How the attic calculation works
Attic insulation is rated by R-value, and R-values add up. If there's already some insulation on your attic floor, you only need to add the difference:
R to add = recommended R for your zone − R of what's there now
- Find your target. ENERGY STAR recommends R-30 for attics in climate zone 1, R-49 in zones 2–3 and R-60 in zones 4–8. Enter your ZIP code and the calculator picks the right one.
- Measure what's there. Push a ruler down to the ceiling drywall in several spots and use a typical depth. Note the type of material.
- Convert depth to R. Multiply the depth by ENERGY STAR's per-inch value for older insulation:
| Existing material | R per inch |
|---|---|
| Fiberglass batts | 3.2 |
| Loose fiberglass | 2.5 |
| Cellulose | 3.7 |
| Rock wool (loose) | 2.8 |
| Vermiculite | 2.7 |
- Turn the R you need into product. For blown-in insulation, the calculator reads the manufacturer's coverage chart for depth and square feet per bag. For batts, it picks the fewest layers of standard batt sizes that reach the target, then the thinnest of those stacks.
The area is the attic floor, measured from the inside of the exterior walls. Length × width is fine for a simple rectangle. For an L-shaped house, add up the rectangles and enter the total.
Worked example: a 1,200 sq ft attic in zone 5A
A 40 × 30 ft attic floor (1,200 sq ft) in climate zone 5A, with 3.5 in of old fiberglass batts between the joists:
- Existing: 3.5 in × 3.2 = R-11
- Target for zone 5A: R-60
- R to add: about R-48.8
- Area with 10% waste: 1,320 sq ft
| What you add | Depth | To buy | Material cost | You end up at |
|---|---|---|---|---|
| Blown cellulose | 14.2 in, settles to 13.2 in | 94 bags | $1,503–$1,689 | R-60 |
| Blown fiberglass | 16.3 in | 35 bags | $1,515–$1,702 | R-60 |
| Fiberglass batts | One layer of R-49 (14 in) | 28 bags | $2,996 | R-60 |
| Mineral wool batts | R-30 (7.25 in) + R-23 (5.5 in) | 79 bags | $7,423 | R-64 |
In this example, blown-in insulation costs roughly half as much as batts for the same result. Mineral wool batts are thinner per R, but they come in narrower bags with less coverage, so you'd need two layers and many more bags.
If you go with batts over existing insulation, buy unfaced batts and lay the new layer across the joists. A facing sandwiched between two layers can trap moisture. Our batt prices are for kraft-faced batts, so check the shelf price for unfaced and enter it under More options.
Get the attic ready first
Seal air leaks
Insulation slows heat flow, but air carries heat straight through it. Before adding insulation, seal the gaps where warm air rises into the attic: around plumbing vents, wiring holes, duct boots, light fixtures and the top plates of interior walls. Once a foot of insulation covers them, they're very hard to find. Around chimneys and flues, use fire-rated materials.
Keep soffit vents clear with baffles
If your attic is vented at the eaves, staple baffles (rafter vents) between the rafters at the eave before you add insulation. They hold the insulation back so outside air can still flow from the soffit vents up under the roof deck. Blocked vents can lead to moisture problems and ice dams.
Recessed lights, chimneys and flues
Don't cover recessed light fixtures unless they're rated IC (insulation contact). Non-IC fixtures get hot. Either keep insulation clear of them or have them replaced with IC-rated, airtight fixtures. Keep insulation away from chimneys, flues and furnace vents, and follow the product's instructions for building a noncombustible dam around them.
Wiring and the hatch
Have an electrician look at any old knob-and-tube wiring before you bury it. Insulate and weatherstrip the attic hatch, and build a dam around the opening so loose-fill doesn't spill when you open it.
Vermiculite
If the existing insulation is pebbly and gray-brown or silvery-gold, it may be vermiculite, which can contain asbestos. The calculator warns you if you choose it. The EPA's advice is to leave it undisturbed and to hire a trained, accredited asbestos professional before any work that would disturb it, including adding insulation on top.
Common mistakes
- Measuring in one spot. Insulation is often thinner near the eaves and around the hatch. Measure in several places.
- Compressing batts. A batt squeezed under a walkway or into a tight space gets thinner and loses part of its R-value.
- Blocking the soffits. See baffles above.
- Stepping between the joists. The ceiling drywall won't hold your weight. Walk on the joists or lay boards across them.
- Skipping the bag count. For blown-in, hitting the right depth with too few bags means the layer is too fluffy to reach its rated R. See the blown-in insulation calculator for how to check both.
To see how thick each R-value is for every material, use the R-value chart. For other parts of the house, start from the insulation calculator.
Sources
- ENERGY STAR: Recommended home insulation R-values
- ENERGY STAR: DIY checks and inspections (depth multipliers)
- U.S. DOE: Guide to Home Insulation
- GreenFiber SANCTUARY attic coverage chart
- Knauf EcoFill Wx blowing insulation product data
- Owens Corning PINK Next Gen Fiberglas fact sheet
- ROCKWOOL Comfortbatt R-value fact sheet
- U.S. EPA: Asbestos-contaminated vermiculite insulation