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How to Add Attic Insulation

Stuck in your house? Make the most of your time with some basic home improvements. We’ll be making regular updates to the HIR blog with project recommendations for those of you under mandatory isolation (like us!).

First up…. Do you have the required amount of attic insulation? If not, add some. Insulation protects your conditioned indoor air from the outdoor changes in temperature. It helps keep your home warm during winter and cool during summer, and by doing so it saves energy, thus lowering your bills.

But how do you know if you need more insulation?

Home Insulation R-Values Insulation levels are specified by R-Value, which is a measure of the insulation’s ability to resist heat traveling through it. The higher the R-Value, the better the thermal performance of the insulation. The geographic area of your home determines its required insulation level. For example, the Department of Energy recommends that the warm, southern regions of the country should have enough attic insulation to achieve a value of R-30 to R-60. The northern, colder areas of the U.S. require at least R-49 for the system to be cost-effective.

So, your first step is to refer to the chart below to determine your region’s recommended R-value.

The amount of insulation needed to achieve a certain R-value depends on the type of insulation being used. Materials vary. For example, 3-1/2 in. of fiberglass batt insulation will achieve R-13, while 2 inches of rigid foam polystyrene achieves R-10. The R-value will be printed on the package of the insulation product, so you can easily calculate how much is required. R-Values of individual products can be added to achieve recommended levels. An R-38 added to an R-11 results in R-49.

So, your next step will be to measure the thickness of your existing insulation and, based on the type of material, determine its current R-value. If your attic insulation’s R-value is below the recommended level, then you should probably add more.

This attic’s insulation level came up a wee bit short.

You can supplement the existing insulation with unfaced fiberglass rolls or with loose-fill cellulose–or a combination of both.

For a new attic with no insulation whatsoever, you can rent a blowing machine to apply the loose-fill (or “blow-in”) insulation, available at many local home centers. For big jobs, the blowing machines can usually be found at the same place you buy the insulation, often with no additional charge to the insulation you purchase.

But a blowing machine can have its drawbacks. The machines require two people to operate them, they can be messy, and they’re not always an accurate means of applying the product. An alternative for smaller jobs is to simply fill up the joist bays by hand with the loose-fill cellulose. This was my approach on a recent project–followed by an additional layer of roll insulation.

On this project, I filled the joist bays with Greenfiber loose-fill cellulose insulation, then topped it all with rolls of Knauf mineral wool insulation.

First, I filled the attic joist bays with the loose-fill cellulose, leveling it with the surface of the attic joists. Blow-in insulation and loose-fill insulation are essentially the same product, the primary difference being the way you install it. On this job, I simply shoveled it into the joist bays by hand as needed. The cellulose material is very compressed inside the package, however, so break it up as you install it to stretch the product’s coverage and to create lots of internal air pockets that will help achieve a thermal break.

When dealing with cellulose or fiberglass insulation, I recommend using eye protection, a face mask, gloves and long sleeves to avoid itching and irritation. Also, when working in an attic, be very careful. If you step between the attic joists, your foot will likely plunge through the ceiling drywall. To avoid this, use plywood (at least 1/2″ thick) as a work platform to bridge over the joists and support your weight while you work.

After filling the joist bays with cellulose, I had a flat attic floor but still needed more insulation, since I now had roughly 5-1/2 inches of insulation and wanted it about 12 inches deep.

I then rolled out the unfaced “glass mineral wool” insulation on top of the cellulose and across the floor joists. Make sure to use an unfaced product if adding extra insulation, so the new layer does not trap moisture. I installed the rolls perpendicular to the joists so they would cover the wood framing. (Wood is not a very effective thermal break by itself.)

To cut rolls or batts of mineral wool (or fiberglass) insulation, use a utility knife that has an extendable blade, and guide the cut with a rigid straight edge.

When all is said and done, you’ll have a much thicker bed of attic insulation with stronger R-value to save energy in your home.


  1. If you’re installing additional insulation near the eaves of a home with soffit vents, consider using soffit vent baffles to allow the vent to stay open above the insulation for proper airflow.

  2. Electrical codes generally require that insulation be kept at least 3 inches away from heat-producing fixtures. A common solution is to nail 2x blocking around the fixtures perpendicular to and flush with the ceiling joists, which creates a small protective enclosure for the fixture.

  3. After you’ve finished adding attic insulation, complete the job by installing an attic access-door cover or insulating it with batt or foam-board insulation.

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