What R-value do I need in an attic?

The Department of Energy suggests R-49 to R-60 across most of the United States, and R-30 to R-49 in the warmest zones. It is a recommendation rather than a code requirement — your local building code sets the minimum, and it is often lower than the recommendation.

Can I add insulation on top of what is there?

Over unfaced insulation, yes, and it is the usual approach: R-values add, so R-19 plus R-30 is R-49. Do not lay faced batts on top of existing insulation with the vapour barrier facing up — a second vapour barrier traps moisture between the layers. Cut the facing or use unfaced material.

Does more depth keep adding value?

It adds, but with diminishing returns. Going from R-0 to R-19 cuts heat loss through that surface by about 95%; going from R-19 to R-49 cuts what remains by about 60%, which is a much smaller share of the original. Air sealing before insulating is usually worth more than the last ten R-values.

Why do the bags cover less than the label says?

Because the coverage figure is per inch of depth. A bag that covers 240 square feet at one inch covers 24 at ten inches and 20 at twelve. Reading the headline number without the depth is the commonest reason an attic job runs short by half.

Insulation Calculator

With area to insulate 1200 sq ft, r-value wanted 49, r-value already there 19, r-value per inch of the material 2.5 and 2 more fields, insulation comes to 60 bags — bags to buy. It is reached in 6 steps, the last of which is ceil(1200 / 20), and each one is printed on the page with its numbers filled in. The formula is the one published by ENERGY STAR, not an approximation fitted to it.

Bags or batts of insulation for an attic or a wall, from the area and the R-value you are aiming at.

Formula and sources checked 27 August 2026 · How we check

Area to insulate 1200 sq ft, R-value wanted 49, R-value already there 19, R-value per inch of the material 2.5

60 bags

Bags to buy for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.

It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.

Area to insulatesq ftAttic floor area, or wall area for a wall.R-value wantedThe US Department of Energy suggests R-49 to R-60 for attics in most of the country.R-value already thereSix inches of old fibreglass is roughly R-19.R-value per inch of the materialBlown fibreglass 2.2–2.7, blown cellulose 3.2–3.8, closed-cell foam 6–7.Coverage per bag at 1 inchsq ftFrom the bag: manufacturers print a coverage chart by depth.Price per bagusd

Bags to buy

60 bags

R-value to add: max(0, 49 - 19)30
Depth to add: 30 / 2.512 in
Coverage per bag at that depth: 240 / 1220 sq ft
Bags needed: ceil(1200 / 20)60 bags
Material cost: 60 * 45$2,700
R-value when finished: 19 + 12 * 2.549

Several at once

A job is rarely one of anything. Work out the first, add it, change the figures and add the next — the list keeps what each line was, and totals them.

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Worked example

A 1,200 sq ft attic with R-19 already in it, going to R-49: 30 more R-value at 2.5 per inch is 12 inches of blown fibreglass. A bag covering 240 sq ft at one inch covers 20 sq ft at twelve, so it takes 60 bags.

How to work it out yourself

1.Measure the attic floor area, not the roof area — insulation goes on the flat.
2.Estimate what is already there by depth: fibreglass batts are roughly R-3.2 an inch, blown fibreglass 2.2 to 2.7, cellulose 3.2 to 3.8.
3.Read the coverage chart on the actual bag. Coverage per bag is quoted at one inch and falls in proportion to the depth you are spreading it to.

The formula

R-value to addmax(0, 49 - 19)
Depth to add30 / 2.5
Coverage per bag at that depth240 / 12
Bags neededceil(1200 / 20)
Material cost60 * 45
R-value when finished19 + 12 * 2.5

Source: ENERGY STAR — seal and insulate: recommended insulation levels by climate zone, NIST Handbook 44, Appendix C — general tables of units of measurement

Questions people actually ask

What R-value do I need in an attic?: The Department of Energy suggests R-49 to R-60 across most of the United States, and R-30 to R-49 in the warmest zones. It is a recommendation rather than a code requirement — your local building code sets the minimum, and it is often lower than the recommendation.
Can I add insulation on top of what is there?: Over unfaced insulation, yes, and it is the usual approach: R-values add, so R-19 plus R-30 is R-49. Do not lay faced batts on top of existing insulation with the vapour barrier facing up — a second vapour barrier traps moisture between the layers. Cut the facing or use unfaced material.
Does more depth keep adding value?: It adds, but with diminishing returns. Going from R-0 to R-19 cuts heat loss through that surface by about 95%; going from R-19 to R-49 cuts what remains by about 60%, which is a much smaller share of the original. Air sealing before insulating is usually worth more than the last ten R-values.
Why do the bags cover less than the label says?: Because the coverage figure is per inch of depth. A bag that covers 240 square feet at one inch covers 24 at ten inches and 20 at twelve. Reading the headline number without the depth is the commonest reason an attic job runs short by half.

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