The most important parameter for attic insulation is the R-value. Compared to the rest of the house, the attic should have an above-average R insulation value. In the US, we have 7 climate zones with 7 recommended R-values for attics; we’re going to look at each of them in turn.
Here’s why attic insulation thickness is so important:
- In the summer, the attic gets the lion’s share of the sun. Without adequate insulation and proper ventilation, a hot attic can substantially increase the cost of air conditioning.
- In the winter, the hot air rises to the top of the house (to the attic). Without adequate insulation, that heat is slipping away quickly, which increases heating costs.
2 inches of fiberglass bat insulation can add R7.4 to the attic (R3.7 per inch).
To reduce both heating and cooling costs, the US Department Of Energy (DOE) published attic insulation recommendations based on location in the US. Recommended R-values for the attic range from R30 to R60.
Here is the full DOE’s recommended attic insulation R-value chart:
Attic R-Value Chart For US Climate Zones
| Climate Zone: | Recommended Attic R-Value: | Additional Attic R-Value To 3-4 Inch Insulation: | States Examples: |
| Zone 1: | R30 – R49 | R19 – R38 | South Florida, Hawaii |
| Zone 2: | R39 – R60 | R38 – R49 | Florida, South Texas, South Arizona |
| Zone 3: | R49 – R60 | R38 – R49 | California, North Texas, Alabama, Georgia |
| Zone 4: | R60 | R49 | Tennessee, Kentucky, West Oregon, Virginia |
| Zone 5: | R60 | R49 | Nevada, Utah, Oregon, Nebraska, Ohio, Illinois |
| Zone 6: | R60 | R49 | New York, Montana, Wyoming, Iowa, Dakotas |
| Zone 7: | R60 | R49 | North Dakota, North Minnesota |
In this chart, you can notice that colder states (Zone 7, Zone 6, Zone 5, Zone 4) all have the recommended R60 attic insulation. If you are already using the standard 3-4 inches of insulation, you should add insulation with a total R-value of R49.
The premise here is to protect your house from heat loss in the winter as much as possible.
In warmer states like California, Alabama, Georgia, and North Texas, you can use R49 to R60 attic insulation.
In very warm states like Florida, South Arizona, and South Florida, you don’t need as much attic insulation. In this Zone 3, you should use anywhere from R39 to R60 attic insulation. R60, for example, might be a bit overkill here.
In the hottest US states like South Florida and Hawaii (Zone 1), R30 to R49 attic insulation will be enough.
If you want to know exactly what is the recommended minimum R-value attic insulation, you can check this DOE’s map and check the exact climate zone your house is built in. When you know that, you can just check this R-value attic insulation chart to see how well you should insulate your attic.