How Much Attic Insulation Do You Need?

Adding attic insulation is one of the highest-return improvements available in an older American house, and one of the easiest to get wrong at the store — because the number on the bag is not the number you need.

Insulation is specified in R-value, a measure of resistance to heat flow, and the quantity that matters is not your target but the gap between what is already up there and where you want to be.

The calculation

R gap        = target R − current R
added depth  = R gap ÷ R per inch
coverage/bag = bag coverage at R-1 ÷ R gap
bags         = attic area ÷ coverage per bag

A 1,200 square foot attic going from R-19 to R-49 needs an R-30 top-up. In blown cellulose at roughly 3.5 R per inch, that is about 8.6 inches of additional depth. If a bag covers 1,000 square feet at R-1, it covers 33 square feet at R-30 — so 36 bags.

The insulation calculator runs this and adjusts the coverage automatically, which is the step that trips people up in the aisle.

Why coverage is not one number

This is the detail worth understanding, because it is where the ordering error happens.

A bag holds a fixed volume of material. Spread that volume thinly over a large area and you get a low R-value. Pile it deep over a small area and you get a high one. Coverage and R-value are inversely related, which is why every bag label quotes coverage at a stated R-value.

Read "covers 40 square feet" without checking which R-value it refers to and you can easily order half of what the job requires. Doubling your target R roughly halves the area each bag covers.

Working out what you already have

Measure the depth of the existing insulation with a ruler pushed down to the ceiling drywall, and multiply by roughly 3 per inch as a rough average across common materials.

Treat that as an upper bound. Old insulation settles and compresses, and damp or dirty material performs below its nominal rating. If the existing layer is visibly compacted, patchy, or dark with dust tracked by air movement, assume less than the arithmetic suggests — those dark patches are literally a map of where air is leaking through.

Targets by region

Broadly, the Department of Energy recommends R-49 to R-60 for attics across the northern United States and R-30 to R-49 further south. Local code sets a minimum for new work, which is usually below the recommendation.

Material choice affects how much depth that requires:

Material	R per inch	Notes
Blown cellulose	~3.5	Most R per inch, recycled, settles somewhat
Fiberglass batts	~3.2	Cheap, easy to install badly
Blown fiberglass	~2.5	Handles occasional moisture best

Cellulose reaches a target in less depth, which matters in a shallow attic where you do not want insulation spilling into the soffits. Batts are the poorest choice for a top-up over existing material, because they only perform in full contact with no gaps, and an attic floor full of joists, wiring and junction boxes guarantees gaps.

Air sealing first — genuinely

This is the most useful thing on this page and it is not a quantity.

Insulation slows conduction, heat moving through material. It does very little about convection, air moving through gaps. A typical attic leaks around recessed lights, plumbing stacks, wiring penetrations, the top plates of interior walls, and above all a bare attic hatch.

Blowing insulation over unsealed penetrations buries the problem rather than fixing it, and the installed R-value will underperform its rating. Seal first with foam and caulk, weatherstrip the hatch, then insulate. Sealing typically costs a fraction of the insulation and frequently delivers more comfort per dollar.

Diminishing returns are real

Heat loss is proportional to 1/R, so each increment buys less than the last:

• R-0 to R-13 removes roughly 92% of conductive loss
• R-13 to R-30 removes about 6% more
• R-30 to R-49 removes roughly 1.5% more
• R-49 to R-60 is a fraction of a percent

This is not an argument against reaching code — blown insulation is cheap and the marginal cost of another few inches is small. It is an argument against paying a premium to exceed code while air leaks and uninsulated ducts go untouched. If your ductwork runs through the attic, insulating those ducts often beats another ten points of R on the floor.

What the bag count leaves out

• Baffles at the eaves, keeping insulation out of the soffit vents. Block those and you trade an energy problem for a moisture problem.
• Ventilation, which must be maintained. Insulation should never reach the roof deck.
• Thermal bridging through the joists, which conduct around the insulation, so real assemblies perform slightly below nominal.
• Knee walls and sloped ceilings, which are different assemblies entirely.
• Existing moisture damage. Wet or mouldy insulation must come out, not be covered over.

Timing it with other work

The cheapest moment to do the attic is when the roof is already off. The roofing calculator sizes that job, and how to measure a roof for shingles covers the pitch multiplier that decides the shingle count. Any ceiling opened up in the process is handled by the drywall calculator.

For recommendations by climate zone, the Department of Energy's insulation guidance is the primary US source. Check your utility for rebates before buying — several require pre-approval, and they can change the payback considerably.