Photo: Sarah Chai

How many inches of loose fill insulation do I need?

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).

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Does loose fill insulation need a vapor barrier?

A common misconception is that vapor barriers are required in all insulation installations. With Modern's Cellulose insulation, a vapor barrier is not recommended except in very cold weather or high moisture areas such as spas or pool buildings.

moderninsulationinc.com - Air / Vapor Barrier - Modern Insulation

Air / Vapor Barrier

Air infiltration is almost as important as R-value in the thermal performance of a building. Infiltration of unconditioned ambient air means that heating and cooling systems must expend more energy to compensate for the infiltration. A common misconception is that vapor barriers are required in all insulation installations. With Modern’s Cellulose insulation, a vapor barrier is not recommended except in very cold weather or high moisture areas such as spas or pool buildings. However, many building codes do require an Air barrier or Vapor retarder to be installed. Check your local building codes for compliance. Insulation inhibits the movement of air from warm to cold (and vice versa) by trapping air or gas between its fibers or pores. The higher the density of the material used, the better it is at inhibiting the movement of air. Cellulose insulation has a high density compared to fiberglass. Blow-in Cellulose insulation seals houses better by limiting the air flow, not only through the insulating material, but also around difficult to insulate areas such as the gaps around electrical boxes, wiring and plumbing. Cellulose insulation can also handle non-standard or off-center wall stud spacing areas better than batts. Field tests have shown that Cellulose insulation can provide a building envelope that is 36% tighter than a fiberglass insulation seal. To prove the point, Canadian engineers conducted a test on a new cellulose-insulated home for air tightness. First the engineers measured the air leakage with a polyethylene air/vapor barrier around installed before siding had been installed. The engineers then slit the polyethylene air/vapor barrier in approximately 20 places and retested air leakage of the building. There was absolutely no change in measured air leakage. Additional Smoke Pencil Testing at the slits showed “not a breath” of air leakage.