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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
The honest answer depends on three things: how much area you are covering, how many storeys that area is spread over, and what R-value your climate asks for. Enter your house below and this gives you each surface separately, because they are rarely the same size and almost never take the same material.
With these inputs:
| Result | Value |
|---|---|
| Attic area to insulate | 1,500 sq ft |
| Attic target for your zone | R-60 |
| Settled depth needed | 16.7 in of blown-in cellulose |
| Bags of loose fill | 165 bags |
| Exterior wall area | 1,059 sq ft |
| Wall target for your zone | R-20 |
| Floor target for your zone | R-30 |
| Attic cost estimate | $3,100 |
Attic area = total floor area / number of storeys · Wall area = 4 x sqrt(footprint) x wall height x storeys - openings · Settled depth = target R-value / material R per inch · Bags = (target R x attic area) / bag coverage x (1 + waste)
| House size | Single storey attic | Two storey attic | Depth for R-49 | Depth for R-60 |
|---|---|---|---|---|
| 800 sq ft | 800 sq ft | 400 sq ft | 13.6 in | 16.7 in |
| 1,000 sq ft | 1,000 sq ft | 500 sq ft | 13.6 in | 16.7 in |
| 1,200 sq ft | 1,200 sq ft | 600 sq ft | 13.6 in | 16.7 in |
| 1,500 sq ft | 1,500 sq ft | 750 sq ft | 13.6 in | 16.7 in |
| 1,800 sq ft | 1,800 sq ft | 900 sq ft | 13.6 in | 16.7 in |
| 2,000 sq ft | 2,000 sq ft | 1,000 sq ft | 13.6 in | 16.7 in |
| 2,500 sq ft | 2,500 sq ft | 1,250 sq ft | 13.6 in | 16.7 in |
| 3,000 sq ft | 3,000 sq ft | 1,500 sq ft | 13.6 in | 16.7 in |
Depths shown for blown cellulose at R-3.6 per inch. A two storey house of the same floor area has half the attic — which is why 'how much for a 2,000 sq ft house' has two different right answers.
| Zone | Attic | Walls | Floors |
|---|---|---|---|
| Zone 1 — S. Florida, Hawaii | R-30 | R-13 | R-13 |
| Zone 2 — Gulf Coast, S. Texas | R-49 | R-13 | R-13 |
| Zone 3 — Southeast, S. California | R-49 | R-20 | R-19 |
| Zone 4 — Mid-Atlantic, Pacific NW | R-60 | R-20 | R-25 |
| Zone 5 — Midwest, New England | R-60 | R-20 | R-30 |
| Zone 6 — N. Midwest, N. New England | R-60 | R-21 | R-30 |
| Zone 7 — N. Minnesota, Maine | R-60 | R-21 | R-38 |
| Zone 8 — Alaska interior | R-60 | R-21 | R-38 |
These follow the DOE recommendations by climate zone. Local code may require more; it rarely allows less.
If it is single storey, the attic is about 1,500 sq ft; if it is two storeys, the attic is about 750 because it only covers the top floor. That distinction changes the answer by half and is the most common mistake in DIY estimates. At the R-49 typical of much of the country, 1,500 sq ft of attic needs roughly 13.6 inches of settled cellulose. Put your own figures in above for the exact numbers.
Around 1,000 sq ft of attic for a single storey, or 500 if it is spread over two. Walls are the other half of the job and work out at roughly 4 times the square root of the footprint, times wall height — for a 1,000 sq ft single storey that is about 1,000 sq ft of gross wall before you deduct windows and doors.
Both, depending on the material, and mixing them up is how people over-order. Batts and boards are sold by area, so square feet is the unit. Loose fill is sold by bag, and a bag's coverage depends on the R-value you are aiming for — the same bag covers far less area at R-60 than at R-30, because you are laying it deeper. The calculator gives whichever unit applies to your choice.
Measure the settled depth in the attic with a ruler pushed down to the ceiling drywall, then divide by the R per inch of whatever is up there. Old fibreglass that has compressed over decades performs closer to its actual depth than its original rating, so measure what is there rather than trusting what was installed. You only need to add the difference between that and your target.
No, and the recommendations differ a lot. Attics get the highest target because heat rises and attic space is cheap to fill deeply; walls are limited by cavity depth so their target is lower; floors sit in between and matter most over an unheated crawlspace or garage. The zone table above shows all three for every zone.
The attic, almost always. It is the cheapest surface per unit of R-value because you can simply add depth without opening anything up, and it is where the largest temperature difference is. Walls need the cavity opened or drilled and are limited to what fits, which makes them a much larger job for a smaller gain.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.