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Attic insulation, vapor retarder, radiant barrier: Advice needed

NS62960 | Posted in Energy Efficiency and Durability on

Hi, I live in the 4a climate zone. Looking to insulate a small (16 x 30) attic, which currently has little to no insulation and no HVAC in the attic. I’m planning to remove the existing (minimal) insulation, prior to sealing and insulating. I’d like to do this properly but I don’t know what is the best way to insulate this attic? Does it need a vapor retarder? Is a radiant barrier needed? Should I use Batts, blown-in or spray foam? I think I’d prefer blown in over Batts, which raises the vapor retarder question.

Any advice would be greatly appreciated.

As a side note the attic only has a gable vent at one end of the attic with no “soffit” vents. If you have any advice on how to best remedy the ventilation issue too, that would be great.


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "I don't know what is the best way to insulate this attic."

    A. These two articles will explain what you need to know (click the links):

    Air Sealing an Attic

    How to Insulate an Attic Floor

    Q. "Does it need a vapor retarder?"

    A. In colder climates, building codes require the installation of a vapor retarder -- a less stringent layer than a vapor barrier. Vapor retarder paint fulfills this requirement. For more information, see Do I Need a Vapor Retarder?

    Q. "Is a radiant barrier needed?"

    A. No.

    Q. "Should I use batts, blown-in or spray foam?"

    A. Blown-in cellulose would be best.

    Q. "I think I'd prefer blown-in over batts, which raises the vapor retarder question."

    A. Use vapor-retarder paint on your ceiling. Actually, if a code official isn't involved, you can skip this step. Just pay attention to air sealing and you will be fine.

    Q. "If you have any advice on how to best remedy the ventilation issue too, that would be great."

    A. Read this article: All About Attic Venting.

  2. NS62960 | | #2

    Thank you for the links.

    I have reviewed the 'Air Sealing an attic' link and I want to mention that the ceiling of the rooms below is 3" toungue and groove boards, which to my way of thinking, offers up an opportunity to leak/seal, every 3 inches. How would you recommend sealing this?

    Thanks again.

  3. GBA Editor
    Martin Holladay | | #3

    The best approach is usually to remove all of the ceiling boards -- carefully, of course, because you will be re-installing them. This is often easy, as long as the boards weren't installed with ring-shank nails. If the ceiling boards are hard to remove, and you end up damaging or splitting the boards, this obviously isn't a good approach.

    Once the ceiling is removed, you can install a drywall ceiling to act as your air barrier. Then you can tape the drywall seams and re-install the ceiling boards.

    If you don't want to do this, and you want to work from above, you might try a cut-and-cobble approach from above with rigid foam. (For more on the cut-and-cobble approach, see Cut-and-Cobble Insulation.)

    The only other way I know of creating a good air seal from above would be to insert cardboard rectangles in each joist bay (to prevent spray foam from expanding between the cracks between the boards) and then installing a thin layer of spray foam from above -- either just at the perimeter of each piece of cardboard, or over the entire field of the cardboard (and the edges, of course).

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