Photo: Engin Akyurt

How deep should blown-in insulation be?

The recommended level for most attics is to insulate to R-38 or about 10 to 14 inches, depending on insulation type.

energystar.gov - Adding Attic Insulation | ENERGY STAR

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Now that you've air-sealed your attic and basement, check your attic insulation levels and add more if necessary. The attic is the easiest place to add insulation to improve your comfort and the energy efficiency of your home.

Do I Have Enough?

No matter what kind of insulation you currently have in your attic, one quick way to determine if you need more is to look across the span of your attic. If your insulation is just level with or below your floor joists (i.e., you can easily see your joists), you should add more. If you cannot see any of the floor joists because the insulation is well above them, you probably have enough and adding more may not be cost-effective. It is important that the insulation be evenly distributed with no low spots; sometimes there is enough insulation in the middle of the attic and very little along the eaves. To see how to add insulation out to the eaves, see Installing Rafter Vents. If your attic insulation covers your joists and is distributed evenly, you probably have enough. Insulation levels are specified by R-Value. R-Value is a measure of insulation's ability to resist heat flow. The higher the R-Value, the better the thermal performance of the insulation. The recommended level for most attics is to insulate to R-38 or about 10 to 14 inches, depending on insulation type. When adding additional insulation, you do not have to use the same type of insulation that currently exists in your attic. You can add loose fill on top of fiberglass batts or blankets, and vice-versa. If you use fiberglass over loose fill, make sure the fiberglass batt has no paper or foil backing; it needs to be "unfaced." If you choose to add loose fill, it may be wise to hire a professional, as the application requires the use of a blowing machine, although some home improvement stores offer rentals of this machine. For additional details and images, download the Seal and Insulate DIY Guide (PDF, 12.8 MB).