Quick answer: ENERGY STAR recommends R-60 for uninsulated attics in climate zones 4–8, R-49 in zones 2–3 and R-30 in zone 1. R-60 is about 24 inches of blown-in fiberglass or about 17 inches of blown-in cellulose.
Recommended Attic R-Values
| Climate zone | Uninsulated attic | Attic with 3–4 in existing | Floor |
|---|---|---|---|
| 1 | R-30 | R-25 | R-13 |
| 2 | R-49 | R-38 | R-13 |
| 3 | R-49 | R-38 | R-19 |
| 4A, 4B | R-60 | R-49 | R-19 |
| 4C, 5, 6 | R-60 | R-49 | R-30 |
| 7, 8 | R-60 | R-49 | R-38 |
Zone 1 is South Florida and Hawaii; zones 2–3 cover the South; zones 4–6 cover the Mid-Atlantic, Midwest and Northeast; zones 7–8 are northern Minnesota and Alaska.
How Many Inches Is That?
| Target | Blown fiberglass (≈R-2.5/in) | Blown cellulose (≈R-3.5/in) | Fiberglass batts (≈R-3.2/in) |
|---|---|---|---|
| R-30 | 12 in | 9 in | 9 in |
| R-38 | 15 in | 11 in | 12 in |
| R-49 | 20 in | 14 in | 15 in |
| R-60 | 24 in | 17 in | 19 in |
Topping Up Existing Insulation
- Measure the depth of your current insulation in several places.
- Estimate its R-value (older fiberglass and cellulose are roughly R-2.5 to R-3.5 per inch).
- Subtract from the target to find the R-value to add.
- Divide by the R-value per inch of your new material.
Example: a zone 5 attic with 4 inches (about R-12) needs about R-48 more, which is roughly 19 inches of blown-in fiberglass.
Skip the math: choose your zone, enter what you have now and get the depth and volume to add. Open the free Insulation Calculator →
Before You Insulate
- Air-seal gaps around plumbing, wiring, chimneys and attic hatches first.
- Install baffles so soffit vents stay open.
- Keep insulation away from recessed lights not rated for contact (IC) and from chimneys.
- Use unfaced batts or loose-fill over existing insulation; never add a second vapor barrier.
Better insulation shrinks heating and cooling loads; see how with the furnace size calculator.
How to Check Your Current Insulation
- Open the attic hatch safely and use a flashlight; walk only on joists or boards.
- Push a ruler down to the drywall in several spots.
- If you can see the tops of the ceiling joists, you almost certainly need more insulation.
- Look for gaps, compressed areas and damp or stained insulation, which signal leaks.
Blown-In vs Batts vs Spray Foam
| Type | Pros | Cons |
|---|---|---|
| Blown-in fiberglass | Fills gaps, low cost, no settling issues | Needs a blower machine |
| Blown-in cellulose | Higher R per inch, recycled content | Settles slightly over time |
| Batts | Easy DIY in open framing | Gaps around wiring reduce performance |
| Spray foam | Air seals and insulates | Expensive; professional install |
DIY or Professional?
Adding blown-in insulation to an open attic is a manageable DIY job; many home centers lend a blower machine when you buy enough bags. Hire a professional for spray foam, attics with vermiculite (which may contain asbestos), knob-and-tube wiring, or poor access.
Typical Savings
Insulating and air-sealing an under-insulated attic is one of the most cost-effective energy upgrades. Savings depend on climate and how much insulation you start with; homes with little or no attic insulation see the biggest improvement.
Sources
Frequently Asked Questions
How many inches of insulation should be in my attic?
In most of the US, enough for R-49 to R-60: about 20–24 inches of blown-in fiberglass or 14–17 inches of cellulose.
Is R-38 enough for an attic?
R-38 is the recommended top-up level in zones 2–3 with existing insulation; colder zones benefit from R-49 to R-60.
Can you have too much attic insulation?
Beyond the recommended level, savings drop off, but extra insulation does not cause harm if ventilation stays clear.
Is blown-in insulation better than batts?
Blown-in fills gaps around framing better in attics; batts are easier in open walls and floors.
How many bags of blown-in insulation do I need?
It depends on the brand. Use the coverage chart on the bag for your target R-value and attic area.
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Try the free Insulation Calculator or browse all home improvement calculators.