🧱 Insulation Calculator

How to Calculate Insulation

Every insulation estimate is the same three questions in order: what R-value do I need, how many inches of this material does that take, and how many batts or bags is that. Get them in that order and the arithmetic is trivial. Get them out of order — picking a product first and hoping it hits code — and you end up with a wall you cannot fix without opening it again.

Two things about that second line matter more than anything else on this page.

Batt coverage is fixed; bag coverage is not. A 15 in × 93 in batt covers 9.7 sq ft no matter what it is rated at. A bag of blown-in covers a completely different area depending on how deep you blow it — the same bag might cover 60 sq ft at R-19 and 25 sq ft at R-49, because you are spreading the same weight of fibre thinner or thicker. There is no universal "bag covers X" number, and any calculator that hard-codes one is guessing on your behalf.

Read the coverage chart printed on the bag. Every bag of loose-fill fiberglass or cellulose carries a table: target R-value, minimum thickness, maximum net coverage per bag, and minimum weight per square foot. That chart is product-specific and it is the number this calculator asks you to type in. Cellulose and fiberglass differ, brands differ, and the same brand changes it between production runs.

Step by step

• Find your climate zone and the recommended R-value for the assembly you are insulating — attic, wall, and floor targets are all different.
• Measure the area. For an attic, that is the footprint of the ceiling below. For walls, it is perimeter × height minus windows and doors. Our square footage calculator handles irregular shapes.
• Divide the target R by the material's R per inch to get the depth.
• Check the depth fits. In an attic it almost always does. In a 2×4 wall you have 3.5 in and that is the end of the conversation — the only way to more R is a material with a higher R per inch, or rigid board over the framing.
• Convert to units. Batts by coverage, bags by the coverage chart, spray foam by the board foot (1 sq ft at 1 in deep).
• Add 5–10% for batts if the framing is irregular. You do not need waste allowance on blown-in — a partial bag is not wasted, and the chart already builds in settling.

Recommended R-Values by Climate Zone

These are the ranges the US Department of Energy and ENERGY STAR publish for retrofitting existing homes, alongside what recent editions of the IECC require in new construction. Codes are adopted state by state and change every few cycles, so treat this as the planning number and confirm the requirement with your local building department before you buy for a permitted job.

Zone 8 covers the far north of Alaska and is treated as zone 7 or higher — R-60 in the attic is the floor, not the ceiling, and R-49 walls are common.

A few notes that the table cannot carry:

• Wall numbers above R-21 usually mean two layers. A 2×6 cavity tops out around R-21 with fiberglass or R-23 with mineral wool. Getting to R-30 means cavity insulation plus continuous rigid board on the outside — written in code as "R-20 + R-5ci" or similar.
• "ci" means continuous insulation — an unbroken layer over the studs. It is worth far more than its R-value suggests because it covers the thermal bridge the framing creates.
• Attic is the cheapest R you will ever buy. If your budget only stretches to one assembly, do the attic. Blown-in over an accessible attic floor costs a fraction of anything you can do to a finished wall.
• Marine zone 4 (coastal Pacific Northwest) follows zone 5 requirements for walls in most code editions.

R-Value per Inch by Material

R-value per inch is the number that decides whether your target fits in the space you have. Manufacturers publish slightly different figures and test conditions vary, so these are working ranges — the value printed on the product you buy wins.

The two rigid boards come with asterisks. XPS is labelled around R-5 per inch when new, but the blowing agent diffuses out over years and long-term performance drifts toward R-4.5. Polyiso is the highest R per inch of any board — until it gets cold, where its R-value drops noticeably; in a zone 6 winter an inch of polyiso may perform closer to R-4.5 than its labelled R-6. If you are sheathing a cold-climate wall, EPS is the honest, boring, stable choice.

Closed-cell foam is the only product that buys you real R in a shallow cavity. In a 2×4 wall it delivers roughly what a 2×6 fiberglass bay does. It is also a vapour retarder and an air barrier at about 1.5 to 2 in, which is why it dominates rim joists and cathedral roofs. It costs several times what batts do, and DIY kits are unforgiving about temperature and mix ratio.

Mineral wool is the quiet upgrade. Same install as fiberglass, roughly 30% more R per inch, holds its shape instead of slumping, does not burn, and cuts cleanly with a bread knife. It costs more per square foot but it is the batt to buy if you can only pick one.

Batt Sizes, Framing and Coverage

Batts are cut to fit standard framing, which is why the framing spacing on your house dictates which product you buy. Get the width wrong and you are trimming every single piece.

Why 93 inches and not 96? An 8 ft wall is 8 ft of stud plus plates, and the actual clear height between the bottom plate and the top plates is about 92.5 in. A 93 in batt fills it. Buy 96 in batts for that wall and you are folding three and a half inches over at the top of every bay — which compresses the material and costs you R-value in exactly the spot where it matters.

The R-19 trap. R-19 fiberglass is 6.25 in thick and is designed for a 6.25 in cavity. Stuff it into a 5.5 in 2×6 wall — which is what almost everyone does — and it compresses to about R-18, sometimes less. R-21 is manufactured at 5.5 in for exactly this reason. Same bay, more R, no downside.

Once the insulation is in, the drywall goes on. Our drywall calculator gives sheet counts and our drywall mud calculator covers compound and tape by finish level.

Air Seal First: The Order That Actually Matters

This is the section that saves people the most money and gets skipped the most often. Insulation slows conductive heat flow. It does nothing to stop air moving through it. Fiberglass in particular is a filter, not a barrier — air passes straight through it, and air carrying moisture through insulation is how attics get frost and walls get mould.

A house with R-60 in the attic and unsealed penetrations underneath will underperform a properly sealed house at R-38. Do these in order:

1. Get in the attic and look before you buy anything. Note the joist spacing, the existing depth, whether there are baffles at the eaves, and whether any wiring looks like it predates 1950.
2. Seal the big holes first. Open chases around chimneys and flues, dropped soffits over kitchen cabinets, the top plates of interior walls, and the gap around plumbing stacks. These leak far more than everything else combined. Use rigid material and fire-rated sealant at flues — never expanding foam against a metal flue pipe.
3. Caulk or foam the small penetrations. Wiring holes, ceiling boxes, recessed can housings, bath fan bodies, attic hatch perimeter.
4. Weatherstrip and insulate the attic hatch. An uninsulated pull-down stair is a 10 sq ft hole in your R-60 ceiling.
5. Install rafter baffles at every eave bay before insulating, so soffit air can still reach the ridge. Staple them to the roof deck between rafters, running from the soffit up past the top of the finished insulation depth.
6. Build dams around anything that must not be buried — non-IC-rated recessed lights, knob-and-tube wiring, the chimney (3 in clearance to masonry chimneys and B-vent is the usual requirement).
7. Check the bath and dryer vents terminate outside, not into the attic. Every year plenty of people insulate over a bath fan duct that has been dumping humid air into the attic all along.
8. Now insulate. Mark the target depth on the rafters or on a stake before you start blowing, so you can actually see when you have hit it.
9. Leave the depth markers in place. The next person to go up there will thank you, and so will an appraiser.

If you are only doing one thing this weekend, do steps 2 through 4. Air sealing is cheap, it is measurable, and it makes every dollar of insulation you add afterwards work harder.

Where Insulation Goes Wrong

Compression destroys R-value

R-value comes from trapped still air, so squashing insulation reduces it roughly in proportion to the thickness you lose. Fold an R-19 batt over at the top of a wall and that section is worth maybe R-13. Cram an R-30 batt into a 2×8 floor bay and you have paid for R-30 and installed R-25. Rules: cut batts to length rather than folding, split them around wires and pipes instead of compressing them behind, and never staple a batt so tightly that it bows.

Gaps are worse than compression

A batt that covers 95% of a cavity does not deliver 95% of its R-value — closer to 70%, because the uncovered strip becomes a convective bypass. Cut batts slightly wide so they friction-fit wall to wall, and split them around every obstruction: electrical boxes, cables, plumbing. This single detail is the biggest difference between a professional batt job and an amateur one.

Never bury a non-IC-rated recessed light

Old recessed cans are rated "IC" (insulation contact) or they are not. Non-IC housings need 3 in of clearance on all sides, and covering one is a genuine fire risk — the thermal cutout will cycle the light off, and if it fails, the housing gets hot enough to matter. Look for the label inside the can. If it does not say IC, either build a dam around it or, better, replace it with an airtight IC-rated LED housing before you insulate.

Do not block the soffit vents

A vented attic works by drawing air in at the soffits and exhausting it at the ridge. Blow insulation right out to the eaves without baffles and you plug that path — the attic stops drying, the roof deck gets damp, and in cold climates you get ice dams on the very eaves you just insulated. Install a baffle in every rafter bay along the eave, not just a few.

Vapour retarder facing: which way and when

The kraft or foil facing on a batt is a vapour retarder, and the rule is that it goes toward the side that is warm in winter — the interior face in most of the US. In heating-dominated climates (roughly zones 4 through 7) that means facing the living space. In hot, humid, cooling-dominated climates (zones 1 and 2), the warm-humid side is outdoors for most of the year, and an interior vapour retarder can trap moisture inside the wall; unfaced batts are the normal choice there. Zone 3 sits in between and is usually built with unfaced or with a smart membrane.

Two absolutes regardless of zone: never install two vapour retarders on opposite faces of the same assembly — the cavity then cannot dry in either direction — and never lay faced batts over existing attic insulation with the facing down between the two layers. In an attic top-up, the new layer goes on unfaced.

Older homes: stop and check first

Three things to identify before you insulate a house built before about 1980:

• Vermiculite. Loose, pebbly, grey-brown granules in the attic — often Zonolite. A large share of it is contaminated with asbestos. Do not disturb it, do not blow over it without advice, and do not vacuum it. Get it tested.
• Knob-and-tube wiring. Ceramic knobs and cloth-sheathed conductors running through porcelain tubes. It is designed to shed heat into open air, and burying it in insulation is prohibited by code in most jurisdictions. It needs replacing, not covering.
• Asbestos pipe lagging and old duct tape. White or grey wrapped insulation on heating pipes and boiler ducts. Leave it alone and get it assessed.

None of these are reasons to abandon the project — they are reasons to make one phone call before you spend a weekend in a respirator.

Wear the gear

Fiberglass and mineral wool shed fibres that irritate skin, eyes and airways. Cellulose creates dense dust. Wear an N95 or better, sealed goggles, gloves and long sleeves, work with the attic hatch open for airflow, and shower in cool water afterwards — hot water opens pores and drives fibres in. Never step between joists; a foot through a ceiling is the most common attic injury after heat exhaustion.

Frequently Asked Questions

How much insulation do I need?

It depends on your climate zone and what you are insulating. Attics run R-30 to R-49 in the deep south and R-60 across most of the northern half of the country. Walls are usually R-13 to R-21 in the cavity, more where continuous exterior insulation is required. Floors over unconditioned space run R-13 to R-38. Divide the target R by your material R per inch to get the depth.

How many bags of blown-in insulation do I need?

Divide your area by the coverage figure on the bag at your target R-value. There is no single answer, because coverage per bag drops as depth increases — the same bag might cover 60 sq ft at R-19 and 25 sq ft at R-49. Every bag of loose fill carries a coverage chart listing minimum thickness, maximum coverage and minimum weight per square foot for each R-value. Read that chart and type the number into the calculator.

What is the R-value per inch of fiberglass insulation?

Fiberglass batts run about R-3.1 to R-3.4 per inch, which is why 3.5 in gives R-13 and 5.5 in gives R-21. Fiberglass blown into an attic is looser and gives only about R-2.2 to R-2.7 per inch, so it needs noticeably more depth than batts for the same R. Blown cellulose sits between them at roughly R-3.2 to R-3.8.

Should I air seal before insulating?

Yes, and it is not optional. Insulation slows conductive heat flow but does almost nothing to stop air movement — fiberglass is effectively a filter. A well sealed attic at R-38 outperforms a leaky one at R-60. Seal open chases, dropped soffits, top plates, plumbing stacks, wiring holes and the attic hatch before a single bag goes in, because once it is covered you cannot get at any of it.

Which way should the vapour barrier face?

Toward the side that is warm in winter — the interior face — in heating-dominated climates, roughly zones 4 through 7. In hot humid zones 1 and 2 the warm humid side is outdoors most of the year, so unfaced batts are normal and an interior vapour retarder can trap moisture in the wall. Never install two vapour retarders on opposite faces of one assembly, and never lay faced batts over existing attic insulation.

Does compressing insulation reduce R-value?

Yes, roughly in proportion to the thickness you lose. R-value comes from trapped still air, so a folded-over batt at the top of a wall is worth only a fraction of its rating in that spot. The classic example is R-19 fiberglass squeezed into a 5.5 in 2×6 wall, which performs around R-18. Use R-21 instead — it is manufactured at 5.5 in for that cavity.

Can I put insulation over recessed lights?

Only if the housing is IC-rated — the label inside the can will say so. Non-IC housings require 3 inches of clearance on every side, and burying one is a real fire risk. The best fix is to swap the old cans for airtight IC-rated LED housings before you insulate; it costs less than building dams and it removes a large air leak at the same time.

Do I need baffles at the soffit vents?

In a vented attic, yes — one in every rafter bay along the eave, not just a few. A vented attic dries by pulling air in at the soffits and exhausting it at the ridge. Blowing insulation out to the eaves without baffles plugs that path, the roof deck stays damp, and in cold climates you get ice dams. Baffles cost a couple of dollars each and take minutes to staple in.

What size are insulation batts?

Batts are made to fit framing. 15 in wide for 16 in on-centre framing and 23 in wide for 24 in on-centre. Common lengths are 93 in for an 8 ft wall, 96 in, and 105 in for a 9 ft wall. A 15 × 93 in batt covers 9.7 sq ft; a 23 × 93 in batt covers 14.9 sq ft. Use 93 in for standard 8 ft walls — the clear height between plates is about 92.5 in, so a 96 in batt folds over and loses R-value at the top of every bay.

Is mineral wool better than fiberglass?

For most jobs, yes. Mineral wool gives roughly R-4.0 to R-4.3 per inch versus R-3.1 to R-3.4 for fiberglass, holds its shape instead of slumping, cuts cleanly with a serrated blade, does not burn, and blocks sound noticeably better. It costs more per square foot and it is heavier. If the budget allows and the space is tight, it is the better batt.

How much R-value can I get in a 2x4 wall?

With batts, about R-13 to R-15 in the 3.5 in cavity — R-15 high-density fiberglass or R-15 mineral wool. Closed-cell spray foam gets you to roughly R-21 to R-24 in the same space. Beyond that the only route is continuous rigid board over the studs on the outside, which also fixes the thermal bridging through the framing.

What is the difference between open-cell and closed-cell spray foam?

Density and what it does. Open-cell is about R-3.5 to R-3.9 per inch, soft, vapour-permeable, and cheaper — good where you want air sealing plus moderate R and the assembly needs to dry inward. Closed-cell is R-6.0 to R-7.0 per inch, rigid, and is its own vapour retarder at around 1.5 to 2 in — the choice for rim joists, shallow cavities and below-grade walls. Closed-cell also adds structural rigidity; open-cell does not.

What if my attic has vermiculite or knob-and-tube wiring?

Stop and get it assessed. Vermiculite — loose grey-brown pebbles, often Zonolite — is frequently contaminated with asbestos; do not disturb it, sweep it or vacuum it. Knob-and-tube wiring sheds heat into open air by design, and burying it in insulation is prohibited by code in most jurisdictions; it needs replacing before you insulate over it. Neither one kills the project, but both need a professional opinion first.

Is polyiso or XPS better for exterior rigid insulation?

It depends on your climate. Polyiso has the highest labelled R per inch of the boards at R-5.6 to R-6.5, but its performance drops as temperature falls, so in a cold-climate winter it can behave closer to R-4.5 per inch. XPS is labelled around R-5 when new but drifts toward R-4.5 as the blowing agent diffuses out. EPS is the lowest per inch at R-3.6 to R-4.2 but its value is stable and it is the cheapest per R — often the sensible pick for cold-climate exterior sheathing.