My recommendation for adding thermal insulation to the roof of a house without disturbing the interior.

If you have a house having a ceiling just under the roof, as is the case of a finished attic, garret room, etc., the following outline for the installation of thermal insulation might be right for you. Without mess or disruption to the interior space, you can increase the R-value of your roof. In fact, no invasiveness to the ceiling is necessary. R-value is the numeration-scale reference to an insulator’s resistance to heat transfer. This will alter the the appearance of the facia area; the result will be a wider facia dimension. So, this wouldn't be applicable to your antique house, for example, if you want to preserve the original look.

Directions:

Remove the existing shingles and fascia board.

Install the rafters, constructed as a frame atop the old roof, using the same dimensions, therefor matching the existing roof-area. The rafters should probably be spaced at 16" intervals (16" centers) which will accommodate the standard 14 ½"-width-size fiberglass thermal insulation.

For installation of high R-value insulation, I recommend using boards of the 6-inch-size for the rafters, "two by six"—which are actually 1½" x 5½". The additional cost over a smaller board- and insulation-size is negligible especially when considering the HVAC operational-cost savings. So, with these-sized rafters, the "6-inch" thickness insulation would be used to fill the spaces between the rafters.

Then, the rafter framework should be covered by, at least, 5/8-inch CDX plywood to which the roofing felt and shingles will be applied. The designation, CDX, means that the plies are stuck together with water-resistant glue.

Lightly colored shingles should be used to further prevent solar heat transfer.

Then, install the appropriate, new-width fascia board.

Install vents to allow condensation evaporation. These would probably be located between each rafter through the fascia board—say, round and louvered approximately about 6 cm, diameter.

If you confer with a roofer or general contractor who is hesitant, or has little imagination about this or any construction matters, move on to another.

Before the plywood covering is installed, make sure that photos are taken of the new frame and insulation! This will certainly be of great value upon transference of the house to the next owner.

I guess that this improvement will reduce heating and cooling costs by approximately 30%.

Richard Stuart Otto, 13 July 2007