🏠 Attic Insulation Calculator

How to Calculate Attic Insulation

Topping up an attic is different from insulating a bare one, because you are not buying the whole R-value — you are buying the difference between what is already there and what you want. Get that subtraction wrong and you either overspend by a third or fall short and never notice.

That last line is where most estimates go wrong, so it is worth being blunt about it.

There is no fixed square footage per bag. Coverage depends entirely on how deep you blow the material, because you are spreading a fixed weight of fibre over a variable area. The same bag of cellulose might cover 50 sq ft at R-19 and 22 sq ft at R-49. Every bag carries a printed coverage chart with a row for each R-value listing the minimum installed thickness, the maximum net coverage per bag, and the minimum weight per square foot. Look up the R-value you are ADDING — not your final target — and type that coverage figure into the calculator.

The minimum-weight column on that chart is the one nobody reads and the one that actually governs performance. Blowing the material too fluffy hits the depth without hitting the density, and the insulation settles below rated R within a year or two.

Measuring the attic

• Use the footprint of the ceiling below, not the roof area. A 40 × 30 ft house has a 1,200 sq ft attic floor regardless of roof pitch.
• Deduct anything you will not be covering — a walk-boarded storage platform, the mechanical platform under an air handler, the hatch.
• Measure existing depth in three or four places with a ruler pushed down to the drywall, and average them. Insulation is always deepest in the middle and thinnest at the eaves, and the deep spot is the one you will instinctively measure.
• Look at the joists. If the tops of the joists are visible, you have about 5.5 to 9 in of material at most, and there is a lot of headroom to gain.
• For an irregular or multi-wing attic, break it into rectangles — our square footage calculator does the arithmetic.

Attic R-Value Targets by Climate Zone

The attic is the highest-return insulation in almost any house: heat rises, the assembly is accessible, and the material is cheap. These are the ranges the US Department of Energy and ENERGY STAR publish for existing homes, with the higher end reflecting what recent IECC editions require for new construction. Codes vary by state and change every few cycles, so confirm with your local building department if the job is permitted or tied to a rebate.

Depths above use R-3.5 per inch for cellulose and R-2.5 per inch for blown fiberglass — reasonable working figures, but check the chart on the product you buy. Zone 8, the far north of Alaska, treats R-60 as a floor rather than a target.

Diminishing returns are real, but they arrive later than people think. Going from R-11 to R-30 is a large, obvious change in comfort and bills. R-30 to R-49 is a smaller but still worthwhile gain. R-49 to R-60 is marginal in a mild climate and clearly worth it in zone 5 and colder. What is never marginal is the air sealing underneath — that is where the first big win lives, and it costs a fraction of the insulation.

Identifying What Is Already Up There

You need two things about the existing layer: what it is, and how deep it is. Identification matters because R per inch varies by nearly a factor of two between materials, and because two of the possibilities are reasons to stop work.

Measure depth honestly. Push a ruler down through the material until it touches the ceiling drywall. Do this near the middle of the attic, halfway out, and about 4 ft in from the eaves, then average. Older blown jobs settle 10 to 20% over their first few years, so what looks like R-19 is frequently performing like R-15.

A note on old batts. If the existing batts are compressed, sagging, damaged by rodents, or wet, their rated R is fiction. Wet insulation in particular loses almost all its value and points at a roof leak that needs fixing before anything else happens up there — our roofing calculator is the next stop if that is what you find.

Bags of Blown-In: What the Coverage Chart Actually Says

The coverage chart is a table on the side of every bag of loose-fill insulation. It typically has four columns: R-value, minimum installed thickness, maximum net coverage per bag, and minimum weight per square foot. There will also be a "bags per 1,000 sq ft" figure, which is the same information stated the other way round.

The figures below are the general ballpark you can expect. They are ranges across brands and products, not a substitute for the chart on the bag you are holding — which is exactly why the calculator asks you to type the number in rather than assuming one.

Two practical consequences:

• Cellulose needs fewer inches but often more bags by weight, because it is denser. Fiberglass needs more depth for the same R but the bags go further in area terms. Which is cheaper depends entirely on local pricing.
• Most big-box stores lend the blower free if you buy a minimum number of bags — usually somewhere around 10 to 20. Factor that in; renting a blower otherwise runs real money for a one-day job.

Do not add a waste factor to blown-in. Unlike sheet goods, a partial bag is not wasted and the chart already accounts for settled density. Buy what the math says plus one or two bags, and return the unopened extras.

Batts in an attic top-up go on unfaced and perpendicular to the joists, laid across the tops so they break the thermal bridge the joists create. Never use faced batts for the second layer — the facing between two layers of insulation is a vapour trap in exactly the wrong place.

Do It In This Order

The sequence matters more than the product choice. Everything on this list becomes unreachable the moment insulation goes down, so front-load the work that can only be done once.

1. Inspect first, on a cool morning. Look for roof leak stains on the underside of the deck, rodent activity, disconnected bath fan ducts, and any wiring that looks pre-1950. Fix leaks before insulating, always.
2. Air seal the big holes. Open chases around chimneys and flues, dropped soffits above kitchen cabinets, the tops of interior partition walls, and the gap around plumbing stacks. These few openings leak more than every small penetration combined. Use rigid material plus fire-rated sealant at flues, never expanding foam against a metal flue.
3. Seal the small penetrations. Wiring holes, ceiling boxes, recessed can housings, bath fan bodies. Caulk or low-expansion foam.
4. Confirm every exhaust fan and dryer duct terminates outdoors. A bath fan dumping into the attic will soak whatever you install over it.
5. Weatherstrip and insulate the hatch or pull-down stair. An uninsulated 10 sq ft hatch is a hole straight through your R-60.
6. Staple a baffle into every rafter bay along the eaves. Not a few — every one. They keep the soffit-to-ridge air path open and stop loose fill washing into the soffit.
7. Dam anything that must stay uncovered: non-IC-rated recessed lights, knob-and-tube runs, and a 3 in clearance around masonry chimneys and B-vent flues. Sheet metal or rigid board, not cardboard.
8. Mark the target depth. Staple rulers or write the depth on the rafters at several points before you start. Depth is impossible to judge by eye once the attic is white.
9. Lay walk boards. Once the joists are buried you cannot see where to step, and a foot through the ceiling is the most common attic accident.
10. Blow from the far corner back toward the hatch, keeping the hose moving so density stays even. Two people is far easier than one — someone feeding the hopper, someone on the hose.
11. Check depth as you go against your markers, and top up the thin spots at the end.

Covering, Not Covering, and Older-Home Safety

What you cover

• The whole ceiling plane, right out over the top plate of the exterior wall — that junction is a classic weak spot and it is where heat loss shows up as ice dams.
• Existing insulation, provided it is dry and not vermiculite. Blowing over old fiberglass is normal and effective.
• Joists. Once you are past joist depth, burying them is the point — wood conducts far better than insulation, and covered joists eliminate that bridge.
• IC-rated, airtight recessed housings, which are built and labelled for direct contact.

What you never cover

• Non-IC recessed lights. The label inside the can tells you. Non-IC needs 3 in of clearance all round; burying one is a fire risk and the thermal cutout will make the light flicker off long before anything worse happens. Replacing old cans with airtight IC LED housings is cheaper than building dams and fixes an air leak too.
• Knob-and-tube wiring. It relies on open air to shed heat. Code in most jurisdictions prohibits covering it. Replace it first.
• Chimneys and metal flues. Keep 3 in clearance to masonry chimneys and B-vent, dammed with sheet metal and sealed with high-temperature sealant.
• Soffit vents. Baffles, every bay.
• Anything hot: older transformers, some attic-mounted equipment, recessed fan housings not rated for contact.

Ventilation still has to work

Adding insulation makes the attic colder in winter, which is the entire point — but it also means the attic has less heat available to dry it out. If ventilation is marginal, more insulation can turn a dry attic into a damp one. The usual rule of thumb is 1 sq ft of net free vent area per 150 sq ft of attic floor, halved to 1 in 300 when there is a proper vapour retarder or when the vents are split roughly evenly between soffit and ridge. Balance matters more than total: intake at the soffits should at least equal exhaust at the ridge, or the attic will pull its make-up air from the house through every gap you failed to seal.

Older homes: check before you start

In a house built before roughly 1980, identify three things before any work begins:

• Vermiculite — loose, pebbly, grey-brown granules with a slight shine, often sold as Zonolite. A large share of it came from a mine contaminated with asbestos. Do not disturb it, sweep it, vacuum it, or blow over it without professional advice. Have it tested.
• Knob-and-tube wiring — ceramic knobs and cloth-sheathed conductors. Replacement is the only correct answer before insulating.
• Asbestos duct and pipe lagging — white or grey wrapping on heating pipes. Leave it and get an assessment.

None of these should stop a project permanently. All three should stop it for a phone call.

Protect yourself

Wear an N95 or better, sealed goggles, gloves, and long sleeves taped at the wrists. Work in the cool part of the day — attic temperatures in summer are genuinely dangerous, and heat exhaustion up there is far more common than any fibre exposure. Keep the hatch open for airflow, take breaks, and shower in cool water afterwards; hot water opens skin pores and drives fibres in. Once the insulation is done and the ceilings need patching, our drywall calculator and drywall mud calculator cover the repair side.

Frequently Asked Questions

How many bags of insulation do I need for my attic?

Divide the attic floor area by the coverage per bag listed on the bag at the R-value you are adding. A 1,200 sq ft attic adding R-38 with cellulose at roughly 28 sq ft per bag is about 43 bags. There is no universal number — coverage per bag falls as you blow deeper, so the chart on the bag is the only figure that counts.

How deep should attic insulation be?

Depends on the material and your target. For R-49, plan on roughly 14 in of cellulose or 20 in of blown fiberglass. For R-60 it is about 17 in of cellulose or 24 in of blown fiberglass. Fiberglass batts run about 12 in for R-38 and 14 in or more for R-49. Mark the depth on the rafters before you blow — you cannot judge it by eye.

What R-value should my attic be?

R-30 to R-49 in the deep south (zone 1), R-49 to R-60 across the middle of the country (zones 2 and 3), and R-60 from zone 4 north. Local codes vary and change, so confirm with your building department if the work is permitted or tied to a utility rebate. If your attic is currently under R-30, almost any climate justifies topping it up.

Can I add new insulation over old insulation?

Yes, in most cases, and it is the normal way to do it. Blow loose fill straight over old fiberglass or cellulose, or lay unfaced batts across the joists. Two exceptions: do not add over insulation that is wet — find and fix the leak first — and do not add over vermiculite until it has been tested for asbestos.

Should attic insulation cover the joists?

Yes, once you are past joist depth. Wood conducts heat far better than insulation, so exposed joist tops are continuous thermal bridges across your ceiling — in a cold climate you can often see them as stripes in frost or melted snow on the roof. Burying them is the whole reason a second layer is laid perpendicular to the joists.

Should I air seal the attic before adding insulation?

Yes, and it is the highest-value hour you will spend up there. Insulation slows conductive heat flow; it does not stop air. A sealed attic at R-38 outperforms a leaky one at R-60. Seal the big holes first — open chases, dropped soffits over cabinets, interior wall top plates, plumbing stacks — then the small ones. All of it becomes unreachable the moment insulation goes down.

Do I need baffles when I insulate an attic?

In a vented attic, yes — one in every rafter bay along the eave. Baffles hold the soffit-to-ridge air path open and stop loose fill washing forward into the soffit. Skip them and the attic loses its ability to dry, the roof deck stays damp, and in cold climates you get ice dams at the eaves. They cost a couple of dollars each and staple in with a hammer stapler.

Blown-in or batts for an attic?

Blown-in, in almost every case. It flows around wiring, ducts, bracing and irregular joist spacing, fills the awkward triangle at the eaves, and reaches high R without gaps. Batts only make sense in a small, clean, perfectly regular attic, or where you cannot get a blower hose in. Batts also depend entirely on install quality — a 5% gap around a batt costs far more than 5% of its R.

Cellulose or fiberglass for attic insulation?

Cellulose gives more R per inch (about R-3.5 versus R-2.5), packs denser so it resists air movement better, and is made from recycled paper treated with borate fire retardant. Blown fiberglass is lighter, does not absorb water, and each bag covers more area. If ceiling joists limit your depth, cellulose wins. If you have unlimited headroom and the price per R is lower, fiberglass is fine.

Can I cover recessed lights with attic insulation?

Only IC-rated housings, which are labelled as such inside the can. Non-IC housings need 3 inches of clearance on every side and covering one is a genuine fire risk. The cleanest fix is to replace old cans with airtight IC-rated LED housings before you insulate — usually cheaper than building dams and it seals a significant air leak at the same time.

Should attic insulation be faced or unfaced?

Unfaced whenever you are adding over existing insulation. A kraft or foil facing is a vapour retarder, and sandwiching one between two layers traps moisture inside the assembly. Faced batts are only appropriate as the first layer directly on the ceiling, with the facing down toward the heated space, and only in heating-dominated climates.

How do I measure my existing attic insulation?

Push a ruler straight down through the material until it hits the ceiling drywall, and read the depth. Do it in three or four spots — near the middle, halfway out, and about 4 ft in from the eaves — then average them. Insulation is always deepest in the centre. Multiply the average depth by the material R per inch to estimate what you have.

What if my attic has vermiculite insulation?

Stop and have it tested. Vermiculite — loose, pebbly, grey-brown granules, often branded Zonolite — largely came from a mine whose ore was contaminated with asbestos. Do not disturb it, sweep it, vacuum it, or blow insulation over it until a lab confirms what it is. Testing is inexpensive relative to the risk, and abatement is a licensed job, not a weekend one.

Does attic insulation settle over time?

Loose fill does, typically 10 to 20% in the first few years, which is why coverage charts specify a minimum installed weight per square foot as well as a depth. Insulation blown too fluffy hits the depth target on day one and falls below the rated R-value soon after. Cellulose settles more than fiberglass; batts do not settle but they do sag and get displaced by anyone walking through.