Many homes with cathedral ceilings or flat roofs have little or no insulation. The IRC 2009 building code requires a ventilated space of at least one inch above the roof insulation. Building scientists sometimes ask how useful the ventilation is and how much ventilation could actually flow in the one-inch space.
Many cathedral roof cavities have been dense-packed with fiberglass insulation without providing space for ventilation and some experts believe this is an effective solution. However, this solution usually requires engineered plans approved by your building department.

The Best Type of Roof Insulation

The dense-packed roof cavity usually resists moisture problems because:

  • Air can’t circulate through the dense-packed insulation to bring moisture or to precipitate it
  • Fiberglass insulation isn’t damaged by water
  • Fiberglass insulation drains water out to the lowest point

Dense-packing a roof cavity with cellulose isn’t a good option because of the high moisture absorption of cellulose.

Preparing for Roof Insulation

To prepare for roof-cavity insulation, consider these suggestions.

  • In cold climates, install a vapor barrier at the ceiling. At a minimum, paint an oil-based primer over the interior drywall or plaster.
  • Repair roof leaks or install a new water-tight roof. Replace moisture-damaged sheathing as part of the roof replacement.
  • When replacing the roof, consider installing 2-to-12 inches of rigid, high-density foam insulation on top of the roof deck. If you choose this option, dense-pack the existing roof cavity first with fiberglass.
  • Install an air-barrier ceiling (drywall) if the existing ceiling isn’t an adequate air barrier, for example: tongue-and-groove paneling.
  • Remove recessed light fixtures and replace them with surface-mounted fixtures.
  • Seal other air leaks with great care, especially at the perimeter and around the ridge beams.

I know of dozens of jobs where this method has been used with a higher R-value and lower cost compared to building an air space. Many roofs would have never been insulated without this allowance for the unventilated roof.