Attic Insulation Buying Guide: R-Values, Materials and How Much You Need

Buy attic insulation to reach R-38 to R-60 in most US climate zones — roughly 12 to 18 in (30 to 46 cm) of fibreglass batts or 10 to 14 in (25 to 36 cm) of blown cellulose — and choose blown-in insulation for an existing attic with obstacles, batts for an open attic you can walk and roll in. Measure your attic floor area first with the Square Footage Calculator, then price the material with the Insulation Calculator: a 1,000 sq ft attic topped up from R-19 to R-49 typically needs 10 to 12 bags of blown cellulose or about 1,000 sq ft of R-30 batts, and costs $1,200 to $2,500 installed. If rooms stay cold in winter, read our winter maintenance checklist for the draught and heating checks that go with insulation, and our renovation budgeting guide before committing to a larger energy project.

What R-value your attic actually needs

R-value measures resistance to heat flow, and higher is better, but only up to the point your climate justifies. The US Department of Energy recommends R-49 to R-60 for attics in northern zones 5 to 8, R-38 to R-49 in zones 3 to 4, and R-30 to R-49 in warm zones 1 to 2, for homes with existing insulation of R-11 or less. Most older homes fall far short: a 1970s attic often carries 3 to 4 in of old fibreglass, about R-11, which means two-thirds of the recommended insulation is simply missing. Adding insulation is the rare home improvement with a payback measured in heating bills within 3 to 7 years in cold climates, and it also cuts summer cooling load because the same barrier slows heat entering from a hot roof.

Do not chase R-value past the point of diminishing returns. Going from R-11 to R-49 typically cuts ceiling heat loss by roughly 70 percent, while going from R-49 to R-60 saves only a few percent more for a real extra cost in material and depth. Depth is the practical limit: R-49 in fibreglass batts needs about 15 in of material, which buries joists and makes the attic hard to use for storage. If you need the attic floor for storage, build up the joists with a raised platform or accept R-38 with a storage deck over part of the floor and full depth elsewhere. Always check your local code — some jurisdictions now require R-49 as a minimum for new work, and a permit may be needed if you are changing ventilation at the same time.

Batts, blown-in, or spray foam: choosing the material

Fibreglass batts are the cheapest material, about $0.60 to $1.20 per sq ft for the material alone, and they suit an open attic with standard 16 in or 24 in joist spacing and few obstructions. Their weakness is fit: a batt compressed around a wire, a pipe, or a junction box loses R-value exactly where it is squeezed, and gaps between batts become thermal bypasses. If you choose batts, buy the width that matches your joist spacing exactly, split batts around obstacles rather than compressing them, and lay a second layer perpendicular across the joists to cover the thermal bridging of the joists themselves.

Blown-in cellulose or fibreglass, at $1.00 to $2.00 per sq ft installed, is the better choice for most existing attics because it flows around obstacles, fills odd corners, and can be topped up over old insulation without removing it. Cellulose, made from recycled paper treated with borates, settles about 10 to 20 percent over time, so a good installer blows it 10 to 20 percent deeper than the target settled depth and notes the settled R-value on the job card. Fibreglass blow-in settles less but is lighter and can drift in a ventilated attic if baffles are missing. Spray foam, at $2.50 to $5.00 per sq ft, gives the highest R-value per inch (R-6 to R-7 per inch for closed-cell) and air-seals at the same time, but it is a pro-only job, it changes how the roof assembly dries, and it costs two to three times as much — reserve it for attics being converted to living space or where the roofline itself must be insulated.

Air-seal first, insulate second

Insulation without air-sealing is like a winter coat left unzipped. Warm air escapes through gaps around light fixtures, plumbing stacks, chimney chases, attic hatches, and the tops of interior walls, carrying moisture that condenses in the insulation and the roof deck. Before adding a single batt, seal those bypasses with caulk for gaps under 1/4 in, expanding foam for larger gaps, and fire-rated materials around chimneys and flues — never ordinary foam against a hot flue. Seal the attic hatch with weatherstripping and an insulated hatch cover, because an unsealed hatch is a hole roughly the size of a small window in your thermal envelope.

Ventilation must be preserved while you insulate. Soffit vents need baffles (rafter vents) that keep insulation at least 2 in clear of the roof deck so air can flow from eaves to ridge. Blocking soffits with blown insulation is the most common DIY insulation mistake, and it causes exactly the ice dams and roof rot the insulation was meant to prevent. The rule of thumb is 1 sq ft of net free vent area per 150 sq ft of attic floor, split evenly between intake at the eaves and exhaust at the ridge, or 1 per 300 sq ft with a vapour barrier and balanced vents. If your attic has no ridge vent, add one or use gable vents — but do not mix vent types that short-circuit airflow.

How much material to buy, and what it costs

Work from the attic floor area, not the roof area. A 1,200 sq ft attic at R-49 needs roughly 1,200 sq ft of batts plus 10 percent waste for cutting, or about 40 to 50 bags of blown cellulose at typical coverage of 25 to 30 sq ft per bag at R-49 depth. Run your exact dimensions through the Insulation Calculator, then add the waste allowance — batts cut around obstacles waste 10 to 15 percent, blown-in waste is near zero because it fills every cavity. Material-only costs run $700 to $1,500 for a 1,000 sq ft attic in batts or blown cellulose, $1,200 to $2,500 professionally installed, and $3,000 to $6,000 for spray foam on the same area. DIY blown-in is realistic for one weekend: most home centres lend the blowing machine free with a minimum bag purchase, and two people can blow 1,000 sq ft in 3 to 4 hours.

Watch for the extras that change the quote. Baffles cost $2 to $5 each and a typical attic needs 20 to 40. An insulated attic hatch cover is $50 to $150. If old insulation is wet, mouldy, or vermiculite — the grey-brown pebble insulation used before 1990 that may contain asbestos — do not disturb it yourself; get it tested (about $50 to $100 per sample) and removed professionally if needed, which can add $1,500 to $3,000 to the job. Knob-and-tube wiring buried under new insulation is a fire risk and must be evaluated by an electrician first, since insulation over it traps heat. A reputable quote itemises baffles, air-sealing, hatch treatment, and disposal separately, so you can see what you are paying for beyond the bags.

Installation mistakes that waste the whole job

The five mistakes that ruin attic insulation are all avoidable. First, compressing batts to fit a shallow cavity — compressed insulation loses R-value in direct proportion to the compression, so a 9 in batt squeezed into 6 in performs like R-19, not R-30. Second, leaving gaps at eaves, around fixtures, and at the hatch, which become the thermal bypasses that dominate heat loss once the field is insulated. Third, covering recessed lights that are not rated IC (insulation contact) — non-IC lights need a 3 in clearance box, and burying them is a fire hazard. Fourth, blocking soffit vents, as above. Fifth, adding a second vapour barrier on top of old faced insulation, which traps moisture between layers; if the old insulation is faced, either remove the facing, slash it, or use unfaced material on top.

After installation, verify the work before paying the final invoice. Depth markers should be visible every 300 sq ft showing the installed depth, the job card should state the material, the installed depth, and the settled R-value, and a quick attic walk (on boards, never on the ceiling drywall) should show even coverage with no bare patches at eaves or around obstacles. In the first winter, check for even snow melt on the roof — patchy melt lines over bypasses mean air is still leaking — and check the attic for frost on nails or sheathing, which signals moisture getting past the air barrier. Insulation done right is invisible, quiet, and boring, and that is exactly how you know it worked.