attic insulation

I finished adding some insulation to the attic today. The attic above the heated apartment is insulated with old gray fiberglass batts partially covered with 1/2″ plywood. The plywood provides a surface that allow the attic to be used for storage. After climbing around up there and checking things out I determined that many of the batts were cut too short and didn’t go all the way to the wall. There were also areas along each side of the attic between the joists and exterior wall that had not been insulated at all. Gaps and and thin spots in the insulation were common along the exterior edges, while the insulation under the plywood seemed to be in pretty good shape. All of these areas of thin and no insulation can drastically reduce the overall insulating capability of the attic floor.

The local building supply carried bags of loose fill fiberglass that is generally used for blow in applications. One bag of densely packed fiberglass was $26. I figured one bag would be plenty to fix the issues in the attic and would be a really inexpensive way to reduce the heating bill. I had never dealt with loose fill fiberglass and was amazed at how much volume is achieved when removing the insulation from the tightly wrapped package and tearing it into small pieces as instructed. There was more than enough insulation to finish the job; in fact I only used about half the bag or $13 worth of insulation.

Uninsulated area between floor joists and wall studs.

New insulation between floor joists and wall studs, and filling in gaps in existing insulation.

Insulation as shown above was added around the entire perimeter of the attic where needed. It was amazing at how many small gaps there were in existing insulation. I determined that the total volume of insulation added is the equivalent of what would have covered the entire attic floor with just over 1″ of new insulation. This is roughly like adding R 3 to the attic, a very modest gain. However, it may be more beneficial than the R 3 estimate would suggest because small uninsulated sections can significantly reduce the overall insulating ability.

The total insulation on the floor of the attic is still only about R 20. There is insulation in the rafters that someone installed for an unknown reason. This insulation is basically worthless because the attic is vented to the outside on the gable ends, so any heat making it into the attic can just flow with the air out these vents. Now that the existing insulation is in order, the next step is to add more. R 20 in the attic is very low for a cold climate, if the attic was not floored with plywood and used for storage it would be really easy to blow in another 12″ of loose fill fiberglass or cellulose. However, framing in a new floor on top of the old and filling it with a second layer of insulation is probably the best answer. I’m thinking 2×4’s with polyiso.

Insulation was added where some batts of the old were cut short.

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About thomas

I'm currently a graduate Water Resources Engineering student studying at the University of Guelph in Ontario, Canada. I completed a B.S. in Geosciences last year at the University of Montana, while taking additional courses in energy technology and history. I have a strong interest in renewable energy and resource conservation, and plan to continue pursuing interests in this area.
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One Response to attic insulation

  1. Enjoyed the article and the pictures. The insulation that was added to the rafters will help keep your attic cooler when the hot sun beats down on the roof.

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