulose. The calculator doesn't include this, so add it to the depth if you want to allow for it.

What R-value to aim for

The right R-value depends on your climate, the part of the house, and local rules, so the calculator doesn't assume one. These are examples, not code requirements:

Extra insulation for waste

The waste allowance starts at 10% for both types. For blown-in insulation, Greenfiber says job conditions, technique, and machine settings can change the actual coverage by as much as 10%, and the chart coverage is the most a bag may cover. For batts or rolls, it covers cutting them to length and around obstacles, the same as for drywall and flooring.

The area includes the studs or joists, where no insulation goes, so the count is a little high. In walls, a few percent extra remains even with the waste allowance at 0%.

Examples

Topping up an attic

An attic floor 40 ft × 25 ft is 1,000 ft², or 1,100 ft² with 10% added. To go from R-19 to R-49, you add R-30. On the R-30 row, that is 1,100 ÷ 68.5 = 16.06, so 17 bags of 27.5 lb fiberglass at least 10.75 in deep, or 1,100 ÷ 25.9 = 42.47, so 43 bags of 25 lb cellulose at least 8.9 in deep. At $35.00 per fiberglass bag, the cost is $595.00.

Batts for the walls of a room

The walls of a room 30 ft × 20 ft with 8 ft walls are 800 ft². Subtracting six windows of 3 × 4 ft and two doors of 3 × 7 ft leaves 686 ft², or 754.6 ft² with 10% added. With R-13 batts in packages of 106.6 ft², that is 754.6 ÷ 106.6 = 7.08, so 8 packages.

Metric dimensions

An attic floor 10 m × 8 m is 80 m², or 88 m² with 10% added. To go from RSI 2.1 to RSI 8.8, you add RSI 6.7 (R-38.04), which falls between the RSI 6.3 and 7.0 rows of the Canadian AttiCat chart. On the RSI 7.0 row, 88 ÷ 4.40 = 20, so 20 bags of 30 lb at least 372 mm deep.

FAQ