Recommended attic R-values
In the US, ENERGY STAR and the Department of Energy publish recommended attic insulation levels by climate zone. The current recommendations are roughly:
| Climate | US climate zones | Uninsulated attic | Attic with 3 to 4 in already |
|---|---|---|---|
| Hot | 1 | R-30 to R-49 | Add R-25 to R-30 |
| Warm to mixed | 2 to 3 | R-49 to R-60 | Add R-38 to R-49 |
| Mixed to very cold | 4 to 8 | R-60 | Add R-49 |
Building codes set minimums for new construction and major renovations, and they change over time. In Canada the requirements are set by provincial and national codes, and in Australia by the National Construction Code. Check the current rules for your area, and check whether any rebate program requires a minimum level.
How many inches is that?
Depth depends on the material. These use typical R-value per inch figures: about 3.2 for fiberglass batts, 3.5 for blown cellulose and 2.5 for blown fiberglass.
| Target | Fiberglass batts | Blown cellulose | Blown fiberglass |
|---|---|---|---|
| R-30 | 9.4 in | 8.6 in | 12 in |
| R-38 | 11.9 in | 10.9 in | 15.2 in |
| R-49 | 15.3 in | 14 in | 19.6 in |
| R-60 | 18.8 in | 17.1 in | 24 in |
Blown fiberglass needs the most depth for the same R-value, but it is light and settles little. Cellulose is denser, gives more R per inch and is good at filling gaps, but it settles more. Always install to the "initial installed thickness" on the bag so the settled depth still meets the target.
How much do you have now?
- Go into the attic safely, stepping only on joists, with a flashlight and dust mask.
- Measure the depth of insulation at several spots with a ruler, away from the eaves.
- Identify the type: batts are blankets in rolls; loose fill is fluffy or shredded.
- Multiply depth by the R per inch above to estimate the current R-value.
A quick visual test: if you can see the tops of the ceiling joists, the insulation is no deeper than the joists, usually 5.5 to 9.25 in, or roughly R-19 to R-30. That is below the recommendation everywhere except the hottest zone.
Worked example: topping up an older attic
A 1,400 sq ft ranch house in a warm-to-mixed climate (zone 3) has old batts about 3.5 in deep, roughly R-11. The recommendation for adding to 3 to 4 in of existing insulation there is R-38 to R-49. Adding R-38 brings the total to about R-49.
- With blown cellulose at R-3.5 per inch: 38 ÷ 3.5 = 10.9 in on top of the old batts.
- With blown fiberglass at R-2.5 per inch: 38 ÷ 2.5 = 15.2 in.
For bag count, read the coverage chart on the bag at R-38. If it said 20 sq ft per bag (a figure used here only to show the math), 1,400 ÷ 20 = 70 bags. The insulation calculator does both steps; choose R-38 and enter the coverage from your product.
Which material should you use?
For topping up an existing attic, blown-in insulation is usually the easiest choice because it fills around wiring, framing and old insulation without cutting. Cellulose is made largely from recycled paper treated with fire retardant and is dense enough to slow air movement a little. Blown fiberglass is lighter, which matters on ceilings with wide joist spacing, and it does not absorb water as readily. Batts are a good choice for an empty attic with open, evenly spaced joists, and for walls and floors during renovation. Whatever you choose, the R-value on the label, installed at the stated depth without gaps or compression, is what counts.
Do these first
- Air seal. Warm air leaking through gaps around pipes, wires, light fixtures, top plates and the attic hatch carries heat and moisture. Seal with foam or caulk before adding insulation.
- Ventilation. Install baffles at each rafter bay so insulation does not block soffit vents.
- Insulate the hatch. Glue rigid foam to the back of the hatch and add weatherstripping.
- Check for problems. Roof leaks, mold, pests or bathroom fans venting into the attic should be fixed first.
Metric R-values
Australia and other metric countries label insulation with metric R-values, which are RSI. To convert, multiply a US R-value by 0.1761, or divide a metric R-value by 0.1761. US R-49 is about RSI 8.6, and an Australian R6.0 ceiling batt is about US R-34.
Safety
Wear a respirator or well-fitting dust mask, goggles, gloves and long sleeves. Work in the cooler part of the day. Keep insulation clear of chimneys, flues and recessed lights that are not IC-rated, according to local code. Stop and call a professional if you find knob-and-tube wiring or vermiculite, which may contain asbestos.
Frequently asked questions
Is R-30 enough for an attic?
R-30 meets the low end of the US recommendation for hot climates (zone 1). Zones 2 and 3 call for R-49 to R-60, and zones 4 to 8 for R-60, so in most of the US and Canada R-30 is worth topping up.
Can you have too much attic insulation?
Beyond the recommended level, each extra inch saves less, so the payback slows. Make sure added insulation never blocks soffit vents or covers non-IC recessed lights.
How deep should blown-in insulation be for R-49?
About 14 inches of cellulose or about 19.6 inches of blown fiberglass, settled. Follow the bag's chart for the exact initial and settled depths.
Do the math for your own project.
Open the insulation calculator