How the insulation calculator works
Batts are sold by the bag, and each bag label says how many square feet it covers. Blown-in insulation is sold by the bag too, but how far a bag goes depends on how deep you blow it, so every bag carries a coverage chart: bags per 1,000 sq ft for each R-value. The calculator finds your area, then uses the label or the chart.
Wall area = wall length × height − 21 sq ft per door − 15 sq ft per window
Batt bags = area ÷ square feet per bag, rounded up
Existing R = existing depth (in) × R per inch
R to add = target R − existing R
Blown-in bags = attic area × bags per 1,000 sq ft for that R ÷ 1,000, rounded up
When the R-value you need falls between two rows of the chart, the calculator reads between them in a straight line. It does the same for the depth to blow.
Worked example: walls
A house has 120 ft of outside wall, 8 ft high, with 2 doors and 10 windows. That is 960 sq ft, minus 42 sq ft of doors and 150 sq ft of windows, so 768 sq ft. R-13 batts for a 2x4 wall come in bags of 106.56 sq ft, so you need 7.2 bags: 8 bags.
Worked example: topping up an attic
A 1,000 sq ft attic has 3 1/2 in of old loose fiberglass, worth about R-8.75 (3.5 × 2.5). To reach R-49 you need to add about R-40. On the fiberglass chart that sits between R-38 (19.5 bags, 13 in) and R-44 (23 bags, 14 3/4 in), so you need 20.8 bags per 1,000 sq ft: 21 bags, blown about 13 3/4 in deep on top of what is there. With cellulose it is 57 bags, about 11 in deep.
Recommended R-values
ENERGY STAR's attic levels, based on the 2021 International Energy Conservation Code. Find your zone on the ENERGY STAR climate zone map.
| Climate zone | Attic with little or no insulation | Adding to 3 to 4 in already there |
|---|---|---|
| 1 | R-30 | Add R-25 |
| 2 and 3 | R-49 | Add R-38 |
| 4 to 8 | R-60 | Add R-49 |
For walls, the framing sets the limit: R-13 or R-15 batts fill a 2x4 wall and R-19 or R-21 batts fill a 2x6 wall.
Coverage figures used
| R-value | Blown fiberglass, bags per 1,000 sq ft | Blown cellulose, bags per 1,000 sq ft | Batts, sq ft per bag |
|---|---|---|---|
| R-13 | 6.6 (4 1/2 in) | 15.2 (3.6 in) | 106.56 |
| R-19 | 9.4 (6 1/2 in) | 22.6 (5.3 in) | 78.79 |
| R-21 | — | — | 67.81 |
| R-30 | 15.3 (10 1/4 in) | 38.6 (8.3 in) | 58.67 |
| R-38 | 19.5 (13 in) | 52.6 (10.4 in) | 42.67 |
| R-49 | 25.8 (16 1/2 in) | 70.8 (13.3 in) | — |
| R-60 | 32.1 (20 in) | 86.0 (16.1 in) | — |
Fiberglass figures are Owens Corning AttiCat (28.5 lb bags); cellulose figures are Greenfiber SANCTUARY (25 lb bags); batt figures are Owens Corning EcoTouch bag labels. Other brands differ, sometimes a lot, so type the row from your own bag into the calculator when you have it. The chart's bag count and depth are both minimums: blow to at least that depth everywhere.
Measuring
- Walls: measure the length of each outside wall (or each wall you are insulating) along the floor and add them up. Measure height from floor to ceiling.
- Attic: measure the attic floor, or use the outside size of the house minus the overhang. Split L-shaped houses into rectangles with Add a section.
- Existing insulation: push a ruler down to the ceiling drywall in a few spots away from the eaves and use a typical depth.
Before you buy
- Seal air leaks first: gaps around light fixtures, pipes, chimneys and the attic hatch. Insulation does not stop air moving through those holes.
- Fit baffles at the eaves so blown insulation does not block the soffit vents, and keep insulation the required distance from recessed lights that are not rated IC and from chimneys.
- Do not disturb vermiculite (gray-brown, pebbly loose fill) until it has been tested for asbestos.
- Wear a respirator, goggles, long sleeves and gloves. Fiberglass and cellulose dust both irritate.
Sources
- ENERGY STAR: Recommended home insulation R-values
- ENERGY STAR: DIY checks, including R-value per inch of existing insulation
- Owens Corning: AttiCat blown-in insulation coverage chart (PDF)
- Greenfiber: SANCTUARY cellulose attic coverage chart (PDF)
- Owens Corning R-13 kraft-faced batts, 15 x 93 in: bag coverage
- Owens Corning R-38 unfaced batts, 16 x 48 in: 42.67 sq ft per bag
Common questions
How many bags of blown-in insulation do I need for 1,000 square feet?
It depends on the R-value and the product. On Owens Corning's AttiCat chart, R-38 takes 19.5 bags per 1,000 sq ft and R-49 takes 25.8. Cellulose bags are smaller and denser: Greenfiber's chart needs 52.6 bags for R-38 and 70.8 for R-49. Always use the chart printed on the bag you buy.
What R-value should my attic have?
ENERGY STAR recommends R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8 for an attic with little or no insulation. If you already have 3 to 4 inches, it suggests adding R-25 in zone 1, R-38 in zones 2 and 3, and R-49 in zones 4 to 8.
How do I work out the R-value of the insulation I already have?
Measure its depth in inches and multiply. ENERGY STAR uses about R-2.5 per inch for loose fiberglass, R-2.8 for rock wool, R-3.7 for cellulose and R-3.2 for fiberglass batts. Six inches of old loose fiberglass is roughly R-15.
What insulation goes in a 2x4 wall?
R-13 or R-15 batts, which are 3 1/2 inches thick and fill the cavity without squashing. A 2x6 wall takes R-19 or R-21 batts.
Can I put new insulation on top of old insulation?
Usually, yes, as long as the old insulation is dry and not moldy. Use unfaced batts or loose fill on top so you do not trap a vapor barrier between layers. Have vermiculite tested for asbestos before you disturb it.