Insulation Calculator

How to use this tool

Pick what you are insulating, enter its area and choose your climate zone. For an attic or a floor the target R value fills in from the ENERGY STAR table; you can type your own. Tell the page what is there now and how deep it is, then choose the material you plan to add. The depth to add, the volume and, for blown insulation, the number of bags from the coverage on the bag chart appear at the top.

For walls the depth is set by the studs, so the page gives the R value each material reaches in a 2×4 or 2×6 cavity instead. The section drawing shows the old and new layers, and the bar chart shows how deep each material would need to be for the same job.

How the depth is worked out

Insulation is rated by R value, its resistance to heat flow, and layers add up: 3.5 inches of old batts at about R3.1 per inch give R10.9, and anything you add on top simply adds its own R value. So the sum is the target R value minus what is there now, and the depth of new material is that difference divided by its R per inch. The volume is the area times the depth, and for spray foam the page also gives board feet, one square foot one inch thick, which is how foam is priced.

The targets come from ENERGY STAR guidance for existing homes, which follows the 2021 International Energy Conservation Code. The attic figure is the total for an empty attic, so it is the right figure to aim for however much is there now.

Recommended R values by climate zone

ENERGY STAR recommended levels for existing wood framed homes. The attic column is the total for an attic with no insulation; the next column is what to add over 3 to 4 inches of old insulation. In the mild marine part of zone 4, west of the Cascades, the floor level is R30.

Walls are harder to reach in an existing home. For empty wood framed walls, ENERGY STAR suggests blowing insulation into the cavities and, when the house is resided, adding R5 of insulated sheathing in zone 3 and R5 to R10 in zones 4 to 8. Walls that already have insulation in 2×4 cavities gain R10 of sheathing in zones 4 to 8.

R value per inch and depth by material

Typical values; products vary. Blown depths are settled depths. Batts are sold by rated R value, so the depths for batts are a guide to how much height they need.

A worked example you can check on paper

A 1,000 sq ft attic in zone 5 has 3.5 inches of old fiberglass batts. The target is R60. The old layer gives 3.5 times 3.1, R10.85, so the new layer must add 60 minus 10.85, R49.15. Blown cellulose at R3.7 per inch needs 49.15 divided by 3.7, 13.3 inches, and the volume is 1,000 times 13.28 divided by 12, about 1,107 cubic feet.

For the bag count, read the bag chart on the product you plan to buy: it lists how many square feet one bag covers at each R value. If it gives, for example, 35 sq ft per bag at R49, the attic takes 1,000 divided by 35, 28.6, so 29 bags. The same job in blown fiberglass at R2.5 per inch needs 19.7 inches of depth.

Measuring what is there now

Take a ruler into the attic and measure the depth in several places, between the joists rather than over them, and use a typical figure. Fluffy pink, yellow or white fibers are fiberglass; gray, shredded paper is cellulose; dense gray or brown fibers are rock wool. Flattened, wet or patchy insulation gives less than its depth suggests, and if the joists stick up above the insulation, there is clearly room for more.

If you find gray brown or silvery pebbles, it may be vermiculite, some of which contains asbestos. Do not disturb it; have it tested and ask a professional before adding anything on top.

Mistakes that catch people out

Skipping air sealing. Gaps around pipes, wires, flues and the attic hatch leak warm air straight past the insulation. Seal them before adding depth.

Blocking the vents. Loose fill blown to the eaves can smother soffit vents. Fit baffles at every rafter bay first.

Forgetting settling. Blown insulation settles after it goes in. The bag chart shows the installed depth that settles to the rated depth; blow to that.

Compressing batts. A batt squashed into a shallower space loses R value. Buy batts that fit the joist or stud depth.

Covering what should not be covered. Keep insulation clear of chimneys, flues and recessed lights not rated for contact with insulation, and never cover knob and tube wiring.

Popular calculations

Each opens the calculator with its own address, and every value can be changed once it is open.

Empty attics in zone 5

Attics in other climate zones

Topping up existing insulation

Other materials

Floors over a crawl space or garage

Walls

Metric

How this was checked

The engine was tested in Node against the hand calculation above: 3.5 inches of fiberglass batts give R10.85, R49.15 is left to add and blown cellulose needs 13.28 inches, 1,107 cubic feet over 1,000 sq ft. An empty zone 5 attic needs 16.22 inches of cellulose, a 2×6 wall of closed cell foam reaches R35.75, and metric inputs were checked by converting square meters and centimeters by hand. The target values were checked against the ENERGY STAR table.

Every address in the lists above was then opened and checked for a distinct title, the right answer, charts that render, and clean CSV, image and print output containing only that calculation, at desktop, tablet and phone widths.

Common questions

How much insulation should I have in my attic?

ENERGY STAR recommends R60 in climate zones 4 to 8, R49 in zones 2 and 3 and R30 in zone 1, for an attic with no insulation. That is about 16 inches of blown cellulose or 24 inches of blown fiberglass for R60.

How many inches of insulation do I need for R49?

About 13.2 inches of blown cellulose at R3.7 per inch, 19.6 inches of blown fiberglass at R2.5, or 15.8 inches of fiberglass batts at R3.1. The bag chart or batt label for your product gives the exact figure.

Can I put new insulation on top of old insulation?

Yes, in most attics, as long as the old layer is dry, shows no mold and is not vermiculite. Loose fill can go straight on top of batts. If you add batts over old ones, use unfaced batts so no vapor retarder ends up trapped between layers, and lay them across the joists.

How do I work out how much insulation I have now?

Measure the depth in several places with a ruler, avoiding spots right over joists, and identify the material: fluffy pink, yellow or white fibers are fiberglass, gray shredded paper is cellulose, and dense gray or brown fibers are rock wool. Depth times R per inch gives the R value.

Is cellulose or fiberglass better for the attic?

Both work. Cellulose gives more R per inch and blocks air movement a little better; fiberglass is lighter, which matters on a ceiling with widely spaced joists, and settles less. For the same R value you need about half again as much depth of blown fiberglass as cellulose.

How much can insulation save?

The EPA estimates that homeowners can save an average of 15 percent on heating and cooling costs by air sealing and adding insulation in attics, floors over crawl spaces and basements. The saving is largest when the existing insulation is thin.

Related Calculators and Reference

• Furnace Size Calculator: how better insulation shrinks the heat loss and the furnace you need.
• R Value Converter: R values and metric RSI.
• BTU Calculator: air conditioner size by room.
• Square Footage Calculator: the area of an attic or floor from its shape.
• Plywood Calculator: sheets for attic decking and subfloors.
• Dew Point Calculator: check whether a cold surface will collect condensation.
• ENERGY STAR recommended home insulation R values: the levels by climate zone used here.